DeGarmo's Materials and Processes in Manufacturing 11th E2d - PDFCOFFEE.COM (2024)

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DeGarmo’s

MATERIALS AND PROCESSES IN MANUFACTURING ELEVENTH EDITION J T. Black

Auburn University-Emeritus

Ronald A. Kohser Missouri University of Science & Technology

John Wiley & Sons, Inc.

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Vice President & Executive Publisher: Executive Editor: Production Manager: Senior Production Editor: Marketing Manager: Creative Director: Cover Designer: Production Management Services: Senior Illustration Editor: Photo Researcher: Editorial Assistant: Executive Media Editor: Media Editor: Cover Photo Credit:

Don Fowley Linda Ratts Dorothy Sinclair Valerie A. Vargas Chris Ruel Harry Nolan Wendy Lai Integra-Chicago Anna Melhorn Sheena Goldstein Christopher Teja Thomas Kulesa Wendy Ashenberg # Ford Motor Company, # Mark Evans/iStockphoto, Handke-Neu Handke-Neu/Photolibrary

This book was set in 10/12 Times Ten by Thomson Digital and printed and bound by Courier Graphics-Kendallville. The cover was printed by Courier Graphics-Kendallville. Copyright # 2012 (2008, 2004, 1997) John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc. 222 Rosewood Drive, Danvers, MA 01923, website www.copyright .com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030-5774, (201)748-6011, fax (201)748-6008, website http://www.wiley.com/go/permissions. Founded in 1807, John Wiley & Sons, Inc. has been a valued source of knowledge and understanding for more than 200 years, helping people around the world meet their needs and fulfill their aspirations. Our company is built on a foundation of principles that include responsibility to the communities we serve and where we live and work. In 2008, we launched a Corporate Citizenship Initiative, a global effort to address the environmental, social, economic, and ethical challenges we face in our business. Among the issues we are addressing are carbon impact, paper specifications and procurement, ethical conduct within our business and among our vendors, and community and charitable support. For more information, please visit our website: www.wiley.com/go/citizenship. Evaluation copies are provided to qualified academics and professionals for review purposes only, for use in their courses during the next academic year. These copies are licensed and may not be sold or transferred to a third party. Upon completion of the review period, please return the evaluation copy to Wiley. Return instructions and a free of charge return shipping label are available at www.wiley.com/go/returnlabel. Outside of the United States, please contact your local representative. ISBN-13 978-0-470-92467-9 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

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ABOUT THE COVER The four vehicles pictured on the cover were selected to represent a spectrum of manufacturing conditions, where the products may be similar, but the ‘‘materials’’ and ‘‘processes’’ are often quite different. The 1912 Model T Ford is representative of an era of great manufacturing change. Early models had leather fenders and wooden-spoke wheels, a carry-over from the days of the horse-and-buggy or carriage. Those vehicles were often hand-made by skilled artisans. Later models had wooden floors and roofs made from chicken wire, cotton mattress batting, and a sheet of rubberized vinyl stretched over wooden spars, and were produced on a moving assembly line. The soapbox racer, a one-of-a-kind item, and formula racer are both limited-quantity products, for which part-specific tooling may be prohibitive. A $5000 forming die to make 50,000 identical pieces only costs 10 cents a part. If only 10 parts are to be produced, however, each part has incurred a $500 tooling expense. Processes such as direct-digital manufacturing can economically address such limited quantities. While both are racers, the operating conditions and demands are worlds apart. One is gravity propelled while the other is powered by hundreds of horsepower and reaches speeds in excess of 200 miles per hour. The stresses that the materials must endure are extremely different. For the formula car, performance clearly overshadows cost when making manufacturing decisions. In contrast, the production of a popular family sedan, like the depicted Ford Taurus, clearly justifies part-specific tooling, but these parts must be made quickly and economically using the principles of lean manufacturing. Passenger safety, fuel economy, comfort, and even recyclability may be as important as performance. Nearly all of the materials and processes described in this book find themselves employed on one or more of the four depicted vehicles. Each of those material-process combinations was selected because it offered the best overall match to the needs of the specific product. As new materials and processes are developed, these ‘‘best’’ solutions will be constantly changing. We invite the reader to open the text and explore this fascinating area of engineering and technology.

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CONTENTS Preface

v

Chapter 1 Introduction to DeGarmo's Materials and Processes in Manufacturing 1 1.1

Materials, Manufacturing, and the Standard

4.12 Plastic Deformation in Polycrystalline Metals

100

4.14 Fracture of Metals

101

4.15 Cold Working, Recrystallization, and Hot Working

102 102

1

4.17 Alloys and Alloy Types

Manufacturing and Production Systems

3

4.18 Atomic Structure and Electrical

Case Study Famous Manufacturing Engineers

28

Chapter 2 Design

Manufacturing Systems 30

2.1

Introduction

30

2.2

Manufacturing Systems

30

2.3

Control of the Manufacturing System

32

2.4

Classification of Manufacturing Systems

33

2.5

Summary of Factory Designs

49

Case Study Jury Duty for an Engineer

Chapter 3

Properties of Materials

58

59

Properties

103

Chapter 5 Equilibrium Phase Diagrams and the Iron–Carbon System 106 5.1

Introduction

106

5.2

Phases

106

5.3

Equilibrium Phase Diagrams

106

5.4

Iron–Carbon Equilibrium Diagram

114

5.5

Steels and the Simplified Iron–Carbon Diagram

116

Cast Irons

117 120

3.1

Introduction

59

5.6

3.2

Static Properties

61

Case Study Fish Hooks

3.3

Dynamic Properties

74

3.4

Temperature Effects (Both High and Low)

79

Chapter 6

3.5

Machinability, Formability, and Weldability

83

3.6

Fracture Toughness and the Fracture Mechanics Approach

83

3.7

Physical Properties

85

3.8

Testing Standards and Testing Concerns

85

Case Study Separation of Mixed Materials

Chapter 4 Nature of Metals and Alloys 4.1

88

89

Structure–Property–Processing–Performance Relationships

89

4.2

The Structure of Atoms

90

4.3

Atomic Bonding

90

4.4

Secondary Bonds

92

4.5

Atom Arrangements in Materials

92

4.6

Crystal Structures of Metals

93

4.7

Development of a Grain Structure

94

4.8

Elastic Deformation

95

4.9

Plastic Deformation

96

4.10 Dislocation Theory of Slippage

97

4.11 Strain Hardening or Work Hardening

98

101

4.16 Grain Growth

of Living 1.2

99

4.13 Grain Shape and Anisotropic Properties

Heat Treatment

121

6.1

Introduction

6.2

Processing Heat Treatments

122

6.3

Heat Treatments Used to Increase Strength

125

6.4

121

Strengthening Heat Treatments for Nonferrous Metals

125

6.5

Strengthening Heat Treatments for Steel

128

6.6

Surface Hardening of Steel

143

6.7

Furnaces

145

6.8

Heat Treatment and Energy

147

Case Study A Carpenter’s Claw Hammer

150

Chapter 7 Alloys 7.1

Ferrous Metals and 152

Introduction to History-Dependent Materials

152

7.2

Ferrous Metals

152

7.3

Iron

153

7.4

Steel

154

7.5

Stainless Steels

168

7.6

Tool Steels

171

7.7

Cast Irons

173

7.8

Cast Steels

178

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The Role of Processing on Cast Properties

Case Study The Paper Clip

Chapter 8 Alloys

179

11.4 The Solidification Process

271

181

11.5 Patterns

281

11.6 Design Considerations in Castings

284

11.7 The Casting Industry

287

Case Study The Cast Oil-Field Fitting

290

Nonferrous Metals and 182

8.1

Introduction

182

8.2

Copper and Copper Alloys

183

8.3

Aluminum and Aluminum Alloys

188

8.4

Magnesium and Magnesium Alloys

196

8.5

Zinc and Zinc Alloys

8.6

Chapter 12 Expendable-Mold Casting Processes

291

12.1 Introduction

291

198

12.2 Sand Casting

292

Titanium and Titanium Alloys

199

12.3 Cores and Core Making

307

8.7

Nickel-Based Alloys

201

12.4 Other Expendable-Mold Processes with

8.8

Superalloys, Refractory Metals, and

Multiple-Use Patterns

Other Materials Designed for

311

12.5 Expendable-Mold Processes Using Single-Use

High-Temperature Service

201

Lead and Tin, and Their Alloys

203

12.6 Shakeout, Cleaning, and Finishing

320

8.10 Some Lesser-Known Metals and Alloys

204

12.7 Summary

320

8.11 Metallic Glasses

204

Case Study Movable and Fixed Jaw Pieces for a

8.12 Graphite

205

Case Study Hip Replacement Prosthetics

207

8.9

Chapter 9 Nonmetallic Materials: Plastics, Elastomers, Ceramics, and Composites 208 9.1

Introduction

208

9.2

Plastics

209

9.3

Elastomers

222

9.4

Ceramics

224

9.5

Composite Materials

234

Case Study Lightweight Armor

247

Chapter 10

Material Selection

10.1 Introduction

248 248

10.2 Material Selection and Manufacturing Processes

Patterns

Heavy-Duty Bench Vise

Chapter 13 Multiple-Use-Mold Casting Processes

313

322

323

13.1 Introduction

323

13.2 Permanent-Mold Casting

323

13.3 Die Casting

327

13.4 Squeeze Casting and Semisolid Casting

331

13.5 Centrifugal Casting

333

13.6 Continuous Casting

335

13.7 Melting

336

13.8 Pouring Practice

339

13.9 Cleaning, Finishing, and Heat Treating of Castings

339

13.10 Automation in Foundry Operations

341

252

13.11 Process Selection

341

10.3 The Design Process

252

Case Study Baseplate for a Household Steam Iron

344

10.4 Approaches to Material Selection

253

10.5 Additional Factors to Consider

256

10.6 Consideration of the Manufacturing Process

258

Chapter 14 Fabrication of Plastics, Ceramics, and Composites 345

10.7 Ultimate Objective

258

14.1 Introduction

345

260

14.2 Fabrication of Plastics

345

14.3 Processing of Rubber and Elastomers

359

261

14.4 Processing of Ceramics

360

262

14.5 Fabrication of Composite Materials

364

266

Case Study Automotive and Light Truck Fuel Tanks 378

10.8 Materials Substitution 10.9 Effect of Product Liability on Materials Selection 10.10 Aids to Material Selection Case Study Material Selection

Chapter 11 Casting

Fundamentals of 267

Chapter 15 Fundamentals of Metal Forming

379

11.1 Introduction to Materials Processing

267

15.1 Introduction

379

11.2 Introduction to Casting

269

15.2 Forming Processes: Independent Variables

380

11.3 Casting Terminology

270

15.3 Dependent Variables

382

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15.4 Independent–Dependent Relationships

382

18.8 Sintering

489

15.5 Process Modeling

383

18.9 Recent Advances in Sintering

490

15.6 General Parameters

384

18.10 Hot-Isostatic Pressing

491

18.11 Other Techniques to Produce High-Density

15.7 Friction and Lubrication Under Metalworking Conditions

385

15.8 Temperature Concerns

387

15.9 Formability

395

Case Study Interior Tub of a Top-Loading Washing Machine

Chapter 16 Bulk-Forming Processes 16.1 Introduction

397

398 398

16.2 Classification of Deformation Processes

398

16.3 Bulk Deformation Processes

399

16.4 Rolling

399

16.5 Forging

406

16.6 Extrusion

418

16.7 Wire, Rod, and Tube Drawing

424

16.8 Cold Forming, Cold Forging, and Impact Extrusion

427 431

16.10 Other Squeezing Processes

432

16.11 Surface Improvement by Deformation 434

Case Study Handle and Body of a Large Ratchet Wrench

Chapter 17 Sheet-Forming Processes

439

440

492

Molding

493

18.13 Secondary Operations

495

18.14 Properties of Powder Metallurgy Products

497

18.15 Design of Powder Metallurgy Parts

498

18.16 Powder Metallurgy Products

499

18.17 Advantages and Disadvantages of Powder Metallurgy

501

18.18 Process Summary

502

Case Study Steering Gear for a Riding Lawn Mower/Garden Tractor

Chapter 19 Additive Processes: Rapid Prototyping and DirectDigital Manufacturing 19.1 Introduction

16.9 Piercing

Processing

Powder Metallurgy Products 18.12 Metal Injection Molding or Powder Injection

506

507 507

19.2 Rapid Prototyping and Direct-Digital Manufacturing

508

19.3 Layerwise Manufacturing

510

19.4 Liquid-Based Processes

514

19.5 Powder-Based Processes

517

19.6 Deposition-Based Processes

521

19.7 Uses and Applications

524

19.8 Pros, Cons, and Current and Future Trends

528

19.9 Economic Considerations

529

17.1 Introduction

440

17.2 Shearing Operations

440

17.3 Bending

449

Chapter 20 Fundamentals of Machining/Orthogonal Machining

17.4 Drawing and Stretching Processes

456

20.1 Introduction

533

17.5 Alternative Methods of Producing Sheet-Type

533

20.2 Fundamentals

533

471

20.3 Forces and Power in Machining

541

17.6 Pipe Manufacture

472

20.4 Orthogonal Machining (Two Forces)

547

17.7 Presses

472

20.5 Chip Thickness Ratio, rc

551

Case Study Automotive Body Panels

480

20.6 Mechanics of Machining (Statics)

553

20.7 Shear Strain, g, and Shear Front Angle, w

555

20.8 Mechanics of Machining (Dynamics)

557

20.9 Summary

564

Products

Chapter 18

Powder Metallurgy

481

18.1 Introduction

481

18.2 The Basic Process

482

18.3 Powder Manufacture

483

18.4 Microcrystalline and Amorphous Material

Case Study Orthogonal Plate Machining Experiment at Auburn University

568

18.5 Powder Testing and Evaluation

484

Chapter 21 Cutting Tools for Machining

18.6 Powder Mixing and Blending

485

21.1 Introduction

569

18.7 Compacting

485

21.2 Cutting Tool Materials

573

Produced by Rapid Cooling

484

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21.3 Tool Geometry

587

25.7 Introduction to Filing

709

21.4 Tool-Coating Processes

589

Case Study Cost Estimating—Planing vs. Milling

713

21.5 Tool Failure and Tool Life

592

21.6 Flank Wear

593

21.7 Cutting Fluids

599

Chapter 26 Abrasive Machining Processes 714

21.8 Economics of Machining

600

26.1 Introduction

714

26.2 Abrasives

715

26.3 Grinding Wheel Structure and Grade

721

26.4 Grinding Wheel Identification

726

26.5 Grinding Machines

730

26.6 Honing

738

26.7 Superfinishing

740

26.8 Free Abrasives

742

Case Study Comparing Tool Materials Based on Tool Life

Chapter 22 Turning and Boring Processes 22.1 Introduction

608

609 609

22.2 Fundamentals of Turning, Boring, and Facing 611

26.9 Design Considerations In Grinding

746

22.3 Lathe Design and Terminology

617

Case Study Process Planning for the MfE

748

22.4 Cutting Tools for Lathes

625

22.5 Workholding in Lathes

629

Chapter 27 Workholding Devices for Machine Tools

749

636

27.1 Introduction

749

Turning

Case Study Estimating the Machining Time for Turning

27.2 Conventional Fixture Design

749

Chapter 23 Drilling and Related Hole-Making Processes 637

27.3 Tool Design Steps

752

27.4 Clamping Considerations

753

23.1 Introduction

637

27.5 Chip Disposal

755

23.2 Fundamentals of the Drilling Process

638

27.6 Unloading and Loading Time

756

23.3 Types of Drills

640

27.7 Example of Jig Design

756

23.4 Tool Holders for Drills

652

27.8 Types of Jigs

758

23.5 Workholding for Drilling

654

27.9 Conventional Fixtures

759

23.6 Machine Tools for Drilling

654

27.10 Modular Fixturing

761

23.7 Cutting Fluids for Drilling

657

27.11 Setup and Changeover

763

27.12 Clamps

765

659

27.13 Other Workholding Devices

766

23.9 Reaming

659

27.14 Economic Justification of Jigs and Fixtures

769

Case Study Bolt-down Leg on a Casting

664

Case Study Fixture versus No Fixture

23.8 Counterboring, Countersinking, and Spot Facing

Chapter 24

Milling

665

in Milling

774

24.1 Introduction

665

24.2 Fundamentals of Milling Processes

665

Chapter 28 Nontraditional Manufacturing Processes

24.3 Milling Tools and Cutters

672

28.1 Introduction

775

24.4 Machines for Milling

678

28.2 Chemical Machining Processes

777

28.3 Electrochemical Machining Processes

783

28.4 Electrical Discharge Machining

790

Case Study Vented Cap Screws

802

Case Study HSS versus Tungsten Carbide Milling

685

Chapter 25 Sawing, Broaching, and Other Machining Processes 686

Chapter 29

Lean Engineering

775

803

25.1 Introduction

686

29.1 Introduction

803

25.2 Introduction to Sawing

686

29.2 The Lean Engineer

803

25.3 Introduction to Broaching

694

29.3 The Lean Production System

804

25.4 Fundamentals of Broaching

695

29.4 Linked-Cell Manufacturing System

25.5 Broaching Machines

703

25.6 Introduction to Shaping and Planing

704

Design Rules 29.5 Manufacturing System Designs

804 806

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Contents 29.6 Preliminary Steps to Lean Production

808

29.7 Methodology for Implementation of Lean Production

808

Chapter 33 Other Welding Processes, Brazing, and Soldering

xv

910

29.8 Design Rule MT < CT

823

33.1 Introduction

910

29.9 Decouplers

825

33.2 Other Welding and Cutting Processes

910

29.10 Integrating Production Control

828

33.3 Surface Modification by Welding-Related

29.11 Integrating Inventory Control

832

29.12 Lean Manufacturing Cell Design

833

29.13 Machine Tool Design for Lean Manufacturing Cells

838

29.14 L-CMS Strategy

841

Case Study Cycle Time for a Manufacturing Cell

844

Chapter 30 Joining

Fundamentals of

30.1 Introduction to Consolidation Processes

845 845

30.3 Some Common Concerns

33.4 Brazing

922

33.5 Soldering

931

Case Study Impeller of a Pharmaceutical Company Industrial Shredder/Disposal

Chapter 34 Adhesive Bonding, Mechanical Fastening, and Joining of Nonmetals

937

938 938

34.2 Mechanical Fastening

947

846

34.3 Joining of Plastics

951

847

34.4 Joining of Ceramics and Glass

953

34.5 Joining of Composites

954

Case Study Golf Club Heads with Insert

956

30.4 Types of Fusion Welds and Types of Joints

919

34.1 Adhesive Bonding

30.2 Classification of Welding and Thermal Cutting Processes

Processes

847

30.5 Design Considerations

850

30.6 Heat Effects

850

30.7 Weldability or Joinability

857

Chapter 35 Measurement and Inspection

30.8 Summary

858

35.1 Introduction

958

35.2 Standards of Measurement

959

35.3 Allowance and Tolerance

964

Chapter 31 Gas Flame and Arc Processes

860

958

35.4 Inspection Methods for Measurement

971

31.1 Oxyfuel-Gas Welding

860

35.5 Measuring Instruments

973

31.2 Oxygen Torch Cutting

864

35.6 Vision Systems for Measurement

982

31.3 Flame Straightening

866

35.7 Coordinate Measuring Machines

983

31.4 Arc Welding

867

35.8 Angle-Measuring Instruments

984

31.5 Consumable-Electrode Arc Welding

869

35.9 Gages for Attributes Measuring

986

31.6 Nonconsumable-Electrode Arc Welding

877

Case Study Measuring An Angle

993

31.7 Welding Equipment

882

31.8 Arc Cutting

884

Case Study Bicycle Frame Construction and Repair

Chapter 32 Resistance- and Solid-State Welding Processes

Quality Engineering 994

36.1 Introduction

994

886

36.2 Determining Process Capability

996

888

36.3 Introduction to Statistical Quality Control

1004

36.4 Sampling Errors

1008

36.5 Gage Capability

1009

36.6 Just in Time/Total Quality Control

1010

31.9 Metallurgical and Heat Effects in Thermal Cutting

Chapter 36

890

32.1 Introduction

890

36.7 Six Sigma

1021

32.2 Theory of Resistance Welding

890

36.8 Summary

1024

32.3 Resistance-Welding Processes

893

Case Study Boring QC Chart Blunders

1029

898

Chapter 37

899

37.1 Introduction

1030

909

37.2 Abrasive Cleaning and Finishing

1040

32.4 Advantages and Limitations of Resistance Welding 32.5 Solid-State Welding Processes Case Study Manufacture of an Automobile Muffler

Surface Engineering 1030

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Contents 41.6 Fabricating Integrated Circuits on Silicon

37.3 Chemical Cleaning

1045

37.4 Coatings

1048

37.5 Vaporized Metal Coatings

1058

41.7 Thin-Film Deposition

1157

37.6 Clad Materials

1058

41.8 Integrated Circuit Packaging

1163

37.7 Textured Surfaces

1058

41.9 Printed Circuit Boards

1170

37.8 Coil-Coated Sheets

1059

41.10 Electronic Assembly

1175

37.9 Edge Finishing and Burrs

1059

Case Study Dana Lynn’s Fatigue Lesson

1064

Chapter 38 Micro/Meso/Nano Fabrication Processes

1067

Wafers

Chapter 42 Thread and Gear Manufacturing (web-based chapter)

1150

1181

(www.wiley.com/college/DeGarmo)

38.1 Introduction

1067

42.1 Introduction

1181

38.2 Additive Processes

1068

42.2 Thread Making

1186

38.3 Metrology at the Micro/Meso/Nano Level

1083

42.3 Internal Thread Cutting–Tapping

1190

42.4 Thread Milling

1193

42.5 Thread Grinding

1195

42.6 Thread Rolling

1195

42.7 Gear Making

1197

42.8 Gear Types

1200

42.9 Gear Manufacturing

1202

42.10 Machining of Gears

1203

42.11 Gear Finishing

1210

42.12 Gear Inspection

1212

Chapter 39 Manufacturing Automation

1086

39.1 Introduction

1086

39.2 The A(4) Level of Automation

1092

39.3 A(5) Evaluation or Adaptive Control

1101

39.4 A(6) Level of Automation and Beyond

1103

39.5 Robotics

1105

39.6 Computer-Integrated Manufacturing Automation

1113

39.7 Computer-Aided Design

1114

39.8 Computer-Aided Manufacturing

1117

39.9 Summary

1118

Chapter 40 NC/CNC Processes and Adaptive Control: A(4) and A(5) Levels of Automation 1122 40.1 Introduction

1122

40.2 Basic Principles of Numerical Control

1122

Chapter 43 Nondestructive Inspection and Testing (web-based chapter) (www.wiley.com/college/DeGarmo) 43.1 Destructive versus Nondestructive Testing Inspection

40.3 Machining Center Features and Trends

1136

40.4 Ultra-High-Speed Machining Centers

1139

40.5 Summary

1140

(www.wiley.com/college/DeGarmo)

(www.wiley.com/college/DeGarmo) 41.1 Introduction

1144

41.2 How Electronic Products Are Made

1144

41.3 Semiconductors

1145

41.4 How Integrated Circuits Are Made

1146

41.5 How the Silicon Wafer Is Made

1149

1215

43.2 Other Methods of Nondestructive Testing and

Chapter 44 The Enterprise (Production Systems) (web-based chapter)

Chapter 41 Microelectric Manufacturing and Assembly 1144 (web-based chapter)

1215

1226

1230

44.1 Introduction

1230

44.2 Axiomatic Design of Systems

1231

44.3 Enterprise System Design Principles

1232

44.4 Functional Areas in the Production System

1235

44.5 Human Resources (Personnel) Department

1240

Index

I-2

Selected References for Additional Study

S1

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PREFACE It’s a world of manufactured goods. Whether we like it or not, we all live in a technological society. Every day we come in contact with hundreds of manufactured items, made from every possible material. From the bedroom to the kitchen, to the workplace, we use appliances, phones, cars, trains, and planes, TVs, cell phones, VCRs, DVD’s, furniture, clothing, sports equipment, books and more! These goods are manufactured in factories all over the world using manufacturing processes. Basically, manufacturing is a value-adding activity, where the conversion of materials into products adds value to the original material. Thus, the objective of a company engaged in manufacturing is to add value and to do so in the most efficient manner, with the least amount of waste in terms of time, material, money, space, and labor. To minimize waste and increase productivity, the processes and operations need to be properly selected and arranged to permit smooth and controlled flow of material through the factory and provide for product variety. Meeting these goals requires an engineer who can design and operate an efficient manufacturing system. Here are the trends that are impacting the manufacturing world. Manufacturing is a global activity Manufacturing is a global activity with companies sending work to other countries (China, Taiwan, Mexico) to take advantage of low-cost labor. Many US companies have plants in other countries and foreign companies have built plants in the United States, to be nearer their marketplace. Automobile manufacturers from all around the globe and their suppliers use just about every process described in this book and some that we do not describe, often because they are closely held secrets. It’s a digital world Information technology and computers are growing exponentially, doubling in power every year. Every manufacturing company has ready access to world-wide digital technology. Products can be built by suppliers anywhere in the world working using a common set of digital information. Designs can be emailed to manufacturers who can rapidly produce a prototype in metal or plastic in a day. Lean manufacturing is widely practiced Most (over 60%) manufacturing companies have restructured their factories (their manufacturing systems) to become lean producers, making goods of superior quality, cheaper, faster in a flexible way (i.e., they are more responsive to the customers). Almost every plant is doing something to make itself leaner. Many of them have adopted some version of the Toyota Production System. More importantly, these manufacturing factories are designed with the internal customer (the workforce) in mind, so things like ergonomics and safety are key design requirements. So while this book is all about materials and processes for making the products, the design of the factory cannot be ignored when it comes to making the external customer happy with the product and the internal customer satisfied with the employer. New products and materials need new processes The number and variety of products and the materials from which they are made continues to proliferate, while production quantities (lot sizes) have become smaller. Existing processes must be modified to be more flexible, and new processes must be developed. Customers expect great quality Consumers want better quality and reliability, so the methods, processes, and people responsible for the quality must be continually improved. The trend toward (improving) zero defects and continuous improvement requires continual changes to the manufacturing system.

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Rapid product development is required Finally, the effort to reduce the time-to-market for new products is continuing. Many companies are taking wholistic or system wide perspectives, including concurrent engineering efforts to bring product design and manufacturing closer to the customer. There are two key aspects here. First, products are designed to be easier to manufacture and assemble (called design for manufacture/assembly). Second, the manufacturing system design is flexible (able to rapidly assimilate new products), so the company can be competitive in the global marketplace.

& HISTORY OF THE TEXT E. Paul DeGarmo was a mechanical engineering professor at the University of California, Berkley when he wrote the first edition of Materials and Processes in Manufacturing, published by Macmillan in 1957. The book quickly became the emulated standard for introductory texts in manufacturing. Second, third, and fourth editions followed in 1962, 1969, and 1974. DeGarmo had begun teaching at Berkeley in 1937, after earning his M.S. in mechanical engineering from California Institute of Technology. DeGarmo was a founder of the Department of Industrial Engineering (now Industrial Engineering and Operations Research) and served as its chair from 1956–1960. He was also assistant dean of the College of Engineering for three years while continuing his teaching responsibilities. Dr. DeGarmo observed that engineering education had begun to place more emphasis on the underlying sciences at the expense of hands on experience. Most of his students were coming to college with little familiarity with materials, machine tools, and manufacturing methods that their predecessors had acquired through the old ‘‘shop’’ classes. If these engineers and technicians were to successfully convert their ideas into reality, they needed a foundation in materials and processes, with emphasis on their opportunities and their limitations. He sought to provide a text that could be used in either a one-or two-semester course designed to meet these objectives. The materials sections were written with an emphasis on use and application. Processes and machine tools were described in terms of what they could do, how they do it, and their relative advantages and limitations, including economic considerations. Recognizing that many students would be encountering the material for the first time, clear description was accompanied by numerous visual illustrations. Paul’s efforts were well received, and the book quickly became the standard text in many schools and curricula. As materials and processes evolved, advances were incorporated into subsequent editions. Computer usage, quality control, and automation were added to the text, along with other topics, so that it continued to provide state-of-the-art instruction in both materials and processes. As competing books entered the market, their subject material and organization tended to mimic the DeGarmo text. Paul DeGarmo retired from active teaching in 1971, but he continued his research, writing, and consulting for many years. In 1977, after the publication of the fourth edition of Materials and Processes in Manufacturing, he received a letter from Ron Kohser, then an assistant professor at the University of Missouri-Rolla who had many suggestions regarding the materials chapters. DeGarmo asked Ron to rewrite those chapters for the upcoming fifth edition. After the 5th edition DeGarmo decided he was really going to retire and after a national search, recruited J T. Black, then a Professor at Ohio State, to co-author the book with Dr. Kohser. For the sixth through tenth editions (published in 1984 and 1988 by Macmillan, 1997 by Prentice Hall and 2003 and 2008 by John Wiley & Sons), Ron Kohser and J T. Black have shared the responsibility for the text. The chapters on engineering materials, casting, forming, powder metallurgy, additive manufacturing, joining and non-destructive testing have been written or revised by Ron Kohser. J T. Black has responsibility for the introduction and chapters on material removal, metrology, surface finishing, quality control, manufacturing systems design, and lean engineering. DeGarmo died in 2000, three weeks short of his 93rd birthday. His wife Mary died in 1995; he is survived by his sons, David and Richard, and many grandchildren. For the

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10th edition, which coincided with the 50th anniversary of the text, we honored our mentor with a change in the title to include his name—DeGarmo’s Materials and Processes in Manufacturing. We recognize Paul for his insight and leadership and are forever indebted to him for selecting us to carry on the tradition of his book for this, the 11th edition!

& PURPOSE OF THE BOOK The purpose of this book is to provide basic information on materials, manufacturing processes and systems to engineers and technicians. The materials section focuses on properties and behavior. Thus, aspects of smelting and refining (or other material production processes) are presented only as they affect manufacturing and manufactured products. In terms of the processes used to manufacture items (converting materials into products), this text seeks to provide a descriptive introduction to a wide variety of options, emphasizing how each process works and its relative advantages and limitations. Our goal is to present this material in a way that can be understood by individuals seeing it for the very first time. This is not a graduate text where the objective is to thoroughly understand and optimize manufacturing processes. Mathematical models and analytical equations are used only when they enhance the basic understanding of the material. So, while the text is an introductory text, we do attempt to incorporate new and emerging technologies like direct-digital-and micro-manufacturing processes as they are introduced into usage.

& ORGANIZATION OF THE BOOK E. Paul DeGarmo wanted a book that explained to engineers how the things they designed are made. DeGarmo’s Materials and Processes in Manufacturing is still being written to provide a broad, basic introduction to the fundamentals of manufacturing. The book begins with a survey of engineering materials, the ‘‘stuff’’ that manufacturing begins with, and seeks to provide the basic information that can be used to match the properties of a material to the service requirements of a component. A variety of engineering materials are presented, along with their properties and means of modifying them. The materials section can be used in curricula that lack preparatory courses in metallurgy, materials science, or strength of materials, or where the student has not yet been exposed to those topics. In addition, various chapters in this section can be used as supplements to a basic materials course, providing additional information on topics such as heat treatment, plastics, composites, and material selection. Following the materials chapters are sections on casting, forming, powder metallurgy, material removal, and joining. Each section begins with a presentation of the fundamentals on which those processes are based. The introductions are followed by a discussion of the various process alternatives, which can be selected to operate individually or be combined into an integrated system. The chapter on rapid prototyping, which had been moved to a web-based supplement in the 10th edition, has been restored to the print text, significantly expanded, and renamed Additive Processes: Rapid Prototyping and Direct-Digital Manufacturing, to incorporate the aspects of rapid prototyping, rapid tooling, and direct-digital manufacturing, and provide updated information on many recent advances in this area. Reflecting the growing role of plastics, ceramics and composites, the chapter on the processes used with these materials has also been expanded. New to this edition is a chapter on lean engineering. The lean engineer works to transform the mass production system into a lean production system. To achieve lean production, the final assembly line is converted to a mixed model delivery system so that the demand for subassemblies and components is made constant. The conveyor type flow lines are dismantled and converted into U-shaped manufacturing cells also capable of one-piece flow. The subassembly and manufacturing cells are linked to the final assembly by a pull system called Kanban (visible record) to form an integrated production and inventory control system. Hence, economy of scale of the mass

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production system changed to the ‘‘Economy of Scope’’, featuring flexibility, small lots, superior quality, minimum-inventory and short throughput times. Later chapters provide an introduction to surface engineering, measurements and quality control. Engineers need to know how to determine process capability and if they get involved in six sigma projects, to know what sigma really measures. There is also introductory material on surface integrity, since so many processes produce the finished surface and residual stresses in the components. Finally, chapters dealing with automation and numerical control conclude the coverage to the 11th edition. With each new edition, new and emerging technology is incorporated, and existing technologies are updated to accurately reflect current capabilities. Through its 50plus year history and 10 previous editions, the DeGarmo text was often the first introductory book to incorporate processes such as friction-stir welding, microwave heating and sintering, and machining dynamics. Somewhat open-ended case studies have been incorporated throughout the text. These have been designed to make students aware of the great importance of properly coordinating design, material selection, and manufacturing to produce cost competitive, reliable products. The text is intended for use by engineering (mechanical, lean, manufacturing, and industrial) and engineering technology students, in both two-and four-year undergraduate degree programs. In addition, the book is also used by engineers and technologists in other disciplines concerned with design and manufacturing (such as aerospace and electronics). Factory personnel will find this book to be a valuable reference that concisely presents the various production alternatives and the advantages and limitations of each. Additional or more in-depth information on specific materials or processes can be found in the expanded list of references that accompanies the text.

& SUPPLEMENTS For instructors adopting the text for use in their course, an instructor solutions manual is available through the book website: www.wiley.com/college/degarmo. Also available on the website is a set of PowerPoint lecture slides created by Philip Appel. Four additional chapters, as identified in the table of contents, are available on the book website. These chapters cover: electronic manufacturing, thread and gear manufacture, nondestructive testing and inspection, and the enterprise (production system). The registration card attached on the inside front cover provides information on how to access and download this material. If the registration card is missing, access can be purchased directly on the website www.wiley.com/college/degarmo, by clicking on ‘‘student companion site’’ and then on the links to the chapter titles.

& ACKNOWLEDGMENTS The authors wish to acknowledge the multitude of assistance, information, and illustrations that have been provided by a variety of industries, professional organizations, and trade associations. The text has become known for the large number of clear and helpful photos and illustrations that have been graciously provided by a variety of sources. In some cases, equipment is photographed or depicted without safety guards, so as to show important details, and personnel are not wearing certain items of safety apparel that would be worn during normal operation. Over the many editions, there have been hundreds of reviewers, faculty, and students who have made suggestions and corrections to the text. We continue to be grateful for the time and interest that they have put into this book. For this edition we benefited from the comments of the following reviewers: Jerald Brevick, The Ohio State University; Zezhong Chen, Concordia University; Emmanuel Enemuoh, University of Minnesota; Ronald Huston, University of Cincinnati; Thenkurussi Kesavadas, University at Buffalo, The State University of New York; Shuting Lei, Kansas State University; Lee Gearhart, University at Buffalo, The State University of New York; ZJ Pei, Kansas State University; Christine Corum,

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Purdue University; Allen Yi, The Ohio State University; Stephen Oneyear, North Carolina State; Roger Wright, Rennselaer Polytechnic Institute. The authors would also like to acknowledge the contributions of Dr. Elliot Stern for the dynamics of machining section in Chapter 20, Dr. Memberu Lulu for inputs to the quality chapter, Dr. Lewis Payton for writing the micro manufacturing chapter, Dr. Subbu Subramanium for inputs to the abrasive chapter, Dr. David Cochran for his contributions in lean engineering and system design, and Mr. Chris Huskamp of the Boeing Company for valuable assistance with the chapter on additive manufacturing. As always, our wives have played a major role in preparing the manuscript. Carol Black and Barb Kohser have endured being ‘‘textbook widows’’ during the time when the book was being were written. Not only did they provide loving support, but Carol also provided hours of expert proofreading, typing, and editing as the manuscript was prepared.

& ABOUT THE AUTHORS J T. Black received his Ph.D. from Mechanical and Industrial Engineering, University of Illinois, Urbana in 1969, an M.S. in Industrial Engineering from West Virginia University in 1963 and his B.S. in Industrial Engineering, Lehigh University in 1960. J T. is Professor Emeritus from Industrial and Systems Engineering at Auburn University. He was the Chairman and a Professor of Industrial and Systems Engineering at The University of Alabama-Huntsville. He also taught at The Ohio State University, the University of Rhode Island, the University of Vermont, and the University of Illinois. He taught his first processes class in 1960 at West Virginia University. J T. is a Fellow in the American Society of Mechanical Engineers, the Institute of Industrial Engineering and the Society of Manufacturing Engineers. J loves to write music (mostly down home country) and poetry, play tennis in the backyard and show his champion pug dog VBo. Ron Kohser received his Ph.D. from Lehigh University Institute for Metal Forming in 1975. Ron is currently in his 37th year on the faculty of Missouri University of Science & Technology (formerly the University of Missouri-Rolla), where he is a Professor of Metallurgical Engineering and Dean’s Teaching Scholar. While maintaining a full commitment to classroom instruction, he has served as department chair and Associate Dean for Undergraduate Instruction. He currently teaches courses in Metallurgy for Engineers, Introduction to Manufacturing Processes, and Material Selection, Fabrication and Failure Analysis. In addition to his academic responsibilities, Ron and his wife Barb operate A Miner Indulgence, a bed-and-breakfast in Rolla, Missouri, and they enjoy showing their three collector cars.

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CHAPTER 1 INTRODUCTION TO DEGARMO’S MATERIALS AND PROCESSES IN MANUFACTURING 1.1 MATERIALS, MANUFACTURING, AND THE STANDARD OF LIVING 1.2 MANUFACTURING AND PRODUCTION SYSTEMS

Production System––The Enterprise Manufacturing Systems Manufacturing Processes Job and Station Operation Treatments

Tools, Tooling, and Workholders Tooling for Measurement and Inspection Integrating Inspection into the Process Products and Fabrications Workpiece and its Configuration Roles of Engineers in Manufacturing Changing World Competition Manufacturing System Designs

Basic Manufacutring Processes Other Manufacturing Operations Understand Your Process Technology Product Life Cycle and Life-Cycle Cost Comparisons of Manufacturing System Design New Manufacturing Systems Case Study: Famous Manufacturing Engineers

& 1.1 MATERIALS, MANUFACTURING, AND THE STANDARD OF LIVING Manufacturing is critical to a country’s economic welfare and standard of living because the standard of living in any society is determined, primarily, by the goods and services that are available to its people. Manufacturing companies contribute about 20% of the GNP, employ about 18% of the workforce, and account for 40% of the exports of the United States. In most cases, materials are utilized in the form of manufactured goods. Manufacturing and assembly represent the organized activities that convert raw materials into salable goods. The manufactured goods are typically divided into two classes: producer goods and consumer goods. Producer goods are those goods manufactured for other companies to use to manufacture either producer or consumer goods. Consumer goods are those purchased directly by the consumer or the general public. For example, someone has to build the machine tool (a lathe) that produces (using machining processes) the large rolls that are sold to the rolling mill factory to be used to roll the sheets of steel that are then formed (using dies) into body panels of your car. Similarly, many service industries depend heavily on the use of manufactured products, just as the agricultural industry is heavily dependent on the use of large farming machines for efficient production. Processes convert materials from one form to another adding value to them. The more efficiently materials can be produced and converted into the desired products that function with the prescribed quality, the greater will be the companies’ productivity and the better will be the standard of living of the employees. The history of man has been linked to his ability to work with tools and materials, beginning with the Stone Age and ranging through the eras of copper and bronze, the Iron Age, and recently the age of steel. While ferrous materials still dominate the manufacturing world, we are entering the age of tailor-made plastics, composite materials, and exotic alloys. A good example of this progression is shown in Figure 1-1. The goal of the manufacturer of any product or service is to continually improve. For a given product or service, this improvement process usually follows an S-shaped curve, as shown in Figure 1-1a, often called a product life-cycle curve. After the initial invention/creation and development, a period of rapid growth in performance occurs, with relatively few resources required. However, each improvement becomes progressively more difficult. For a delta

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Introduction to DeGarmo’s Materials and Processes in Manufacturing Run flat Product Development S-Curve

Growth

Silica (Tread life) Miles to failure

Maturity Design optimization

Performance

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Computer modeling Belt architectures Synthetic rubber

Innovation process: superposition of design enhancements

Development Creation/invention ‘45

Time resource (a)

1950

1965

1980

1995

2010

(b)

FIGURE 1-1 (a) A product development curve usually has an ‘‘S’’-shape. (b) Example of the S-curve for the radial tire. (Courtesy of Bart Thomas, Michelin)

gain, more money and time and ingenuity are required. Finally, the product or service enters the maturity phase, during which additional performance gains become very costly. For example, in the automobile tire industry, Figure 1-1b shows the evolution of radial tire performance from its birth in 1946 to the present. Growth in performance is actually the superposition of many different improvements in material, processes, and design. These innovations, known as sustaining technology, serve to continually bring more value to the consumer of existing products and services. In general, sustaining manufacturing technology is the backbone of American industry and the everincreasing productivity metric. Although materials are no longer used only in their natural state, there is obviously an absolute limit to the amounts of many materials available here on earth. Therefore, as the variety of man-made materials continues to increase, resources must be used efficiently and recycled whenever possible. Of course, recycling only postpones the exhaustion date. Like materials, processes have also proliferated greatly in the past 50 years, with new processes being developed to handle the new materials more efficiently and with less waste. A good example is the laser, invented around 1960, which now finds many uses in machining, measurement, inspection, heat treating, welding, and more. New developments in manufacturing technology often account for improvements in productivity. Even when the technology is proprietary, the competition often gains access to it, usually quite quickly. Starting with the product design, materials, labor, and equipment are interactive factors in manufacturing that must be combined properly (integrated) to achieve low cost, superior quality, and on-time delivery. Figure 1-2 shows a breakdown of costs for a Selling price Engineering cost, 15% Admin. sales marketing, 25%

FIGURE 1-2 Manufacturing cost is the largest part of the selling price, usually around 40%. The largest part of the manufacturing cost is materials, usually 50%.

Profit, ≅20%

Manufacturing cost ≅40% selling price

Equipment plant energy, 12%

Indirect labor, 26% Manufacturing cost, 40%

Direct labor, 10–12%

Subassemblies component parts and other materials

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Manufacturing and Production Systems

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product (like a car). Typically about 40% of the selling price of a product is the manufacturing cost. Because the selling price determines how much the customer is willing to pay, maintaining the profit often depends on reducing manufacturing cost. The internal customers who really make the product are called direct labor. They are usually the targets of automation, but typically they account for only about 10% of the manufacturing cost, even though they are the main element in increasing productivity. In Chapter 2, a manufacturing strategy is presented that attacks the materials cost, indirect costs, and general administration costs, in addition to labor costs. The materials costs include the cost of storing and handling the materials within the plant. The strategy depends on a new factory design and is called lean production. Referring again to the total expenses shown in Figure 1-2 (selling price less profit), about 68% of dollars are spent on people, but only 5 to 10% on director labor, the breakdown for the rest being about 15% for engineers and 25% for marketing, sales, and general management people. The average labor cost in manufacturing in the United States is around $15 per hour for hourly workers (2010). Reductions in direct labor will have only marginal effects on the total people costs. The optimal combination of factors for producing a small quantity of a given product may be very inefficient for a larger quantity of the same product. Consequently, a systems approach, taking all the factors into account, must be used. This requires a sound and broad understanding on the part of the decision makers on the value of materials, processes, and equipment to the company, and their customers, accompanied by an understanding of the manufacturing systems. Materials, processes, and manufacturing systems are what this book is all about.

& 1.2 MANUFACTURING AND PRODUCTION SYSTEMS Manufacturing is the economic term for making goods and services available to satisfy human wants. Manufacturing implies creating value by applying useful mental or physical labor. The manufacturing processes are collected together to form a manufacturing system (MS). The manufacturing system is a complex arrangement of physical elements characterized by measurable parameters (Figure 1-3). The manufacturing system takes inputs and produces products for the external customer. The entire company is often referred to as the enterprise or the production system. The production services the manufacturing system, as shown in Figure 1-4. In this book, a production system will refer to the total company and will include within it the manufacturing system. The production system includes the manufacturing system plus all the other functional areas of the plant for information, design, analysis, and control. These subsystems are connected by various means to each other to produce either goods or services or both. Goods refers to material things. Services are nonmaterial things that we buy to satisfy our wants, needs, or desires. Service production systems include transportation, banking, finance, savings and loan, insurance, utilities, health care, education, communication, entertainment, sporting events, and so forth. They are useful labors that do not directly produce a product. Manufacturing has the responsibility for designing Inputs include Raw materials Component supplies

FIGURE 1-3 The manufacturing system design (aka the factory design) is composed of machines, tooling, material handling equipment, and people.

Subassemblies information Internal customers

Lathe

Broach

Paint

Subassembly

Saw

Lathe

Drill

Final assembly

Cast

Mill

Grinder

Sequence of Processes

Plating

Finished Goods Storage

A Flow Shop Manufacturing System

Receiving

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External customer (consumer goods) Outputs include • Components • Goods • Products • Parts • Subassemblies

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Introduction to DeGarmo’s Materials and Processes in Manufacturing The production system services the manufacturing system (shaded). Legend: information systems Instructions or orders Feedback Material flow External customer of goods

Deliver products

Marketing department (Estimate price and volume forecasts)

Distribution centers, warehouses

Predict demand (Q)

bili ty

Inspect products

Market information

rel ia

Inspection (quality control)

Finance department

Finished products

Manufacturing system (see figure 1-7)

co s

t

Recommend design changes Production budget

tion

% of scrap Iosses

Product design engineering

e

su

Is

Recommend ch anges in desig n to improve man ufacturing Wo rk s che du les

Produc

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ls

ia

er

at

m

Material delivery

Purchasing department

MSD manufacturing department process design

Production and inventory control How to make (schedules) the product

Drawings, specifications, and standards

R&D Ve

Material requisitions

nd

ors

Design and test and redesign new products

FIGURE 1-4 The production system includes and services the manufacturing system. The functional departments are connected by formal and informal information systems, designed to service the manufacturing that produces the goods.

processes (sequences of operations and processes) and systems to create (make or manufacture) the product as designed. The system must exhibit flexibility to meet customer demand (volumes and mixes of products) as well as changes in product design. Chapter 44 on the Web provides more detailed discussions of the production system beyond what is presented here. As shown in Table 1-1, production terms have a definite rank of importance, somewhat like rank in the army. Confusing system with section is similar to mistaking a colonel for a corporal. In either case, knowledge of rank is necessary. The terms tend to overlap because of the inconsistencies of popular usage. An obvious problem exists here in the terminology of manufacturing and production. The same term can refer to different things. For example, drill can refer to the machine tool that does these kinds of operations; the operation itself, which can be

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TABLE 1-1

Manufacturing and Production Systems

5

Production Terms for Manufacturing Production Systems

Term

Meaning

Examples

Production system; the enterprise

All aspects of workers, machines, and information, considered collectively, needed to manufacture parts or products; integration of all units of the system is critical.

Company that makes engines, assembly plant, glassmaking factory, foundry; sometimes called the enterprise or the business.

Manufacturing system (sequence of operations, collection of processes) or factory

The collection of manufacturing processes and operations resulting in specific end products; an arrangement or layout of many processes, materialshandling equipment, and operators.

Rolling steel plates, manufacturing of automobiles, series of connected operations or processes, a job shop, a flow shop, a continuous process.

Machine or machine tool or manufacturing process

A specific piece of equipment designed to accomplish specific processes, often called a machine tool; machine tools linked together to make a manufacturing system. A collection of operations done on machines or a collection of tasks performed by one worker at one location on the assembly line.

Spot welding, milling machine, lathe, drill press, forge, drop hammer, die caster, punch press, grinder, etc.

Operation (sometimes called a process)

A specific action or treatment, often done on a machine, the collection of which makes up the job of a worker.

Drill, ream, bend, solder, turn, face, mill extrude, inspect, load.

Tools or tooling

Refers to the implements used to hold, cut, shape, or deform the work materials; called cutting tools if referring to machining; can refer to jigs and fixtures in workholding and punches and dies in metal forming.

Grinding wheel, drill bit, end milling cutter, die, mold, clamp, three-jaw chuck, fixture.

Job (sometimes called a station; a collection of tasks)

Operation of machines, inspection, final assembly; e.g., forklift driver has the job of moving materials.

done on many different kinds of machines; or the cutting tool, which exists in many different forms. It is therefore important to use modifiers whenever possible: ‘‘Use the radial drill press to drill a hole with a 1-in.-diameter spade drill.’’ The emphasis of this book will be directed toward the understanding of the processes, machines, and tools required for manufacturing and how they interact with the materials being processed. In the last section of the book, an introduction to systems aspects is presented.

PRODUCTION SYSTEM—THE ENTERPRISE The highest-ranking term in the hierarchy is production system. A production system includes people, money, equipment, materials and supplies, markets, management, and the manufacturing system. In fact, all aspects of commerce (manufacturing, sales, advertising, profit, and distribution) are involved. Table 1-2 provides a partial list of TABLE 1-2

Partial List of Production Systems for Producer and Consumer Goods

Aerospace and airplanes Appliances

Foods (canned, dairy, meats, etc.) Footwear

Automotive (cars, trucks, vans, wagons, etc.)

Furniture

Beverages

Glass

Building supplies (hardware)

Hospital suppliers

Cement and asphalt

Leather and fur goods

Ceramics

Machines

Chemicals and allied industries

Marine engineering

Clothing (garments) Construction

Metals (steel, aluminum, etc.) Natural resources (oil, coal, forest, pulp and paper)

Construction materials (brick, block, panels)

Publishing and printing (books, CDs, newspapers)

Drugs, soaps, cosmetics

Restaurants

Electrical and microelectronics

Retail (food, department stores, etc.)

Energy (power, gas, electric)

Ship building

Engineering

Textiles

Equipment and machinery (agricultural, construction and electrical products, electronics, household products, industrial machine tools, office equipment, computers, power generators)

Tire and rubber Tobacco Transportation vehicles (railroad, airline, truck, bus) Vehicles (bikes, cycles, ATVs, snowmobiles)

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TABLE 1-3

Introduction to DeGarmo’s Materials and Processes in Manufacturing

Types of Service Industries

Advertising and marketing Communication (telephone, computer networks) Education Entertainment (radio, TV, movies, plays) Equipment and furniture rental Financial (banks, investment companies, loan companies) Health care

production systems. Another term for them is ‘‘industries’’ as in the ‘‘aerospace industry.’’ Further discussion on the enterprise is found in Chapter 44, on the Web. Much of the information given for manufacturing systems is relevant to the service system. Most require a service production system [SPS] for proper product sales. This is particularly true in industries, such as the food (restaurant) industry, in which customer service is as important as quality and on-time delivery. Table 1-3 provides a short list of service industries.

Insurance Transportation and car rental Travel (hotel, motel, cruise lines)

MANUFACTURING SYSTEMS A collection of operations and processes used to obtain a desired product(s) or component(s) is called a manufacturing system. The manufacturing system is therefore the design or arrangement of the manufacturing processes in the factory. Control of a system applies to overall control of the whole, not merely of the individual processes or equipment. The entire manufacturing system must be controlled in order to schedule and control the factory—all its inputs, inventory levels, product quality, output rates, and so forth. Designs or layouts of factories are discussed in Chapter 2.

MANUFACTURING PROCESSES A manufacturing process converts unfinished materials to finished products, often using machines or machine tools. For example, injection molding, die casting, progressive stamping, milling, arc welding, painting, assembling, testing, pasteurizing, homogenizing, and annealing are commonly called processes or manufacturing processes. The term process can also refer to a sequence of steps, processes, or operations for production of goods and services, as shown in Figure 1-5, which shows the processes to manufacture an Olympic-type medal. A machine tool is an assembly of related mechanisms on a frame or bed that together produce a desired result. Generally, motors, controls, and auxiliary devices are included. Cutting tools and workholding devices are considered separately. A machine tool may do a single process (e.g., cutoff saw) or multiple processes, or it may manufacture an entire component. Machine sizes vary from a tabletop drill press to a 1000-ton forging press.

JOB AND STATION In the classical manufacturing system, a job is the total of the work or duties a worker performs. A station is a location or area where a production worker performs tasks or his job. A job is a group of related operations and tasks performed at one station or series of stations in cells. For example, the job at a final assembly station may consist of four tasks: 1. Attach carburetor. 2. Connect gas line. 3. Connect vacuum line. 4. Connect accelerator rod. The job of a turret lathe (a semiautomatic machine) operator may include the following operations and tasks: load, start, index and stop, unload, inspect. The operator’s job may also include setting up the machine (i.e., getting ready for manufacturing). Other machine operations include drilling, reaming, facing, turning, chamfering, and knurling. The operator can run more than one machine or service at more than one station. The terms job and station have been carried over to unmanned machines. A job is a group of related operations generally performed at one station, and a station is a position or location in a machine (or process) where specific operations are performed. A simple machine may have only one station. Complex machines can be composed of

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How an olympic medal is made using the CAD/CAM process Laser scan model

Artist’s model of medal Model of medal

(1) An oversized 3D plaster model is made from the artist’s conceptual drawings.

(2) The model is scanned with a laser to produce a digital computer called a computer-aided design (CAD).

CAM

Top

CAD

Die set

Cavity in die forms medal

Bottom Computer

CNC machine tool

(3) The computer has software to produce a program to drive numerical control machine to cut a die set.

(4) Blanks are cut from bronze metal sheet stock using an abrasive water jet under 2-axis CNC control.

Air supply port valve

High-pressure water inlet

Abrasive cutting head

(5) The blanks are heated and placed between the top die and bottom die. Very high pressure is applied by a press at very slow rates. The blank plastically deforms into the medal. This press Abrasive is called hot metering isostatic pressing. system

Formed medal Blank

Abrasive feed line Additional finishing steps in the process include chemical etching; gold or silver plating; packaging

Sheet stock (bronze) Blank

FIGURE 1-5 The manufacturing process for making Olympic medals has many steps or operations, beginning with design and including die making. (Courtesy J T. Black)

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Introduction to DeGarmo’s Materials and Processes in Manufacturing Simplified Sequence of Operations (Typical Machine Tool Used)

GE

STA

1

GE

STA

2

Raw material bar stock cylinder with flat ends

Cut bar stock to length; centerdrill ends. (saw and drill press)

Multiple cylinders made by turning (see Figure 1-12)

Turn and face rough turn and finish turn. (Lathe)

External cylindrical

GE

STA

3

Three external cylinders and four flats

Turn the smaller external cylindrical surfaces. (Lathe)

Flat Flat Three cylinders and six flats

Slot Four internal holes

Mill the flat on the right end. Mill the slot on the left end. (Milling Machine)

Drill four holes on left end. Tap (internal threads) holes. (Drill press)

Holes Internal cylindrical

FIGURE 1-6 The component called a pinion shaft is manufactured by a ‘‘sequence of operations’’ to produce various geometric surfaces. The engineer figures out the sequence and selects the tooling to perform the steps.

many stations. The job at a station often includes many simultaneous operations, such as ‘‘drill all five holes’’ by multiple spindle drills. In the planning of a job, a process plan is often developed (by the engineer) to describe how a component is made using a sequence of operation. The engineer begins with a part drawing and a piece of raw material. Follow in Figure 1-6 the sequence of machining operations that transforms the cylinder in a pinion shaft. This information can be embedded in a computer program, in a machine tool called a lathe.

OPERATION An operation is a distinct action performed to produce a desired result or effect. Typical manual machine operations are loading and unloading. Operations can be divided into suboperational elements. For example, loading is made up of picking up a part, placing part in jig, closing jig. However, suboperational elements will not be discussed here. Operations categorized by function are: 1. Materials handling and transporting: change in position of the product. 2. Processing: change in volume and quality, including assembly and disassembly; can include packaging.

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3. Packaging: special processing; may be temporary or permanent for shipping. 4. Inspecting and testing: comparison to .the standard or check of process behavior 5. Storing: time lapses without further operations. These basic operations may occur more than once in some processes, or they may sometimes be omitted. Remember, it is the manufacturing processes that change the value and quality of the materials. Defective processes produce poor quality or scrap. Other operations may be necessary but do not, in general, add value, whereas operations performed by machines that do material processing usually do add value.

TREATMENTS Treatments operate continuously on the workpiece. They usually alter or modify the product-in-process without tool contact. Heat treating, curing, galvanizing, plating, finishing, (chemical) cleaning, and painting are examples of treatments. Treatments usually add value to the part. These processes are difficult to include in manufacturing cells because they often have long cycle times, are hazardous to the workers’ health, or are unpleasant to be around because of high heat or chemicals. They are often done in large tanks or furnaces or rooms. The cycle time for these processes may dictate the cycle times for the entire system. These operations also tend to be material specific. Many manufactured products are given decorative and protective surface treatments that control the finished appearance. A customer may not buy a new vehicle because it has a visible defect in the chrome bumper, although this defect will not alter the operation of the car.

TOOLS, TOOLING, AND WORKHOLDERS The lowest mechanism in the production term rank is the tool. Tools are used to hold, cut, shape, or form the unfinished product. Common hand tools include the saw, hammer, screwdriver, chisel, punch, sandpaper, drill, clamp, file, torch, and grindstone. Basically, machines are mechanized versions of such hand tools and are called cutting tools. Some examples of tools for cutting are drill bits, reamers, single-point turning tools, milling cutters, saw blades, broaches, and grinding wheels. Noncutting tools for forming include extrusion dies, punches, and molds. Tools also include workholders, jigs, and fixtures. These tools and cutting tools are generally referred to as the tooling, which usually must be considered (purchased) separate from machine tools. Cutting tools wear and fail and must be periodically replaced before parts are ruined. The workholding devices must be able to locate and secure the workpieces during processing in a repeatable, mistake-proof way.

TOOLING FOR MEASUREMENT AND INSPECTION Measuring tools and instruments are also important for manufacturing. Common examples of measuring tools are rulers, calipers, micrometers, and gages. Precision devices that use laser optics or vision systems coupled with sophisticated electronics are becoming commonplace. Vision systems and coordinate measuring machines are becoming critical elements for achieving superior quality.

INTEGRATING INSPECTION INTO THE PROCESS The integration of the inspection process into the manufacturing process or the manufacturing system is a critical step toward building products of superior quality. An example will help. Compare an electric typewriter with a computer that does word processing. The electric typewriter is flexible. It types whatever words are wanted in whatever order. It can type in Pica, Elite, or Orator, but the font (disk or ball that has the appropriate type size on it) has to be changed according to the size and face of type wanted. The computer can do all of this but can also, through its software, set italics; set bold, dark type; vary the spacing to justify the right margin; plus many other functions. It checks immediately for incorrect spelling and other defects like repeated words. The software system provides a signal to the hardware to flash the word so that the operator will know something is wrong and can make an immediate correction. If the system

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were designed to prevent the typist from typing repeated words, then this would be a poka-yoke, a term meaning defect prevention. Defect prevention is better than immediate defect detection and correction. Ultimately, the system should be able to forecast the probability of a defect, correcting the problem at the source. This means that the typist would have to be removed from the process loop, perhaps by having the system type out what it is told (convert oral to written directly). Poka-yoke devices and source inspection techniques are keys to designing manufacturing systems that produce superior-quality products at low cost.

PRODUCTS AND FABRICATIONS In manufacturing, material things (goods) are made to satisfy human wants. Products result from manufacture. Manufacture also includes conversion processes such as refining, smelting, and mining. Products can be manufactured by fabricating or by processing. Fabricating is the manufacture of a product from pieces such as parts, components, or assemblies. Individual products or parts can also be fabricated. Separable discrete items such as tires, nails, spoons, screws, refrigerators, or hinges are fabricated. Processing is also used to refer to the manufacture of a product by continuous means, or by a continuous series of operations, for a specific purpose. Continuous items such as steel strip, beverages, breakfast foods, tubing, chemicals, and petroleum are ‘‘processed.’’ Many processed products are marketed as discrete items, such as bottles of beer, bolts of cloth, spools of wire, and sacks of flour. Separable discrete products, both piece parts and assemblies, are fabricated in a plant, factory, or mill, for instance, a textile or rolling mill. Products that flow (liquids, gases, grains, or powders) are processed in a plant or refinery. The continuous-process industries such as petroleum and chemical plants are sometimes called processing industries or flow industries. To a lesser extent, the terms fabricating industries and manufacturing industries are used when referring to fabricators or manufacturers of large products composed of many parts, such as a car, a plane, or a tractor. Manufacturing often includes continuous-process treatments such as electroplating, heating, demagnetizing, and extrusion forming. Construction or building is making goods by means other than manufacturing or processing in factories. Construction is a form of project manufacturing of useful goods like houses, highways, and buildings. The public may not consider construction as manufacturing because the work is not usually done in a plant or factory, but it can be. There is a company in Delaware that can build a custom house of any design in its factory, truck it to the building site, and assemble it on a foundation in two or three weeks. Agriculture, fisheries, and commercial fishing produce real goods from useful labor. Lumbering is similar to both agriculture and mining in some respects, and mining should be considered processing. Processes that convert the raw materials from agriculture, fishing, lumbering, and mining into other usable and consumable products are also forms of manufacturing.

WORKPIECE AND ITS CONFIGURATION In the manufacturing of goods, the primary objective is to produce a component having a desired geometry, size, and finish. Every component has a shape that is bounded by various types of surfaces of certain sizes that are spaced and arranged relative to each other. Consequently, a component is manufactured by producing the surfaces that bound the shape. Surfaces may be: 1. Plane or flat. 2. Cylindrical (external or internal). 3. Conical (external or internal). 4. Irregular (curved or warped). Figure 1-6 illustrates how a shape can be analyzed and broken up into these basic bounding surfaces. Parts are manufactured by using a set or sequence of processes that

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will either (1) remove portions of a rough block of material (bar stock, casting, forging) so as to produce and leave the desired bounding surface or (2) cause material to form into a stable configuration that has the required bounding surfaces (casting, forging). Consequently, in designing an object, the designer specifies the shape, size, and arrangement of the bounding surface. The part design must be analyzed to determine what materials will provide the desired properties, including mating to other components, and what processes can best be employed to obtain the end product at the most reasonable cost. This is often the job of the manufacturing engineer.

ROLES OF ENGINEERS IN MANUFACTURING Many engineers have as their function the designing of products. The products are brought into reality through the processing or fabrication of materials. In this capacity designers are a key factor in the material selection and manufacturing procedure. A design engineer, better than any other person, should know what the design is to accomplish, what assumptions can be made about service loads and requirements, what service environment the product must withstand, and what appearance the final product is to have. To meet these requirements, the material(s) to be used must be selected and specified. In most cases, to utilize the material and to enable the product to have the desired form, the designer knows that certain manufacturing processes will have to be employed. In many instances, the selection of a specific material may dictate what processing must be used. On the other hand, when certain processes must be used, the design may have to be modified in order for the process to be utilized effectively and economically. Certain dimensional sizes can dictate the processing, and some processes require certain sizes for the parts going into them. In converting the design into reality, many decisions must be made. In most instances, they can be made most effectively at the design stage. It is thus apparent that design engineers are a vital factor in the manufacturing process, and it is indeed a blessing to the company if they can design for manufacturing, that is, design the product so that it can be manufactured and/or assembled economically (i.e., at low unit cost). Design for manufacturing uses the knowledge of manufacturing processes, and so the design and manufacturing engineers should work together to integrate design and manufacturing activities. Manufacturing engineers select and coordinate specific processes and equipment to be used or supervise and manage their use. Some design special tooling is used so that standard machines can be utilized in producing specific products. These engineers must have a broad knowledge of manufacturing processes and material behavior so that desired operations can be done effectively and efficiently without overloading or damaging machines and without adversely affecting the materials being processed. Although it is not obvious, the most hostile environment the material may ever encounter in its lifetime is the processing environment. Industrial and lean engineers are responsible for manufacturing systems design (or layout) of factories. They must take into account the interrelationships of the factory design and the properties of the materials that the machines are going to process as well as the interreaction of the materials and processes. The choice of machines and equipment used in manufacturing and their arrangement in the factory are key design tasks. The lean engineer has expertise in cell design, setup reduction (tool design), integrated quality control devices (poka-yokes and decouplers) and reliability (maintenance of machines and people) for the lean production system. See Chapter 29 for discussion of cell design and lean engineering. Materials engineers devote their major efforts to developing new and better materials. They, too, must be concerned with how these materials can be processed and with the effects that the processing will have on the properties of the materials. Although their roles may be quite different, it is apparent that a large proportion of engineers must concern themselves with the interrelationships of materials and manufacturing processes. As an example of the close interrelationship of design, materials selection, and the selection and use of manufacturing processes, consider the common desk stapler. Suppose that this item is sold at the retail store for $20. The wholesale outlet sold the stapler for $16 and the manufacturer probably received about $10 for it. Staplers typically

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consist of 10 to 12 parts and some rivets and pins. Thus, the manufacturer had to produce and assemble the 10 parts for about $1 per part. Only by giving a great deal of attention to design, selection of materials, selection of processes, selection of equipment used for manufacturing (tooling), and utilization of personnel could such a result be achieved. The stapler is a relatively simple product, yet the problems involved in its manufacture are typical of those that manufacturing industries must deal with. The elements of design, materials, and processes are all closely related, each having its effect on the performance of the device and the other elements. For example, suppose the designer calls for the component that holds the staples to be a metal part. Will it be a machined part rather than a formed part? Entirely different processes and materials need to be specified depending on the choice. Or, if a part is to be changed from metal to plastic, then a whole new set of fundamentally different materials and processes would need to come into play. Such changes would also have a significant impact on cost as well as the service (useful life) of the product.

CHANGING WORLD COMPETITION In recent years, major changes in the world of goods manufacturing have taken place. Three of these are: 1. Worldwide competition for global products and their manufacture. 2. High-tech manufacturing or advanced technology. 3. New manufacturing systems designs, strategies, and management. Worldwide (global) competition is a fact of manufacturing life, and it will get stronger in the future. The goods you buy today may have been made anywhere in the world. For many U.S. companies, suppliers in China, India, and Mexico are not uncommon. The second aspect, advanced manufacturing technology, usually refers to new machine tools or processes controlled by computers. Companies that produce such machine tools, though small, can have an enormous impact on factory productivity. Improved processes lead to better components and more durable goods. However, the new technology is often purchased from companies that have developed the technology, so this approach is important but may not provide a unique competitive advantage if your competitors can also buy the technology, provided that they have the capital. Some companies develop their own unique process technology and try to keep it proprietary as long as they can. The third change and perhaps the real key to success in manufacturing is to implement lean manufacturing system design that can deliver, on time to the customer, superquality goods at the lowest possible cost in a flexible way. Lean production is an effort to reduce waste and improve markedly the methodology by which goods are produced rather than simply upgrading the manufacturing process technology. Manufacturing system design is discussed extensively in Chapters 2 and 29 of the book, and it is strongly recommended that students examine this material closely after they have gained a working knowledge of materials and processes. The next section provides a brief discussion of manufacturing system designs.

MANUFACTURING SYSTEM DESIGNS Five manufacturing system designs can be identified: the job shop, the flow shop, the linked-cell shop, the project shop, and the continuous process. See Figure 1-7. The continuous process deals with liquids and/or gases (such as an oil refinery) rather than solids or discrete parts and is used mostly by the chemical engineer. The most common of these layouts is the job shop, characterized by large varieties of components, general-purpose machines, and a functional layout (Figure 1-8). This means that machines are collected by function (all lathes together, all broaches together, all grinding machines together) and the parts are routed around the shop in small lots to the various machines. The layout of the factory shows the multiple paths through the shop and a detail on one of the seven broaching machine tools. The material is moved from machine to machine in carts or containers and is called the lot or batch.

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(a) Job shop Receiving

Lathes

13

(b) Project shop Lot no. 73

Grinders

Supplies

Heattreating

House

Labor

Component parts Materials Machines

Drill process

Saws

C

Equipment Submachines

B Presses (sheet metal)

A

Milling machines

Painting

Assembly

Storage

Job shop makes components for subassembly using a functional layout.

(c) Flow shop STA 4

STA 3

(d) Continuous process

STA 2

STA 1

Raw materials Process I

Moving assembly line

Energy

Work is devided equally into the stations

Rework line for detects Product

STA 10

Process II

Process II

By-products

Gas

REPAIR

Subassembly feeder lines

Oil STA 14

STA 13

STA 12

STA 11

Continuous process systems make products that can flow like gas and oil.

Finished Product

Flow shop uses line balancing to achieve one piece flow.

(e) Linked-cell Main assembly plant

Supplier Seats Chassis

Frames

Subassembly

Controls Final Motors assembly Mfg cell I

Subassembly

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Lean production uses manufacturing and subassembly cells linked to final assembly.

Final assembly

Subassembly cell II

y

bl

em

ss

a ub

In

S

Kanban link

Out

Start

FIGURE 1-7 Schematic layouts of factory designs: (a) functional or job shop, (b) fixed location or project shop, (c) flow shop or assembly line, (d) continuous process or lean shop or linked-cell design.

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Introduction to DeGarmo’s Materials and Processes in Manufacturing 8 Crack Detection (Outsourced)

Saw

2

4

5

6

7

9 S

I.H. Lathe

Saw Saw

3

Broach

Grinder S

I.H.

Broach

10

S

Draw Furnace

I.H.

Grinder

Lathe

Saw

Broach Lathe

I.H.

S

Broach Broach

I.H.

Lathe Broach Lathe

Broach

S

Draw Furnace S

1

I.H.

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Washer

Grinder

Washer

Broach stroke 90‘‘

Grinder S

S

S

S

Grinder Grinder

I.H.

Draw Furnace

I.H. I.H.

S

Grinder

Floor level

375‘ Raw Material

Finished Parts

10‘ Detail on broach 8‘

FIGURE 1-8 The vertical broaching machine is one of seven machines in this production job shop. IH ¼ induction hardening, S ¼ bar strengthening.

This rack bar machining area is functionally designed so it operates like a job shop, with lathes, broaches, and grinders lined up. Flow shops are characterized by larger volumes of the same part or assembly, special-purpose machines and equipment, less variety, less flexibility, and more mechanization. Flow shop layouts are typically either continuous or interrupted and can be for manufacturing or assembly, as shown in Figure 1-9. If continuous, a production line is built that basically runs one large-volume complex item in great quantity and nothing else. The common light bulb is made this way. A transfer line producing an engine block is another typical example. If interrupted, the line manufactures large lots but is periodically ‘‘changed over’’ to run a similar but different component. The linked-cell manufacturing system (L-CMS) is composed of manufacturing and subassembly cells connected to final assembly (linked) using a unique form of inventory and information control called kanban. The L-CMS is used in lean production systems where manufacturing processes and subassemblies are restructured into U-shaped cells so they can operate on a one-piece-flow basis, like final assembly. As shown in Figure 1-10, the lean production factory is laid out (designed) very differently than the mass production system. At this writing, more than 60% of all manufacturing industries have adopted lean production. Hundreds of manufacturing companies have dismantled their conveyor-based flow lines and replaced them with U-shaped subassembly cells, providing flexibility while eliminating the need for line balancing. Chapter 2 discusses manufacturing system designs. Chapter 29 discusses subassembly cells and manufacturing cells. The project shop is characterized by the immobility of the item being manufactured. In the construction industry, bridges and roads are good examples. In the manufacture of goods, large airplanes, ships, large machine tools, and locomotives are manufactured in project shops. It is necessary that the workers, machines, and materials come to the site. The number of end items is not very large, and therefore the lot sizes of the components going into the end item are not large. Thus, the job shop usually supplies parts and subassemblies to the project shop in small lots. Continuous processes are used to manufacture liquids, oils, gases, and powders. These manufacturing systems are usually large plants producing goods for other

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Finish

Large Storage of Subassemblies

Flow Lines

Subassembly lines make components and subassemblies for the installation into the product, often using conveyors. These lines are examples of the flow shop.

Lines

Conveyor Conveyor

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Parts Storage

Start of final assembly Job Shop

Lathe

Shipping and Receiving

Station

Mill

Drill

Final Inspection

Grind

FIGURE 1-9 Flow shops and lines are common in the mass production system. Final assembly is usually a moving assembly line. The product travels through stations in a specific amount of time. The work needed to assemble the product is distributed into the stations, called division of labor. The moving assembly line for cars is an example of the flow shop.

producers or mass-producing canned or bottled goods for consumers. The manufacturing engineer in these factories is often a chemical engineer. Naturally, there are many hybrid forms of these manufacturing systems, but the job shop is the most common system. Because of its design, the job shop has been shown to be the least cost-efficient of all the systems. Component parts in a typical job shop spend only 5% of their time in machines and the rest of the time waiting or being moved from one functional area to the next. Once the part is on the machine, it is actually being processed (i.e., having value added to it by the changing of its shape) only about 30 to 40% of the time. The rest of the time parts are being loaded, unloaded, inspected, and so on. The advent of numerical control machines increased the percentage of time that the machine is making chips because tool movements are programmed and the machines can automatically change tools or load or unload parts. However, there are a number of trends that are forcing manufacturing management to consider means by which the job shop system itself can be redesigned to improve its overall efficiency. These trends have forced manufacturing companies to convert their batch-oriented job shops into linked-cell manufacturing systems, with the manufacturing and subassembly cells structured around specific products. Another way to identify families of products with a similar set of manufacturing processes is called group technology. Group technology (GT) can be used to restructure the factory floor. GT is a concept whereby similar parts are grouped together into part families. Parts of similar size and shape can often be processed through a similar set of processes. A part family based on manufacturing would have the same set or sequences of manufacturing processes. The machine tools needed to process the part family are

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k

k

= Manufacturing cell (see detail below)

k k k

Subassembly (Sub A) cells (see detail below)

k Final assembly is mixed model

Cells are one-piece flow

= Kanban linking of cell to Sub A or Sub A to Final A

k

k

k = Direct linking, flow, or synchronized

in

out

M

Eliminate conveyor

1

2

3

Polarity check

Solder repair

Measurement

4 Attach and adjust resistor

IN

Standing, walking workers

1

L Attac fram h e

Fasten screws

3 4

12 11 9 Attach lid Pack- External 10 Attach aging inspection label

8 Main measurement

G Cell # 1

6

5

D

5

2

OUT

M

Directly link processes to eliminate in-process inventory

7

L

G

L

A

ing

Cauld

L Subassembly Cell

RM

FG

Manufacturing Cell

FIGURE 1-10 The linked-cell manufacturing system for lean production has subassembly and manufacturing cells connected to final assembly by kanban links. The traditional subassembly lines can be redesigned into U-shaped cells as part of the conversion of mass production to lean production.

gathered into a cell. Thus, with GT, job shops can be restructured into cells, each cell specializing in a particular family of parts. The parts are handled less, machine setup time is shorter, in-process inventory is lower, and the time needed for parts to get through the manufacturing system (called the throughput time) is greatly reduced.

BASIC MANUFACTURING PROCESSES It is the manufacturing processes that create or add value to a product. The manufacturing processes can be classified as:

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Casting, foundry, or molding processes. Forming or metalworking processes. Machining (material removal) processes. Joining and assembly. Surface treatments (finishing). Rapid prototyping. Heat treating. Other. These classifications are not mutually exclusive. For example, some finishing processes involve a small amount of metal removal or metal forming. A laser can be used either for joining or for metal removal or heat treating. Occasionally, we have a process such as shearing, which is really metal cutting but is viewed as a (sheet) metal-forming process. Assembly may involve processes other than joining. The categories of process types are far from perfect. Casting and molding processes are widely used to produce parts that often require other follow-on processes, such as machining. Casting uses molten metal to fill a cavity. The metal retains the desired shape of the mold cavity after solidification. An important advantage of casting and molding is that, in a single step, materials can be converted from a crude form into a desired shape. In most cases, a secondary advantage is that excess or scrap material can easily be recycled. Figure 1-11 illustrates schematically some of the basic steps in the lost-wax casting process, one of many processes used in the foundry industry. Casting processes are commonly classified into two types: permanent mold (a mold can be used repeatedly) or nonpermanent mold (a new mold must be prepared for each casting made). Molding processes for plastics and composites are included in the chapters on forming processes. Forming and shearing operations typically utilize material (metal or plastics) that has been previously cast or molded. In many cases, the materials pass through a series of forming or shearing operations, so the form of the material for a specific operation may be the result of all the prior operations. The basic purpose of forming and shearing is to modify the shape and size and/or physical properties of the material. Metal-forming and shearing operations are done both ‘‘hot’’ and ‘‘cold,’’ a reference to the temperature of the material at the time it is being processed with respect to the temperature at which this material can recrystallize (i.e., grow new grain structure). Figure 1-12 shows the process by which the fender of a car is made using a series of metalforming processes. Metal cutting, machining, or metal removal processes refer to the removal of certain selected areas from a part in order to obtain a desired shape or finish. Chips are formed by interaction of a cutting tool with the material being machined. Figure 1-13 shows a chip being formed by a single-point cutting tool in a machine tool called a lathe. The manufacturing engineer may be called upon to specify the cutting parameters such as cutting speed, feed, or depth of cut (DOC). The engineer may also have to select the cutting tools for the job. Cutting tools used to perform the basic turning on the lathe are shown in Figure 1-14. The cutting tools are mounted in machine tools, which provide the required movements of the tool with respect to the work (or vice versa) to accomplish the process desired. In recent years many new machining processes have been developed. The seven basic machining processes are shaping, drilling, turning, milling, sawing, broaching, and abrasive machining. Each of these basic processes is extensively discussed. Historically, eight basic types of machine tools have been developed to accomplish the basic processes. These machine tools are called shapers (and planers), drill presses, lathes, boring machines, milling machines, saws, broaches, and grinders. Most of these machine tools are capable of performing more than one of the basic machining processes. Shortly after numerical control was invented, machining centers

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Plastic part

Patterns of plastic parts Sprue and runner Polystyrene pattern

Slurry Dipped in refractory slurry

To make the foam parts, metal molds are used. Beads of polystyrene are heated and expanded in the mold to get parts.

A pattern containing a sprue, runners, risers, and parts is made from single or multiple pieces of foamed polystyrene plastic.

The polystyrene pattern is dipped in a ceramic slurry, which wets the surface and forms a coating about 0.005 inch thick.

Unbonded sand Flask

The coated pattern is placed in a flask and surrounded with loose, unbonded sand.

Surrounded with loose unbonded sand Vibration

The flask is vibrated so that the loose sand is compacted around the pattern. Compacted by vibration Molten metal

During the pouring of molten metal, the hot metal vaporizes the pattern and fills the resulting cavity.

Metal poured onto polystyrene pattern Solidified casting The solidified casting is removed from flask and the loose sand reclaimed.

FIGURE 1-11 Schematic of the lost-foam casting process.

Casting removed and sand reclaimed

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Metalforming Process for Automobile Fender Sheet metal bending/forming

Single draw punch and die

Hot rolling Billet

Slab Slide

Blank holder

Upper die

Rolls Slab

Punch Sheet metal Cushion pin

Cold-rolling plate

Punch

Sheet metal

(a) Cast billets of metal are passed through successive rollers to produce sheets of steel rolled stock.

(b) The flat sheet metal is “formed” into a fender, using sets of dies mounted on stands of large presses.

1

Trim punch

Punch

Metal shearing 2

Sheet metal

Die

Fender Punch

Scrap

Die Scrap

(c) The fender is cut out of the sheet metal in the last stage using shearing processes.

Die

(d) Sheet metal shearing processes are like scissors cutting paper. Next, the sheet metal parts are welded into the body of the car.

FIGURE 1-12

The forming process used to make a fender for a car.

were developed that could combine many of the basic processes, plus other related processes, into a single machine tool with a single workpiece setup. Aside from the chip-making processes, there are processes wherein metal is removed by chemical, electrical, electrochemical, or thermal sources. Generally speaking, these nontraditional processes have evolved to fill a specific need when conventional processes were too expensive or too slow when machining very hard materials. One of the first uses of a laser was to machine holes in ultra-high-strength metals. Lasers are being used today to drill tiny holes in turbine blades for jet engines. Because of its ability to produce components with great precision and accuracy, metal cutting, using machine tools, is recognized as having great value-adding capability. In recent years a new family of processes has emerged called rapid prototyping or rapid manufacturing or free-form manufacturing (see Chapter 19). These additive-type processes produce first, or prototype, components directly from the software using specialized machines driven by computer-aided design packages. The prototypes can be field tested and modifications to the design quickly implemented. Early versions of these machines produced only nonmetallic components, but modern machines can

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The Machining Process (turning on a lathe) Cutting tool

Workpiece

The workpiece is mounted in a workholding device in a machine tool (lathe) and is cut (machined) with a cutting tool. Lathe

Workpiece

Original diameter

Cutting speed V

Final diameter

Depth of cut Feed (inch/rev) Tool

The workpiece is rotated while the tool is fed at some feed rate (inches per revolution). The desired cutting speed V determines the rpm of the workpiece. This process is called turning.

Chip

The cutting tool interacts with the workpiece to form a chip by a shearing process. The tool shown here is an indexable carbide insert tool with a chip-breaking groove.

FIGURE 1-13 Single-point metal-cutting process (turning) produces a chip while creating a new surface on the workpiece. (Courtesy J T. Black)

make metal parts. In contrast, the machining processes are recognized as having great value-adding capability, that is, the ability to produce components with great precision and accuracy. Companies have sprung up; you can send your CAD drawing over the Internet and a prototype is made in hours. Perhaps the largest collection of processes, in terms of both diversity and quantity, are the joining processes, which include the following: 1. Mechanical fastening. 2. Soldering and brazing. 3. Welding. 4. Press, shrink, or snap fittings. 5. Adhesive bonding. 6. Assembly processes. Many of these joining processes are often found in the assembly area of the plant. Figure 1-14 provides one example where all but welding are used in the sequence of

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Silicon wafer Microelectronic manufacturing

Level 1 Die

Level 2 Cover chip

Packaging

C S

H

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P

S

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L

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F

O O

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or Integrated circuit (IC) on chip

N

N

IC Package for connection

R

Level 1

PCB fab

Computer

PCB assembly Motherboard

Level 3

Level 5

FIGURE 1-14

Level 4

How an electronic product is made.

operations to produce a computer. Starting in the upper left corner, microelectronic fabrication methods produce entire integrated circuits (ICs) of solid-state (no moving parts) components, with wiring and connections, on a single piece of semiconductor material, usually single-crystalline silicon. Arrays of ICs are produced on thin, round disks of semiconductor material called wafers. Once the semiconductor on the wafer has been fabricated, the finished wafer is cut up into individual ICs, or chips. Next, at level 2, these chips are individually housed with connectors or leads making up ‘‘dies’’ that are placed into ‘‘packages’’ using adhesives. The packages provide protection from the elements and a connection between the die and another subassembly called the printed circuit boards (PCBs). At level 3, IC packages, along with other discrete components (e.g., resistors, capacitors, etc.), are soldered onto PCBs and then assembled with even larger circuits on PCBs. This is sometimes referred to as electronic assembly. Electronic packages at this level are called cards or printed wiring assemblies (PWAs). Next, series of cards are combined on a back-panel PCB, also known as a motherboard or simply a board. This level of packaging is sometimes referred to as card-on-board packaging. Ultimately, card-on-board assemblies are put into housings using mechanical fasteners and snap fitting and finally integrated with power supplies and other electronic peripherals through the use of cables to produce final commercial products. See Chapter 41 for more details on electronic manufacturing. Finishing processes are yet another class of processes typically employed for cleaning, removing burrs left by machining, or providing protective and/or decorative surfaces on workpieces. Surface treatments include chemical and mechanical cleaning, deburring, painting, plating, buffing, galvanizing, and anodizing. Heat treatment is the heating and cooling of a metal for the specific purpose of altering its metallurgical and mechanical properties. Because changing and controlling these properties is so important in the processing and performance of metals, heat treatment is a very important manufacturing process. Each type of metal reacts differently to heat treatment. Consequently, a designer should know not only how a selected metal can be altered by heat treatment but, equally important, how a selected metal will react, favorably or unfavorably, to any heating or cooling that may be incidental to the manufacturing processes.

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OTHER MANUFACTURING OPERATIONS In addition to the processes already described, there are many other fundamental manufacturing operations that must be considered. Inspection determines whether the desired objectives stated by the designer in the specifications have been achieved. This activity provides feedback to design and manufacturing with regard to the process behavior. Essential to this inspection function are measurement activities. In the factory, measurements are either by attributes or variables (see Chapter 35) to inspect the outcomes from the process and determine how they compare to the specifications. The many aspects of quality control are presented in Chapter 36. Chapter 43 (on the Web) covers testing, where a product is tried by actual function or operation or by subjection to external effects. Although a test is a form of inspection, it is often not viewed that way. In manufacturing, parts and materials are inspected for conformance to the dimensional and physical specifications, while testing may simulate the environmental or usage demands to be made on a product after it is placed in service. Complex processes may require many tests and inspections. Testing includes life-cycle tests, destructive tests, nondestructive testing to check for processing defects, wind-tunnel tests, road tests, and overload tests. Transportation of goods in the factory is often referred to as material handling or conveyance of the goods and refers to the transporting of unfinished goods (workin-process) in the plant and supplies to and from, between, and during manufacturing operations. Loading, positioning, and unloading are also material-handling operations. Transportation, by truck or train, is material handling between factories. Proper manufacturing system design and mechanization can reduce material handling in countless ways. Automatic material handling is a critical part of continuous automatic manufacturing. The word automation is derived from automatic material handling. Material handling, a fundamental operation done by people and by conveyors and loaders, often includes positioning the workpiece within the machine by indexing, shuttle bars, slides, and clamps. In recent years, wire-guided automated guided vehicles (AGVs) and automatic storage and retrieval systems (AS/RSs) have been developed in an attempt to replace forklift trucks on the factory floor. Another form of material handling, the mechanized removal of waste (chips, trimming, and cutoffs), can be more difficult than handling the product. Chip removal must be done before a tangle of scrap chips damages tooling or creates defective workpieces. Most texts on manufacturing processes do not mention packaging, yet the packaging is often the first thing the customer sees. Also, packaging often maintains the product’s quality between completion and use. (The term packaging is also used in electronics manufacturing to refer to placing microelectronic chips in containers for mounting on circuit boards.) Packaging can also prepare the product for delivery to the user. It varies from filling ampules with antibiotics to steel-strapping aluminum ingots into palletized loads. A product may require several packaging operations. For example, Hershey Kisses are (1) individually wrapped in foil, (2) placed in bags, (3) put into boxes, and (4) placed in shipping cartons. Weighing, filling, sealing, and labeling are packaging operations that are highly automated in many industries. When possible, the cartons or wrappings are formed from material on rolls in the packaging machine. Packaging is a specialty combining elements of product design (styling), material handling, and quality control. Some packages cost more than their contents (e.g., cosmetics and razor blades). During storage, nothing happens intentionally to the product or part except the passage of time. Part or product deterioration on the shelf is called shelf life, meaning that items can rust, age, rot, spoil, embrittle, corrode, creep, and otherwise change in state or structure, while supposedly nothing is happening to them. Storage is detrimental, wasting the company’s time and money. The best strategy is to keep the product moving with as little storage as possible. Storage during processing must be eliminated, not automated or computerized. Companies should avoid investing heavily in large automated systems that do not alter the bottom line. Have the outputs improved with respect to the inputs, or has storage simply increased the costs (indirectly) without improving either the quality or the throughput time?

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TABLE 1-4

Characterizing a Process Technology

Mechanics (statics and dynamics of the process) How does the process work? What are the process mechanics (statics, dynamics, friction)? What physically happens, and what makes it happen? (Understand the physics.)

Manufacturing and Production Systems

23

By not storing a product, the company avoids having to (1) remember where the product is stored, (2) retrieve it, (3) worry about its deteriorating, or (4) pay storage (including labor) costs. Storage is the biggest waste of all and should be eliminated at every opportunity.

Economics or costs What are the tooling costs, the engineering costs? Which costs are short term, which long term? What are the setup costs? Time spans How long does it take to set up the process initially? What is the throughput time? How can these times be shortened? How long does it take to run a part once it is set up (cycle time)? What process parameters affect the cycle time? Constraints What are the process limits? What cannot be done? What constrains this process (sizes, speeds, forces, volumes, power, cost)? What is very hard to do within an acceptable time/cost frame? Uncertainties and process reliability What can go wrong? How can this machine fail? What do people worry about with this process? Is this a reliable, stable process? Skills What operator skills are critical? What is not done automatically? How long does it take to learn to do this process? Flexibility Can this process be adapted easily for new parts of a new design or material? How does the process react to changes in part design and demand? What changes are easy to do? Process capability What are the accuracy and precision of the process? What tolerances does the process meet? (What is the process capability?) How repeatable are those tolerances?

UNDERSTAND YOUR PROCESS TECHNOLOGY Understanding the process technology of the company is very important for everyone in the company. Manufacturing technology affects the design of the product and the manufacturing system, the way in which the manufacturing system can be controlled, the types of people employed, and the materials that can be processed. Table 1-4 outlines the factors that characterize a process technology. Take a process you are familiar with and think about these factors. One valid criticism of American companies is that their managers seem to have an aversion to understanding their companies’ manufacturing technologies. Failure to understand the company business (i.e., its fundamental process technology) can lead to the failure of the company. The way to overcome technological aversion is to run the process and study the technology. Only someone who has run a drill press can understand the sensitive relationship between feed rate and drill torque and thrust. All processes have these ‘‘know-how’’ features. Those who run the processes must be part of the decision making for the factory. The CEO who takes a vacation working on the plant floor and learning the processes will be well on the way to being the head of a successful company.

PRODUCT LIFE CYCLE AND LIFE-CYCLE COST

Manufacturing systems are dynamic and change with time. There is a general, traditional relationship between a product’s life cycle and the kind of manufacturing system used to make the product. Figure 1-15 simplifies the product life cycle into these steps, again using an S-shaped curve. 1. Startup. New product or new company, low volume, small company. 2. Rapid growth. Products become standardized and volume increases rapidly. Company’s ability to meet demand stresses its capacity. 3. Maturation. Standard designs emerge. Process development is very important. 4. Commodity. Long-life, standard-of-the-industry type of product or 5. Decline. Product is slowly replaced by improved products. The maturation of a product in the marketplace generally leads to fewer competitors, with competition based more on price and on-time delivery than on unique product features. As the competitive focus shifts during the different stages of the product life cycle, the requirements placed on manufacturing—cost, quality, flexibility, and delivery dependability—also change. The stage of the product life cycle affects the product design stability, the length of the product development cycle, the frequency of

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Introduction to DeGarmo’s Materials and Processes in Manufacturing Time odity)

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Cost per unit

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Saturation

Manufacturing cost per unit

Time Startup

Rapid growth

Maturation

Commodity or decline

Manufacturing system design

Job shop

Production job shop with some flow

Production job shop with some flow lines and assembly lines

More flow mass-produce

Product variety:

Great variety; product innovation great

Increasing standardization; less variety

Emergence of a dominant standard design

High standardization "Commodity" characteristics

Industry structure:

Many small competitors

Fallout and consolidation

Few large companies

"Survivors" become commodities

Form of competition:

Product Product quality, cost, characteristics and availability

Price and quality with reliability

Price with consistent quality

Process innovation: Low

Medium to high

High

Medium

Automation:

Medium

Medium to high

High

FIGURE 1-15

Low

Product life-cycle costs change with the classic manufacturing system designs.

engineering change orders, and the commonality of components—all of which have implications for manufacturing process technology. During the design phase of the product, much of the cost of manufacturing and assembly is determined. Assembly of the product is inherently integrative as it focuses on pairs and groups of parts. It is crucial to achieve this integration during the design phase because about 70% of the life-cycle cost of a product is determined when it is designed. Design choices determine materials; fabrication methods; assembly methods; and, to a lesser degree, material-handling options, inspection techniques, and other aspects of the production system. Manufacturing engineers and internal customers can influence only a small part of the overall cost if they are presented with a finished design that does not reflect their concerns. Therefore, all aspects of production should be included if product designs are to result in real functional integration. Life-cycle costs include the costs of all the materials, manufacture, use, repair, and disposal of a product. Early design decisions determine about 60% of the cost, and all activities up to the start of full-scale development determine about 75%. Later decisions can make only minor changes to the ultimate total unless the design of the manufacturing system is changed. In short, the concept of product life-cycle provides a framework for thinking about the product’s evolution through time and the kind of market segments that are

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25

likely to develop at various times. Analysis of life-cycle costs shows that the design of the manufacturing system determines the cost per unit, which generally decreases over time with process improvements and increased volumes. For additional discussion on reliability and maintainability of manufacturing equipment, see the Society of Automotive Engineers SAE publication M-110.2. The linked-cell manufacturing system design discussed in Chapters 2 and 29 (known as lean production or the Toyota Production System) has transformed the automobile industry and many other industries to be able to make a large variety of products in small volumes with very short throughput times. Thousands of companies have implemented lean to reduce waste, decrease cost per unit significantly while maintaining flexibility and making smooth transitions. This is a new business model affecting product development, design, purchasing, marketing, customer service and all other aspects of the company. Low-cost manufacturing does not just happen. There is a close, interdependent relationship among the design of a product, the selection of materials, the selection of processes and equipment, the design of the processes, and tooling selection and design. Each of these steps must be carefully considered, planned, and coordinated before manufacturing starts. Some of the steps involved in getting the product from the original idea stage to daily manufacturing are discussed in more detail in Chapter 10. The steps are closely related to each other. For example, the design of the tooling is dependent on the design of the parts to be produced. It is often possible to simplify the tooling if certain changes are made in the design of the parts or the design of the manufacturing systems. Similarly, the material selection will affect the design of the tooling or the processes selected. Can the design be altered so that it can be produced with tooling already on hand and thus avoid the purchase of new equipment? Close coordination of all the various phases of design and manufacture is essential if economy is to result. With the advent of computers and computer-controlled machines, the integration of the design function and the manufacturing function through the computer is a reality. This is usually called CAD/CAM (computed-aided design/computer-aided manufacturing). The key is a common database from which detailed drawings can be made for the designer and the manufacturer and from which programs can be generated to make all the tooling. In addition, extensive computer-aided testing and inspection (CATI) of the manufactured parts is taking place. There is no doubt that this trend will continue at ever-accelerating rates as computers become cheaper and smarter, but at this time, the computers necessary to accomplish complete computer-integrated manufacturing (CIM) are expensive and the software very complex. Implementing CIM requires a lot of manpower as well.

COMPARISONS OF MANUFACTURING SYSTEM DESIGN When designing a manufacturing system, two customers must be taken into consideration: the external customer who buys the product and the internal customer who makes the product. The external customer is likely to be global and demand greater variety with superior quality and reliability. The internal customer is often empowered to make critical decisions about how to make the products. The Toyota Motor Company is making vehicles in 25 countries. Their truck plant in Indiana has the capacity to make 150,000 vehicles per year (creating 2300 new jobs), using the Toyota Production System (TPS). An appreciation of the complexity of the manufacturing system design problem is shown in Figure 1-16, where the choices between the system designs are reflected against the number of different products, or parts being made in the system, often called variety. Clearly, there are many choices regarding which method (or system) to use to make the goods. A manufacturer never really knows how large or diverse a market will be. If a diverse and specialized market emerges, a company with a focused flow-line system may be too inflexible to meet the varying demand. If a large but homogeneous market develops, a manufacturer with a flexible system may find production costs too high and the flexibility unexploitable. Another general relationship between manufacturing system designs and production volumes is shown in Figure 1-17.

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Introduction to DeGarmo’s Materials and Processes in Manufacturing Transfer line Dedicated flow lines Job shop with standalone CNC

1000 System production rate parts per hour

C01

100

10

Job shop No families of parts, stand-alone machines

1 .1

FMS Project shop

.01 10 1 Large volume Low variety

FIGURE 1-16

Continuous flow processes Transfer lines Continuous flow shop Assembly line

100 1000 10,000 Variety of parts per system

100,000 Large variety

Different manufacturing system designs produce goods at different production rates.

Oil, Gas Beer Bricks Not flexible; more efficient

Disconnected line Interrupted flow shop Batch processing Job shop

Autos Cycles TVs Trucks Airplanes Ships Shuttles

Job shop ad hoc flow

Less efficient; more flexible Commodity High volume

FIGURE 1-17

This part variety-production rate matrix shows examples of particular manufacturing system designs. This matrix was developed by Black based on real factory data. Notice there is a large amount of overlap in the middle of the matrix, so the manufacturing engineer has many choices regarding which method or system to use to make the goods. This book will show the connection between the process and the manufacturing system used to produce the products, turning raw materials into finished goods.

Manned and unmanned cells

Few types High volume

Multiple products Low volume

Space station

The figure shows in a general way the relationship between manufacturing systems and production volumes. The upper left represents systems with low flexibility but high efficiency compared to the lower right, where volumes are low and so is efficiency. Where a particular company lies in this matrix is determined by many forces, not all of which are controllable. The job of manufacturing and industrial engineers is to design and implement a system which can achieve low unit cost, superior quality, with on-time delivery in a flexible way.

Very low volume

This figure shows in a general way the relationship between manufacturing systems and production volumes.

NEW MANUFACTURING SYSTEMS The manufacturing process technology described in this text is available worldwide. Many countries have about the same level of process development when it comes to manufacturing technology. Much of the technology existing in the world today was developed in the United States, Germany, France, and Japan. More recently Taiwan, Korea, and China have been making great inroads into American markets, particularly in the automotive and electronics industries. Many companies have developed and promoted a different kind of manufacturing system design. This new manufacturing system, called lean manufacturing, will take its place with the American Armory System and the Ford System for mass production. This new manufacturing system, developed by the Toyota Motor Company, has been successfully adopted by many American companies. For lean production to work, units with no defects (100% good) must flow rhythmically to subsequent processes without interruption. In order to accomplish this, an integrated quality control (IQC) program has to be developed. The responsibility for quality has been given to manufacturing. All the employees are inspectors and are

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Review Questions

27

empowered to make it right the first time. There is a companywide attitude toward constant quality improvement. Make quality easy to see, stop the line when something goes wrong, and inspect things 100% if necessary to prevent defects from occurring. The results of this system are astonishing in terms of quality, low cost, and on-time delivery of goods to the customer. The most important factor in economical and successful manufacturing is the manner in which the resources—labor, materials, and capital—are organized and managed so as to provide effective coordination, responsibility, and control. Part of the success of lean production can be attributed to a different management approach. This approach is characterized by a holistic attitude toward people. The real secret of successful manufacturing lies in designing a manufacturing system in which everyone who works in the system understands how the system works and how goods are controlled, with the decision making placed at the correct level. The engineers also must possess a broad fundamental knowledge of design, metallurgy, processing, economics, accounting, and human relations. In the manufacturing game, low-cost mass production is the result of teamwork within an integrated manufacturing/ production system. This is the key to producing superior quality at less cost with on-time delivery.

& KEY WORDS assembly casting construction consumer goods continuous process design engineer fabricating finishing process flow shop forming goods group technology (GT)

heat treatment inspection job job shop joining process lean engineer lean production linked-cell manufacturing system (L-CMS) machine tool machining manufacturing

manufacturing cost manufacturing engineer manufacturing process manufacturing system materials engineer molding numerical control operation packaging processing producer goods product

product life cycle production system project shop shearing shelf life station storage sustaining technology tooling tools Toyota Production System treatments

& REVIEW QUESTIONS 1. What role does manufacturing play relative to the standard of living of a country? 2. Aren’t all goods really consumer goods, depending on how you define the customer? Discuss. 3. Is the Subway sandwich shop an example of a job shop, flow shop, or project shop? 4. How does a system differ from a process? From a machine tool? From a job? From an operation? 5. Is a cutting tool the same thing as a machine tool? 6. What are the major classifications of basic manufacturing processes? 7. Casting is often used to produce a complex-shaped part to be made from a hard-to-machine metal. How else could the part be made? 8. In the lost-wax casting process, what happens to the foam? 9. In making a gold medal, what do we mean by a ‘‘relief image’’ cut into the die? 10. How is a railroad station like a station on an assembly line? 11. Because no work is being done on a part when it is in storage, it does not cost you anything. True or false? Explain. 12. What forming processes are used to make a paper clip? 13. What is tooling in a manufacturing system?

14. It is acknowledged that chip-type machining is basically an inefficient process. Yet it is probably used more than any other to produce desired shapes. Why? 15. Compare Figure 1-1 and Figure 1-15. What are the stages of the product life cycle for a computer? 16. In a modern safety razor with three or four blades that sells for $1, what do you think the cost of the blades might be? 17. List three purposes of packaging operations. 18. Assembly is defined as ‘‘the putting together of all the different parts to make a complete machine.’’ Think of (and describe) an assembly process. Is making a club sandwich an assembly process? What about carving a turkey? Is this an assembly process? 19. What are the physical elements in a manufacturing system? 20. In the production system, who usually figures out how to make the product? 21. In Figure 1-8, what do the lines connecting the processes represent? 22. Characterize the process of squeezing toothpaste from a tube (extrusion of toothpaste) using Table 1-4 as a guideline. See the index for help on extrusion. 23. What difficulties would result if production planning and scheduling were omitted from the procedure outlined in Chapter 9 for making a product in a job shop?

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24. It has been said that low-cost products are more likely to be more carefully designed than high-priced items. Do you think this is true? Why or why not? 25. Proprietary processes are closely held or guarded company secrets. The chemical makeup of a lubricant for an extrusion process is a good example. Give another example of a proprietary process. 26. If the rolls for the cold-rolling mill that produces the sheet metal used in your car cost $300,000 to $400,000, how is it that your car can still cost less than $20,000? 27. Make a list of service systems, giving an example of each. 28. What is the fundamental difference between a service systems and a manufacturing system?

29. In the process of buying a calf, raising it to a cow, and disassembling it into ‘‘cuts’’ of meat for sale, where is the ‘‘value added’’? 30. What kind of process is powder metallurgy: casting or forming? 31. In view of Figure 1-2, who really determines the selling price per unit? 32. What costs make up manufacturing cost (sometimes called factory cost)? 33. What are major phases of a product life cycle? 34. How many different manufacturing systems might be used to make a component with annual projected sales of 16,000 parts per year with 10 to 12 different models (varieties)? 35. In general, as the annual volume for a product increases, the unit cost decreases. Explain.

& PROBLEMS 1. The Toyota truck plant in Indiana produces 150,000 trucks per year. The plant runs one eight-hour shift, 300 days per year, and makes 500 trucks per day. About 1300 people work on the final assembly line. Each car has about 20 labor hours per car in it. a. Assuming the truck sells for $16,000 and workers earn $30 per hour in wages and benefits, what percentage of the cost of the truck is in direct labor? b. What is the production rate of the final assembly line? 2. Suppose you wanted to redesign a stapler to have fewer components. (You should be able to find a stapler at a local discount store.) How much did it cost? How many parts does it have? Make up a ‘‘new parts’’ list and indicate which parts would have to be redesigned and which parts would be eliminated. Estimate the manufacturing cost of the stapler assuming that manufacturing costs are 40% of the selling price.

Chapter 1

3.

4.

5.

6.

What are the disadvantages of your new stapler design versus the old stapler? A company is considering making automobile bumpers from aluminum instead of from steel. List some of the factors it would have to consider in arriving at its decision. Many companies are critically examining the relationship of product design to manufacturing and assembly. Why do they call this concurrent engineering? We can analogize your university to a manufacturing system that produces graduates. Assuming that it takes four years to get a college degree and that each course really adds value to the student’s knowledge base, what percentage of the four years is ‘‘value adding’’ (percentage of time in class plus two hours of preparation for each hour in class)? What are the major process steps in the assembly of an automobile?

CASE STUDY

Famous Manufacturing Engineers

M

anufacturing engineering is that engineering function charged with the responsibility of interpreting product design in terms of manufacturing requirements and process capability. Specifically, the manufacturing engineer may:

Determine how the product is to be made in terms of specific manufacturing processes. Design workholding and work transporting tooling or containers. Select the tools (including the tool materials) that will machine or form the work materials.

Select, design, and specify devices and instruments that inspect products that have been manufactured to determine their quality. Design and evaluate the performance of the manufacturing system. Perform all these functions (and many more) related to the actual making of the product at the most reasonable cost per unit without sacrifice of the functional requirements or the users’ service life.

There’s no great glory in being a great manufacturing engineer (MfE). If you want to be a manufacturing

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Case Study production methods. In 1848, he rented a plant in Hartford, Connecticut, and the Colt legend spread. In 1853 he had built one of the world’s largest arms plant in Connecticut, which had 1400 machine tools. Colt helped start the careers of E. K. Root, mechanic and superintendent, paying him a salary of $25,000 in the 1800s. Abolished hand work—jigs and fixtures. Francis Pratt and Amos Whitney—famous machine tool builders. William Gleason—gear manufacturer E. P. Bullard—invented the Mult–An–Matic Multiple spindle machine, which cut the time to make a flywheel from 18 minutes to slightly over 1 minute. Sold this to Ford. Christopher Sponer. E. J. Kingsbury—invented a drilling machine to drill holes through toy wheel hubs that had a springloaded cam that enabled the head to sense the condition of the casting and modify feed rate automatically.

engineer, you had better be ready to get your hands dirty. Of course, there are exceptions. There have been some very famous manufacturing engineers. For example:

John Wilkinson of Bersham, England built a boring mill in 1775 to bore the cast iron cylinders for James Watt’s steam engine. How good was this machine? Eli Whitney was said to have invented the cotton gin, a machine to separate seeds from cotton. His machine was patented but was so simple, anyone could make one. He was credited with ‘‘interchangeability’’—but we know Thomas Jefferson observed interchangeability in France in 1785 and probably the French gunsmith LeBlanc is the real inventor here. Jefferson tried to bring the idea to America and Whitney certainly did. He took 10 muskets to Congress, disassembled them, and scattered the pieces. Interchangeable parts permitted them to be reassembled. He was given a contract for 2000 guns to be made in two years. But what is the rest of his story? Joe Brown started a business in Rhode Island in 1833 making lathes and small tools as well as timepieces (watchmaker). Lucian Sharp joined the company in 1848 and developed a pocket sheet metal gage in 1877 and a 1-inch micrometer, and in 1862 developed the universal milling machine. At age 16, Sam Colt sailed to Calcutta on the Brig ‘‘Curve.’’ He whittled a wood model of a revolver on this voyage. He saved his money and had models of a gun built in Hartford by Anson Chase, for which he got a patent. He set up a factory in New Jersey—but he could not sell his guns to the Army because they were too complicated. He sold to the Texas Rangers and the Florida Frontiersmen, but he had to close the plant. In 1846, the Mexican war broke out. General Zachary Taylor and Captain Sam Walters wanted to buy guns. Colt had none but accepted orders for 1000 guns and constructed a model (Walker Colt); he arranged to have them made at Whitney’s (now 40-year-old) plant in Whitneyville. Here he learned about mass

Now here are some more names from the past of famous and not-so-famous manufacturing, mechanical, and industrial engineers. Relate them to the development of manufacturing processes or manufacturing system designs.

Eli Whitney Henry Ford Charles Sorenson Sam Colt John Parsons Eiji Toyoda Elisha Root John Hall Thomas Blanchard Fred Taylor Taiichi Ohno Ambrose Swasey

29

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CHAPTER 2 MANUFACTURING SYSTEMS DESIGN 2.1 INTRODUCTION 2.2 MANUFACTURING SYSTEMS 2.3 CONTROL OF MANUFACTURING SYSTEMS 2.4 CLASSIFICATION OF MANUFACTURING SYSTEMS Job Shop

Flow Shop Project Shop Continuous Process Lean Manufacturing System 2.5 SUMMARY OF FACTORY DESIGNS The Evolution of the First Factory

Factory Revolutions Evolve Evolution of the Second MDS—The Flow Shop The Third MDS—Lean Production Summary

& 2.1 INTRODUCTION In a factory, manufacturing processes are assembled together to form a manufacturing system to produce a desired set of goods. The manufacturing system takes specific inputs and materials, adds value through processes, and transforms the inputs into products for the customer. It is important to distinguish between the manufacturing system and the production system, which is also known as the enterprise system, or the whole company. The production system includes the manufacturing system. As shown in Figure 2-1, the production system services the manufacturing system, using all the other functional areas of the plant for information, design, analysis, and control. These subsystems are connected to each other to produce goods or services, or both. A production system includes all aspects of the business, including design engineering, manufacturing engineering, sales, advertising, production and inventory control (scheduling and distribution), and, most important, the manufacturing system. The enterprise and all its functional areas are discussed in Chapter 44 on the Web.

& 2.2 MANUFACTURING SYSTEMS A sequence of processes and people that actually produce the desired product(s) is called the manufacturing system. In Figure 2-2, the manufacturing system is defined as the complex arrangement of physical elements characterized (and controlled) by measurable parameters (Black, 1991). The relationship among the elements determines how well the system can run or be controlled. The control of a system refers to the entire manufacturing system (which means the control of the operators in a harmonious way relative to the system’s objectives), not merely the individual processes or equipment. The entire manufacturing system must be under daily control to enable the management of material movement, people and processes (scheduling), inventory levels, product quality, production rates, throughput, and, of course, cost. As shown in Figure 2-2, inputs to the manufacturing system include materials, information, and energy. The system is a complex set of elements that includes machines (or machine tools), people, materials-handling equipment, and tooling. Workers are the internal customers. They process materials within the system, which gain value as the material progresses from process to machine. Manufacturing system outputs may be finished or semifinished goods. Semifinished goods serve as inputs to some other process at other locations. Manufacturing systems are dynamic, meaning that they must be designed to adapt constantly to change. Many of the inputs cannot be fully controlled by management, and the effect of disturbances must be counteracted by manipulating the controllable inputs or the system itself. Controlling the input

30

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SECTION 2.2

31

Manufacturing Systems

The Production System

Requirements

Feedback from the Customer

Development Orders

Engineering and Design

Sales and Marketing Information

Part and Product Definition Data

Forecast

Feedback from ICs

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Engineering

07/06/2011

Concurrent

C02

Manufacturing Planning Information

Manufacturing Systems Product Demand data

Information

Engineering –What to order –How to sequence/order –When

–How long

–Rate

–How many

MANUFACTURING SYSTEM (The Factory)

Energy

External

Where value is added Goods and

customer

Materials

The Enterprise or Production System (Dotted line) Components from Subassemblies, Materials recg. From suppliers

FIGURE 2-1 The manufacturing system (shaded) is the heart of the company. It lies within and is served by the production system or the enterprise.

material availability and/or predicting demand fluctuations may be difficult. A national economic decline or recession can cause shifts in the business environment that can seriously change any of these inputs. In manufacturing systems, not all inputs are fully controllable. To understand how manufacturing systems work and be able to design manufacturing systems, computer modeling (simulation) and analysis are used. However, modeling and analysis are difficult because

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CHAPTER 2

Manufacturing Systems Design Unplanned disturbances Inputs to system

Outputs to customers

Materials Subassemblies Energy Demand Social Political Pressure

A manufacturing system is A complex arrangement of physical elements* characterized by measurable parameters†

Good products, good parts, etc. Information Service to customer Defectives and scrap

External customer

C02

Information from Design, Purchasing, Production Control, etc. * Physical

FIGURE 2-2 Here is our manufacturing system with its inputs and outputs. (From Design of the Factory with a Future, 1991, McGraw-Hill, by J T. Black)

elements:

• Machines for processing • Tooling (fixtures, dies, cutting tools) • Material handling equipment (which includes all transportation and storage) • People (internal customers) operators, workers, associates

Feedback

† Measurable

system parameters:

• Throughput time (TPT) • Production rate (PR) • Work-in-process inventory • % on-time delivery • % defective • Daily/weekly/monthly volume • Cycle time or takt time (TT) • Total cost or unit cost

1. In the absence of a system design, the manufacturing systems can be very complex, be difficult to define, and have conflicting goals. 2. The data or information may be difficult to secure, inaccurate, conflicting, missing, or even too abundant to digest and analyze. 3. Relationships may be awkward to express in analytical terms, and interactions may be nonlinear; thus, many analytical tools cannot be applied with accuracy. System size may inhibit analysis. 4. Systems are always dynamic and change during analysis. The environment can change the system, and vice versa. 5. All systems analyses are subject to errors of omission (missing information) and commission (extra information). Some of these are related to breakdowns or delays in feedback elements. Because of these difficulties, digital simulation has become an important technique for manufacturing systems modeling and analysis as well as for manufacturing system design.

& 2.3 CONTROL OF THE MANUFACTURING SYSTEM In general, a manufacturing system should be an integrated whole, composed of integrated subsystems, each of which interacts with the entire system. The critical control functions are production rate and mix control, inventory control, quality control, and machine tool control (reliability).While the system may have a number of objectives or goals, the users of the system may seek to optimize the whole. Optimizing bits and pieces does not optimize the entire system. System control functions require information gathering, communication capabilities, and decision-making processes that are integral parts of the manufacturing system.

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SECTION 2.4

Classification of Manufacturing Systems

33

& 2.4 CLASSIFICATION OF MANUFACTURING SYSTEMS Manufacturing industries vary by the products that they make or assemble. While almost all factories are different, there are five basic manufacturing system designs (MSDs): four classic (or hybrid combinations thereof) and one new manufacturing system design that is rapidly gaining acceptance in almost all of these industries. The classical systems here are the job TABLE 2-1 Types and Examples of Manufacturing shop, the flow shop, the project shop, and the continuous Systems process. The lean shop is a new kind of system. The assembly line is a form of the flow shop, and a system that Examples Type of has batch flow might also be added as another manufacManufacturing turing system. Figure 2-3 shows schematics of the four Servicea Productb System classical systems along with the linked-cell manufacturJob shop Auto repair Machine shop ing system, or the lean shop. Hospital Metal fabrication Table 2-1 lists examples of five types of manufacturRestaurant Custom jewelry ing systems. These lists are not meant to be complete, just University FMS informative as there are hundreds of examples of each of Flow shop or flow line X-ray TV factory the basic system designs. Cafeteria

Auto assembly line

College registration

JOB SHOP

Car wash

The job shop’s distinguishing feature is its functional design. In the job shop, a variety of products are manufactured, which results in small manufacturing lot sizes, Food court at mall Family of turned parts often one of a kind. Job shop manufacturing is commonly 10-minute oil change Composite part families done to specific customer order, but, in truth, many job Design families shops produce to fill finished-goods inventories. Because Project shop Producing a movie Locomotive assembly the plant must perform a wide variety of manufacturing Broadway play Bridge construction processes, general-purpose production equipment is TV show House construction required. Workers must have relatively high skill levels Continuous process Telephone company Oil refinery to perform a range of different work assignments, often Phone company Chemical plant due to a lack of work standardization, defects, and variety a in goods produced. Job shop products include space vehiCustomer receives a service or perishable product. b cles, aircraft, machine tools, special tools, and equipProducts can be for customers or for other companies. ment. Figure 2-4 depicts the functionally arranged job shop. Production machines are grouped according to the general type of manufacturing process. The lathes are in one department, drill presses in another, plastic molding in still another, and so on. The advantage of this layout is its ability to make a wide variety of products. Each different part requiring its own unique sequence of operations can be routed through the respective departments in the proper order. In the job shop, process planning consists of determining the sequence of individual manufacturing processes and operations needed to produce the parts. An overview of the product is developed with a process flow chart, which shows the various levels of the product, subassemblies, and components. Figure 2-5 shows a process flow chart and a bill of materials (BOM), which lists all the parts and components in a product. The route sheet and the operations sheet are the documents that specify the process sequence through the job shop and the sequence of operations to be performed at specific machines. Route sheets are used as the production control device to define the path of the material through the manufacturing system; see Figure 2-6, for example. Forklifts and handcarts are used to move materials from one machine to the next. As the company grows, the job shop evolves into a production job shop making products in large lots or batches. The route sheet lists manufacturing operations and associated machine tools for each workpiece. The route sheet travels with the parts, which move in batches (or lots) between the processes. When a cart of parts has to be moved from one point in the job shop to another, the route sheet provides routing (travel) information, telling the material handler which machine in which department the parts must go to next. Now look at Figure 2-4, which shows the path through the job shop that the punch would take from start to finish as described in the route sheet. Lean shop or linked-cell

Fast-food restaurant (KFC, Wendy’s)

Product families

34 In

Assembly

Heattreating

Start

(c) Lean shop (U-shaped cells)

Kanban link

In

Out

Final Motors assembly

Final assembly

Frames

B

Subassembly cell II

Mfg cell I

Controls

A

B

C

Equipment

Labor

Supplies

Gas Oil

III

II By-products

Process I

(e) Continuous-process layout

Energy

Raw materials

Refinery

(d) Project shop – fixed position layout

House

Lot no. 73

Submachines

Machines

Materials

Component parts

FIGURE 2-3 Schematic layouts of five manufacturing systems: (a) job shop (functional or process layout); (b) flow shop (line or product layout); (c) linked-cell layout, or the lean production system; (d) project shop (fixed-position layout); and (e) continuous process.

ba

m

e ss

y

bl

Team leader Cores housings Subassembly

Out

Storage

Area manager assembly

(b) Flow shop – line or product layout

Seated

Inventory

Assembly Using Conveyors

Stand by

Su

Painting

Grinders

(a) Job shop – functional or process layout

Milling machines

Drill presses

Lathes

Area manager housing

Presses (sheet metal)

Saws

Receiving

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Subassembly

Storage

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SECTION 2.4 Lathe Department

35

Classification of Manufacturing Systems Drilling Department

Milling Department

J&L lathe

L

M1

M

J&L lathe

L

M

M

D

D

Heat Treatment Dept

D

D

F

Grinding Department L

L

L

L

M

Final Inspection and Assembly

Receiving and Shipping Key:

M

Movement of parts in containers Machine tool operators Machine tool

I

P

A

A

G

G

G

G

G

G

Surface grinding 3

Dark arrow shows path for part shown in Figure 2-6

FIGURE 2-4 Schematic layout of a job shop where processes are gathered functionally into areas or departments. Each square block represents a manufacturing process. Sometimes called the ‘‘spaghetti design.’’

The operations sheet describes what machining or assembly operations are done to the parts at particular machines. The operations performed on an engine lathe to make the part shown in Figure 2-7 are shown. Note that the details of speed, feed, and depth of cut are specified (actually, recommended, because the machinist may change them). Over time, many different people may plan the same part; therefore, there can be many different process sequences and many different routes through the factory for the same or similar parts. Process Planning for the Job Shop. The first step in planning is to determine the basic job requirements that must be satisfied. These are usually determined by analysis of the drawings and the job orders. They involve consideration and determination of the following: 1. Size and shape of the geometric components of the workpiece. 2. Tolerances, as applied by the designer. 3. Material from which the part is to be made. 4. Properties of material being machined (hardness). 5. Number of pieces to be produced (see the section in this chapter on quantity versus process and case studies on economic analysis). 6. Machine tools available for this workpiece. Such an analysis for the threaded shaft shown in Figure 2-7 would be as follows: 1. a. Two concentric and adjacent cylinders having diameters of 0.877/0.873 and 0.501/ 0.499, respectively, and lengths of 2 in. and 11/2 in. b. Three parallel plain surfaces forming the ends of the cylinders. c. A 45 1=8 -in. bevel on the outer end of the 7=8 -in. cylinder. d. A 7=8 -in. NF-2 thread cut the entire length of the 7=8 -in. cylinder.

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CHAPTER 2

Manufacturing Systems Design

Personal computer processor flow chart

Cables sockets

PC comp

Video monitor

Video monitor

Processor unit

Keyboard unit

Processor

Sub-assembly level

Cable Keyboard

Memory board (4 req.)

Box casing

Arithmetic board

Switch (4 req.)

S S S S

Boards level

RAM RAM RAM

Ram chip (4 req.)

Switch

Microprocessor

Board

Ram chip (2 req.)

RAM

Switch

Bill of materials for computer Item

Low-level code

Quantity required for one unit

Arithmetic board Box casing Ram chip Keyboard unit Memory board Processor unit Ram chip Switch Video unit Board type X Board type Y Microprocessor

2 2 3 1 2 1 3 3 1 3 3 3

Assembled 1 16 1 Assembled Assembled 2 9 1 4 1 1

FIGURE 2-5

Board

Chip level

Arithmetic

Board Memory card; assemble 4 RAM chips and 1 switch onto card Fabricate chips and boards

C C

S

C C

Final assembly The video unit and the keyboard unit have been preassembled. Connect the sockets on the end of their cables to the corresponding plugs at the rear of the processor unit. Processor subassembly Assemble one switch(s) to the inside of each of the 4 plug connections at the back of the box. Then fit 4 memory cards boards into the 4 identical rows of connectors. Finally, fit the arithmetic cards into the front connector row.

C

The process flow chart and the bill of materials (BOM) for a personal computer. See Chapter 34.

2. The tightest tolerance is 0.002 in., and the angular tolerance on the bevel is 1 . 3. The material is AISI1340 cold-rolled steel, BHN 200. 4. The job order calls for 25 parts. Now the engineer can draw a number of conclusions regarding the processing of the part. First, because concentric, external, cylindrical surfaces are involved, turning operations are required and the piece should be made on some type of lathe. Second, because 25 pieces are to be made, the use of an engine lathe or a computer numerical control (CNC) lathe would be preferred over an automatic screw machine where the setup time will likely be too long to justify this small a lot size. As the company’s numerical control (NC) lathe is in use, an engine lathe will be used. Third, because the maximum required diameter is approximately 7=8 in., 1-in.-diameter cold-rolled stock will be satisfactory; it will provide about 1=16 in. of material for rough and finish turning of the large diameter. From this information, the operations sheet(s) for a particular engine lathe is (are) prepared. The engineer prepares the operations sheet shown in Figure 2-7 listing, in sequence, the operations required for machining the threaded shaft shown in the part drawing. A single operation sheet lists the operations that are done in sequence on a single machine. This sheet is for an engine lathe (see Chapter 22). Operations sheets vary greatly as to details. The simpler types often list only the required operations and the machines to be used. Speeds and feeds may be left to the discretion of the operator, particularly when skilled workers and small quantities are involved. However, it is common practice for complete details to be given regarding

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SECTION 2.4

.000 0.249 .002

3 16

Classification of Manufacturing Systems

37

1 16 D

5 32

.000 0.125 .003

R 3 16

5 32

21 32 3 1 32

Punch Part drawing

Matl. – 0.250 dia. AISI 1040 H.T. to 50 R.C. on 0.249 dia.

DARIC INDUSTRIES ROUTING SHEET

FIGURE 2-6 Part drawing for a punch used in a progressive die set (above) with the route sheet (traveler) for making the punch. The route sheet is used in the job shop (layout in Figure 2-4) to tell the material handler (forklift truck driver) where to take the containers of parts after each operation is completed. The manufacturing engineer designs the process plan or sequence of operations to make the product.

NAME OF PART

Punch

PART NO.

2

QUANTITY

1,000

MATERIAL

SAE 1040

OPERATION NUMBER

DESCRIPTION OF OPERATION

1

5 Turn, 32 , 0.125, and 0.249 diameters

2

3 Cut off to 132 length

3

Mill

4

1 Drill 16 hole

5

Heat treat. 1,700° F for 30 minutes, oil quench

6

Grind (cutting edges)

7

Check hardness

3 16

radius

EQUIPMENT OR MACHINES See Fig 41-4

J & L turret lathe 12

"

#1 Milwaukee

TOOLING

#642 box tools

#6 cutoff in cross turret

Special jaws in vise 3 16 form cutter 4 D

Turret Drill 4

Atmosphere furnace

Surface grinder 3

3 16

end radius on wheel

Rockwell tester

tools, speeds, and often the time allowed for completing each operation. Such data are necessary if the work is to be done on NC machines, and experience has shown that these preplanning steps are advantageous when ordinary machine tools are used. The selection of speeds and feeds required to manufacture the part is discussed in Chapters 20–24 and will depend on many factors such as tool material, workpiece

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CHAPTER 2

Manufacturing Systems Design 7 8

45° 1°

– 14 NF–2 thread BHN 艑 200 0.501 0.499

1 8

0.877 0.873

1

12 1

32 Matl. AISI 1340 Medium carbon steel BHN 艑 200 Threaded shaft made in quantities of 25 units.

DEBLAKOS INDUSTRIES

OPER. NO.

OPERATION SHEET

Threaded Shaft 1340 Cold Rolled Steel

PART NAME:

NAME OF OPERATION

MACH. TOOL

Part No. 7358-267-10 CUTTING SPEED ft/min

rpm

FEED ipr

120

458

Hand

750

Hand

CUTTING TOOL

10

Face end of bar

Engine Lathe

20

Center drill end

"

Combination center drill

30

9 Cut off to 316 length

"

Parting tool

120

458

Hand

40

Face to length

"

RH facing tool (small radius point)

120

458

Hand

50

Center drill end

"

Combination center drill

750

Hand

60

Place between centers, turn .501 diameter, .499 and face shoulder

"

RH turning tool (small radius point)

70

Remove and replace end .877 for end and turn .873 diameter

"

80

Produce 45°-chamfer

"

90

Cut

7 8

- 14 NF-2 thread

Remove burrs and sharp edges

To prevent chattering, keep overhang of work and tool at a minimum and feed steadily. Use lubricant. (R) 18 max. (F) .005

(R) 458 (F) 611

(R) .0089 (R) .081 (3) (F) .0029 (F) .007

RH tools (R) (small radius point) (F) Round nose tool

120 160

(R) 458 (F) 611

(R) .0089 (F) .0029

(R) .057 (F) .005

RH round nose tool

120

458

Hand

(R) 18 max. (F) .005

Threading tool

"

Hand file

60

208

REMARKS

Use 3-jaw Universal chuck

160

"

120

DEPTH OF CUT Inches

(R) .004 (F) .001

Before replacing part in 3-jaw chuck, scribe a line 1 marking the 3 2 inch length.

(1) Swivel compound rest to 30 degrees. (2) Set tool with thread gage. (3) When tool touches outside diamter of work set cross slide to zero. (4) Depth of cut for roughing = .004. (5) Engage thread dial indicator on any line. (6) Depth of cut for finishing = .001 Use compound rest.

Date

FIGURE 2-7 Part drawing (above) and operations sheet, which provides details of the recommended manufacturing process steps.

material, and depth of cut. Tables of machining data will give suggested values for the turning and facing operations for either high-speed-steel or carbide tools. The tables are segregated by workpiece material (medium-carbon-alloy steels, wrought or cold worked) and then by process—in this case, turning. For these materials, additional tables for drilling and threading would have to be referenced. The depth of cut dictates

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SECTION 2.4

Classification of Manufacturing Systems

39

the speed and feed selection. The depth of cut for a roughing pass for operation 60 was 0.081 in., and the finish pass was 0.007 in. Therefore, for operation 60, three roughing cuts and one finishing cut are used. Looking ahead to Chapter 22, could you estimate or determine the cutting time for one of these roughing cuts? While the operation sheet does not specifically say so, high-speed-steel tools are being used throughout. If the job were being done on an NC lathe, it is likely that all the cutting tools would be carbides, which would change all the cutting parameters. Notice also that the job as described for the engine lathe required two setups, a threejaw chuck to get the part to length and produce the centers and a between-centers setup to complete the part. It was necessary to stop the lathe to invert the part. Do you think this part could be manufactured efficiently in an NC lathe? How would you orient the part in the NC lathe? (See Chapter 40 for discussion of NC lathes.) Speed and feed recommendations are often computerized to be compatible with computer-aided manufacturing and computer-aided process planning systems, but the manufacturing engineer is still responsible for the final process plan. As noted earlier, the cutting time, as computed from the machining parameters, represents only 30 to 40% of the total time needed to complete the part. Referring again to Figure 2-6, the operator will need to pick up the part after it is cut off at operation 30, stop the lathe, open up the chuck, take out the piece of metal in the chuck, scribe a line on the part marking the desired 31/2-in. length, place the part back in the chuck, change the cutoff tool to a facing tool, adjust the facing tool to the right height, and move the tool to the proper position for the desired facing cut in operation 40. All these operations take time, and someone must estimate how much time is required for such noncutting operations if an accurate estimate of total time to make the part is to be obtained. When an operation is machine controlled, as in making a lathe cut of a certain length with power feed, the required time can be determined by simple mathematics. For example, if a cut is 10 in. in length and the turning speed is 200 rpm, with a feed of 0.005 in. per revolution, the cutting time required will be 10 minutes. See Chapter 22 for details of this calculation. Procedures are available for determining the time required for people-controlled machining elements, such as moving the carriage of a lathe by hand, back from the end of one cut to the starting point of a following cut. Such determinations of time estimates are generally considered the job of the industrial engineer, and space does not permit us to cover this material, but the techniques are well established. Actual time studies, accumulated data from past operations, or some type of motion-time data, such as MTM (methods-time-measurement), can be employed for estimating such times. Each can provide accurate results that can be used for establishing standard times for use in planning. Various handbooks and books on machine shop estimating contain tables of average times for a wide variety of elemental operations for use in estimating and setting standards. However, such data should be used with great caution, even for planning purposes, and they should never be used as a basis of wage payment. The conditions under which they were obtained may have been very different from those for which a standard is being set. Quantity versus Process and Material Alternatives. Most processes are not equally suitable and economical for producing a wide range of quantities for a given product. Consequently, the quantity to be produced should be considered, and the product design should be adjusted to the process that actually is to be used before the design is finalized. As an example, consider the part shown in Figure 2-8. Assume that, functionally, a brass alloy, a heat-treated aluminum alloy, or stainless steel would be suitable materials. What material and process would be most economical if 10, 100, or 1000 parts were to be made? If only 10 parts were to be made, lathe turning, milling the flat, milling the slot, and drilling and tapping the holes would be very economical. The part could be machined out of bar stock. Casting would require the making of a pattern, which would be about as costly to produce as the part itself. It is likely that a suitable piece of stainless steel, brass, or heat-treated aluminum alloy would be available in the correct diameter and finish so that the largest diameter would not need any additional machining. Brass may

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CHAPTER 2

Manufacturing Systems Design Part drawing for a family of drive pinions, A, C, C.D.

Part no. Part name Quantity Lot size Material Diameter Length

8060 Drive pinion 1000 200 430F stainless cold finished 1.780 ± .003 12 ft

1.375 (44.07)

A Slot 0.030 (0.77)

B

1.100 4.75 (120.7)

16 µin. (400 nm) finish 0.50 (12.7)

1.750 (44.45)

A

B

18.750 (A) 14.750 (B) 10.750 (C) 8.750 (D)

0.50

0.375 (9.5) Tap 38 –16,

1 2

(13) deep

Four places

3D view of part 1.250 diameter View A-A Material: 430F stainless steel cold-finished, annealed

View B-B

8060 Part no. _____________

1000 Ordering quantity _____________

430F Stainless Material __________________

Drive Pinion Part name ___________

200 Lot requirement ______________

steel, 1.780 ± 0.003 in. _________________________ cold - finished 12-ft _________________________ bars = 1000 pieces _________________________ $ 22.47 Unit material cost ___________

Workstation

Operation no.

Description of operations (list tools and gages)

Setup hour

Cycle hour/ 100 units

Unit estimate

Labor rate

Labor + overhead rate

Cost for labor + overhead rate

Engine lathe # 137

10

Face A-A end 0.05 center drill. A-A and rough turn has cast off to length 18.750

3.2

10.067

0.117

18.35

1.70

3.65

Engine lathe # 227

11

Center drill B-B end finish turn 1.100 turn 1.735 dirn

3.2

8.067

0.095

18.35

1.70

2.96

Vertical drill # 357

20

End mill 0.50 stat with 1/2 HSS and mill (collet future)

1.8

7.850

0.088

19.65

1.85

3.20

Horizontal drill # 469

30

Slab mill 4.75 x 3/8 (resting vise HSS tool)

1.3

1.500

0.022

19.65

1.80

0.78

NC rarret drill press # 474

40

Drill 3/8 hcles-4x tap 3/8-16 (collet fixture)

0.66

5.245

0.056

17.40

2.15

2.10

Cylindrical Grinder # 67

50

Grind shaft to 16gm – 1.10

1.0

10.067

0.110

19.65

1.80

3.89

$ 16.58 + 22.47 = 39.051

Estimated mig cost per unit

FIGURE 2-8 (Top) Part drawing with 3D view of part. (Bottom) Process planning sheet based on manufacturing the pinion in a job shop.

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be slightly more expensive than the other materials in low volumes. However, brass material with sawing, turning, milling, and drilling most likely would be the best combination. For 10 parts, the excess cost of brass over stainless steel would not be great, and this combination would require no special consideration on the part of the designer. For a quantity of 100 parts, an effort to minimize machining costs may be worthwhile. Forging discussed in Chapter 16 might be the most economical process, followed by machining. Stainless steel would be cheaper than any of the other permissible materials. Although the design requirements for forging this simple shape would be minimal, the designer would want to consider them, particularly as to whether the part should be forged, as this will reduce machining time. (Can stainless steel be forged?) For 1000 parts, entirely different solutions become feasible. The use of an aluminum extrusion, with the individual units being sawed off, might be the most economical solution, assuming that the lead time required obtaining a special extrusion die was not a detriment. Aluminum can be cut at much higher speeds than stainless steel. What material would you recommend for the part now? How many feet of extrusion would be required, including sawing allowance? How much should be spent on the die so that the per-piece cost would not be great and probably would be more than offset by the savings in machining costs? If extrusion is used, the designer should make sure that any tolerances specified are well within commercial extrusion tolerances. So what did the manufacturing engineer (MfE) decide to do for this part? Figure 2-8 shows the process plan developed for this part using stainless steel bars that were purchased in 12-ft lengths with diameters of 1.70 0.003 in. Follow the processing steps (operations 10–50) to understand how the pinion was made. Notice that the process plan is used to obtain a cost estimate for this part. The total cost estimate takes into account all of the one-time or fixed costs associated with the component plus the labor and material costs (variable costs) required to make the component. Estimating the cost of making a product prior to ever making the first one is usually the job of the industrial engineer (IE) or MfE, and there are many good reference texts available for this work. This simple example clearly illustrates how quantity can interact with material and process selection and how the selection of the process may require special considerations and design revisions on the part of the designer. Obviously, if the dimensional tolerances were changed, entirely different solutions might result. When more complex products are involved, these relations become more complicated, but they also are usually more important and require detailed consideration by the designer. The production job shop becomes extremely difficult to manage as it grows, resulting in long product throughput times and very large in-process inventory levels. In the job shop, parts typically spend 95% of the time waiting (delay) or being transported and only 5% of the time on the machine. Thus, the time spent actually adding value may only be 2 or 3% of the total time available. The job shop typically builds large volumes of products but still builds lots or batches, usually medium-sized lots of 50 to 200 units. The lots may be produced only once, or they may be produced at regular intervals. The purpose of batch production is often to satisfy continuous customer demand for an item. This system usually operates in the following manner: Because the production rate can exceed the customer demand rate, the shop builds an inventory of the item, then changes over the machines to produce other products to fill other orders. This involves tearing down the setups on many machines and resetting them for new products. When the stock of the first item becomes depleted, production is repeated to build the inventory again. See Chapter 44 on the Web for a discussion on inventory control. Some machine tools are designed for higher production rates. For example, automatic lathes capable of holding many cutting tools can have shorter processing times than engine lathes. The machine tools are often equipped with specially designed workholding devices, jigs, and fixtures, which increase process output rate, precision, accuracy, and repeatability. Industrial equipment, furniture, textbooks, and components for many assembled consumer products (household appliances, lawn mowers, and so on) are made in production job shops. Such systems are called machine shops, foundries, plastic-molding

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factories, and pressworking shops. Because the job shop has for years been the dominant factory design, most service companies are also job shops, organized functionally. Does your school have separate engineering or technology departments? Are you routed to different departments for processing? This is how the job shop works.

FLOW SHOP The flow shop has a product-oriented layout composed mainly of flow lines. When the volume gets very large, especially in an assembly line, this is called mass production, shown schematically in Figure 2-9. This kind of system can have (very) high production rates. Specialized equipment, dedicated to the manufacture of a particular product, is used. The entire plant may be designed exclusively to produce the particular product or family of products, using special-purpose rather than general-purpose equipment. The investment in specialized machines and specialized tooling is high. Many production skills are transferred from the operator to the machines so that the manual labor skill level in a flow shop tends to be lower than in a production job shop. Items are made to ‘‘flow’’ through a sequence of operations by material-handling devices (conveyors, moving belts, and transfer devices).The items move through the operations one at a time. The time the item spends at each station or location is fixed or equal (balanced). Figure 2-10 shows a layout of an assembly line or flow line requiring line balancing. Figure 2-11 shows an example of an automated transfer machine for the assembly of engine blocks at the rate of 100 per hour. All the machines are specially designed and built to perform specific tasks and are not capable of making any other products. Consequently, to be economical, such machines must be operated for considerable periods of time to spread the cost of the initial investment over many units. These machines and systems, although highly efficient, can be utilized only to make products in very large volume, hence the term mass production. Changes in product design are often avoided or delayed because it would be too costly to change the process or scrap the machines. However, as we have already noted, products manufactured to meet the demands of free-economy, mass-consumption markets need to incorporate changes in design for improved product performance as well as style changes. Therefore, hard automation systems need to be as flexible as possible while retaining the ability to mass-produce. The incorporation of programmable logic controllers (PLCs) and feedback control devices have made these machines more flexible. Modern PLCs have the functional sophistication to perform virtually any control task. These devices are rugged, reliable, easy to program, and economically competitive with alternative control devices. PLCs have replaced conventional hard-wired relay panels in many applications because they are easy to reprogram. Relay panels have the advantage of being well understood by maintenance people and are invulnerable to electronic noise, but construction time is long and tedious. PLCs allow for mathematical algorithms to be included in the closedloop control system and are being widely used for single-axis, point-to-point control as typically required in straight-line machining, robot handling, and robot-assembly applications. They do not at this time challenge the computer numerical controls used on multi-axis contouring machines. However, PLCs are used for monitoring temperature, pressure, and voltage on such machines. PLCs are used on transfer lines to handle complex material movement problems, gaging, automatic tool setting, online tool wear compensation, and automatic inspection, giving these systems flexibility that they never had before. The transfer line has been combined with CNC machines to form flexible manufacturing systems (FMSs).These systems are discussed in Chapters 39 and 40. In the flow-line manufacturing system, the processing and assembly facilities are arranged in accordance with the product’s sequence of operations; see Figure 2-11. Workstations or machines are arranged in line with only one workstation of a type, except where duplicates are needed for balancing the time products take at each station. The line is organized by the processing sequence needed to make a single product or a regular mix of products. A hybrid form of the flow line produces batches of products moving through clusters of workstations or processes organized by product flow. In most cases, the setup times to change from one product to another are long and often complicated.

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43

Welding, body fitting

Press working

Final assembly line

Cer Bodies Steel fragments Bale

Cupola furnace

Baling machine

Painting Machining Foundry Mechanical components fitting Casting

Briquette plant

Chips

Briquette

Assembly

From forging line

Line-off Electronic parts

Moldings

Lathe

Paint

Assembly

Saw

Lathe

Paint

Assembly

Saw

Mill

Grinder

Plating

Receiving department The job shop is a functionally designed factory with line processes grouped in departments. Parts are routed from process to process in tote boxes.

Storage

Subassembly Lines (See Figure 2-10)

Receiving

C02

Lathes

Grinders

Milling machines

Plating and painting

Heat treatment

A B C

Saws

Assembly

FIGURE 2-9 The mass production system produces large volumes at low unit cost, and personifies the economy of scale where large fixed costs are spread out over many units.

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FLOW SHOP STA 4

STA 3

STA 2

STA 1

Raw Material Moving assembly line Rework line for defects

Work is divided equally into the stations

Product STA 10

Subassembly feeder lines

Finished Product

FIGURE 2-10

Schematic of a flow shop manufacturing system, which requires line balancing.

Various approaches and techniques have been used to develop machine tools that would be highly effective in large-scale manufacturing. Their effectiveness was closely related to the degree to which the design of the products was standardized and the time over which no changes in the design were permitted. If a part or product is highly standardized and will be manufactured in large quantities, a machine that will produce the parts with a minimum of skilled labor can be developed. A completely tooled automatic screw machine is a good example for small parts. Most factories are mixtures of the job shop with flow lines. Obviously, the demand for products can precipitate a shift from batch to high-volume production, and much of the production from these plants is consumed by that steady demand. Subassembly lines and final assembly lines are further extensions of the flow line, but the latter are usually much more labor intensive.

PROJECT SHOP In the typical project shop, or project manufacturing system, a product must remain in a fixed position or location during manufacturing because of its size and/or weight. The materials, machines, and people used in fabrication are brought to the site. Prior to the development of the flow shop, cars were assembled in this way. Today, large products like locomotives, large machine tools, large aircraft, and large ships use fixed-position layout. Obviously, fixed-position fabrication is also used in construction jobs (buildings, bridges, and dams); see Figure 2-12. As with the fixed-position layout, the product is large, and the construction equipment and workers must be moved to it. When the job is completed, the equipment is removed from the construction site. The project shop invariably has job shop/flow shop elements manufacturing all the

FIGURE 2-11

Major intermediate assembly parts

Oil pump

Water pump

Oil pan 2

Baffle plate

Oil pan 1

Oil strainer

Rear retainer

Drive plate

Transfer line for assembly of an engine is an example of a very automated flow line. (From Toyota Technical Review, Vol. 44, no. 1, 1994)

Crank cap

Thrust bearing

Crankshaft

Main bearing

Connecting rod bearing

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FIGURE 2-12 The project shop involves large stationary assemblies (projects), where components are usually fabricated elsewhere and transported to the site.

subassemblies and components for the large complex project and thus has a functionalized production system. The project shop often produces one-of-a-kind products with very low production rates—from one per day to one per year. The work is scheduled using project management techniques like the critical path method (CPM) or program evaluation and review technique (PERT). These methods use precedence diagrams that show the sequence of manufacturing and assembly events or steps and the relationship (precedence) between the steps. There is always a path through the diagram that consumes the most time, called the critical path. If any of the tasks on the longest path are delayed, it is likely the whole project will be delayed. The process engineer must know the relationships (the order) of the processes and be able to estimate the times needed to perform each task. All this (and more) is the responsibility of project management. Project shop manufacturing is labor intensive, with projects typically costing in the millions of dollars.

CONTINUOUS PROCESS In the continuous process, the project physically flows. Oil refineries, chemical processing plants, and food-processing operations are examples. This system is sometimes called flow production, when the manufacture of either complex single parts (such as a canning operation or bottling operation) or assembled products (such as television sets) is described. However, these are not continuous processes but rather high-volume flow lines. In continuous processes, the products really do flow because they are liquids, gases, or powders. Continuous processes are the most efficient but least flexible kinds of manufacturing systems. They usually have the leanest, simplest production systems because these manufacturing systems designs are the easiest to control, having the least work-in-process (WIP). However, these manufacturing systems usually involve complex chemical reactions, and thus a special kind of manufacturing engineer (MfE), called the chemical engineer (ChE), is usually assigned the task of designing, building, and running the manufacturing system.

LEAN MANUFACTURING SYSTEM The lean shop, or the lean manufacturing system, employs U-shaped cells or parallel rows to manufacture components. The entire mass production factory is reconfigured. The final assembly lines are converted to mixed-model, final assembly so that the demand for subassemblies and components is leveled, making the daily demand for components the same every day. Subassembly lines are also reconfigured into volumeflexible, single-piece flow cells; see Figure 2-13. The restructuring of the job shop is difficult; the concept is shown in Figure 2-14. In the cells, the machine tools are upgraded to be single-cycle automatics so parts can be loaded into the machine and the machining cycle started. The operator moves off to the next machine in the sequence, carrying the part from the previous process with him, so

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47

Lean Production

Mass Production

Final assembly

Line

Inventory

Station #38

Out Steering Gear SubA

In K-Link

Rack & Pinion Out SubA In

Parts storage Subassembly cells

K-link

Subassembly

erin g K-l ink

Flow Shop

Ste

Large batches

Mixed Model Final Assembly

Final assembly

Subassembly

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Key M Milling Machine D Driving Machine S Saw G Grinder I Inspection TL Transfer Line services the manned cell

In

Mfg Call with transfer line

Out

S

I Worker 1

Components Job shop Mill

Drill

Lathe

Grind

Rack cell

L Decouptor M2

M1

Four Station Transfer Line

Raw materials

G

Components in Mfg Cells “U” Shaped, one piece flow

M3

TL

M4

D Worker 2

M

M

FIGURE 2-13 The manufacturing system design on the left, called ‘‘mass production,’’ produces large volumes at low unit cost. It can be restructured into a lean manufacturing system design to achieve single-piece flow, on the requirements of a LCMS design. On the right is a manufacturing cell with a small transfer serving one station in the cell.

the lean shop employs standing, walking multiprocess workers. System design requirements are used to design the cells. In a linked-cell system, the key proprietary aspects are the U-shaped manufacturing and assembly cells. In the cells, the axiomatic design concept of decoupling is employed to separate (decouple) the processing times for individual machines from the cycle time for the cell, enabling the lead time for a batch of parts to be independent of the processing times for individual machines. This effect of this is to take all the variation out of the supply chain lead times, so scheduling of the supply system becomes so simple that the supply chain can be operated by a pull system of production control (kanban). Using kanban, the inventory levels can be dropped, which decreases in turn, the throughput time for the manufacturing system. Workers in the cells are also multifunctional; each worker can operate more than one kind of process and also perform inspection and machine maintenance duties according to a standard work pattern. Cells eliminate the job shop concept of one person–one machine and thereby greatly increase worker productivity and utilization. The restriction of the cell to a family of parts makes reduction of setup in the cell possible. The general approach to setup reduction is discussed in Chapter 27 as part of the cell design strategy. In some cells decouplers are placed between the processes, operations, or machines to connect the movement of parts between operators. A decoupler physically holds one unit, decouples the variability in processing time between the machines, and enables the separation of the worker from the machine. Decouplers can provide flexibility, part transportation, inspection for defect prevention (poka-yoke) and quality control, and process delay for the manufacturing cell. Here is how an inspection decoupler might work. The part is removed from a process and placed in a decoupler. The decoupler inspects the part for a critical dimension

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L

L

L

L

L

Lathe dept

M

L

L IN

M

M

Milling dept

M

D

D

G

OUT

Mfg cell 1 M

D

D

M

D

M

Drill dept

D

L

M

M

D

D

G

D

D

Mfg cell 2

D G

Grinding dept

G

M

L

G

D

G Mfg cell 3

Finished good storage or inventory Functional layout of the job shop

Cellular layout of the L-CMS

FIGURE 2-14 The job shop portion of the plant requires a systems level conversion to reconfigure it into manufacturing cells, operated by standing and walking workers.

and turns on a light if a bad (oversized) part is detected. A process delay decoupler delays the part movement to allow the part to cool down, heat up, cure, or whatever is necessary, for a period of time greater than the cycle time for the cell. Decouplers are vital parts of manned and unmanned cells and will be discussed further in Chapter 29. The system design ensures that the right mix and quantity of parts are made according to an averaged customer demand. This system is robust in that it ensures that the right quantity and mix are made, even though there is variation or disturbance in the manufacturing process or other operations in the manufacturing system. If process variation or disturbances to the system occur from outside the manufacturing system’s

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Supplier in Tenn. U

Mfg cell

Pulls

Final Assembly

Subassembly cell

Supplier in Alabhama U

FIGURE 2-15 The linked-cell manufacturing pulls the goods from the suppliers through the assembly to the external customer.

Summary of Factory Designs

49

External customer Production leveling boxes (Hyjunka box)

Plant in Indiana

Customer demand information

Plant’s manufacturing system boundry

boundary (i.e., incoming parts are defective), the system design is robust enough to handle these disturbances due to its feedback control design. A typical linked-cell system is shown in Figure 2-15 with the customer demand information illustrated by dashed lines. A specific level of inventory is established after each producing cell and is held in the kanban links. This set-point level is sometimes called the standard work-in-process (SWIP) inventory. The SWIP defines the minimum inventory necessary for the system to produce the right quantity and mix to the external customer and to be able to compensate for disturbances and variation to the system.

& 2.5 SUMMARY OF FACTORY DESIGNS Let us summarize the characteristics of the lean shop versus the other basic factory designs; see Table 2-2. The cell makes parts one at a time in a flexible design. Cell capacity (the cycle time) can be altered quickly to respond to changes in customer demand by increasing or decreasing operators. The cycle time does not depend on the machining times. Families of parts with similar designs, flexible workholding devices, and tool changers in programmable machines allow rapid changeover from one component to another. Rapid change are over means that quick or one-touch setup is employed, often like flipping a light switch. This leads to small lot production. Significant inventory reductions between the cells are possible and the inventory level can be directly controlled by the user. Quality is controlled within the cell, and the equipment within the cell is

TABLE 2-2

Characteristics of Basic Manufacturing Systems

Characteristics Types of machines

Job Shop

Flow Shop

Project Shop

Continuous Process

Lean Shop

Flexible

Single purpose

General purpose

Single function

Mobile, manual

Specialized, high technology

Simple, customized

General purpose

Automation

Single-cycle automatic

Design of processes

Functional or process Process

Product flow layout

Project or fixed-position layout

Product

Linked U-shaped cells

Setup time

Long, variable frequent

Long and complex

Variable, every job different

Skill level varies

Multifunctional, multipurpose

Workers

Single function, highly skilled: one worker–one machine

One function, lower-skilled: one worker–one machine

Specialized, highly skilled

Skill level varies

Multifunctional, multiprocess

Inventories (WIP)

Large inventory to provide for large variety

Large to provide buffer storage

Variable, usually large

Very small

Small

Lot sizes

Small to medium

Large lot

Small lot

Very large

Small

Manufacturing lead time

Long, variable

Short, constant time

Long, variable lead

Very fast, constant

Short, constant

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maintained routinely by the workers. The utilization of individual machines is not optimized, but the cell as a whole operates to exactly meet the pace of the customer’s demand. These characteristics of the lean shop are different from that of the other manufacturing systems because it has a different design. In the next section, we will review a bit of the history of factory designs.

THE EVOLUTION OF THE FIRST FACTORY There was a time when there were no factories. From 1700 to 1850, most manufacturing was performed by skilled craftsmen in home-based workshops, what historians call cottage industry. These artisans were gunsmiths, blacksmiths, toolmakers, silversmiths, and so on. The machines they had for forming and cutting materials were manually powered. Amber & Amber (1962) in their yardstick for automation pointed out that the first step in mechanizing and automating the factory was to power the machines. The need to power the machine tools efficiently created the need to gather the machine tools into one location where the power source was concentrated. By this time, the steel and cast iron to build machine tools were becoming economically available. Railroads were built to transport goods. These were the technologies that influenced the factory design and are now called enabling technologies; see Table 2-3. A functional design evolved because of the method needed to drive or power the machines. The first factories were built next to rivers, and the machines were powered by flowing water driving waterwheels, which drove overhead shafts that ran into the factory. See Figure 2-16. Machines of like type were set, all in a line, underneath the appropriate power shaft (i.e., the shaft that turned at the speed needed to drive this type of machine). Huge leather belts were used to take power off the shaft to the machines. Thus, a factory design of collected like processes evolved, and it came to be known as the job shop. So, all the lathes were collected under their own power shaft, likewise all the milling machines and all the drill presses. Later, the waterwheel was replaced by a steam engine, which allowed the factory to be built somewhere other than next to a river. Eventually, large electric motors were used, and later individual electric motors for each machine replaced the large steam and electric engines. Nevertheless, the job shop design was replicated, and the functional design held. Many early factories were in the northeastern part of the United States and were manufacturing guns. Filing jigs were used by laborers to create interchangeable parts. In time, this design became

TABLE 2-3

The Evolution of Manufacturing System Designsa First Factory Revolution

Second Factory Revolution

Third Factory Revolution

Fourth Factory Revolution

Time period

1840–1910

1910–1970

1960–2010

2000–???

Manufacturing system design

Job shop

Flow shop

Lean shop

Global Information Technology

Layout

Functional layout

Product layout

Linked-cells to Mixed Model Final Assembly line

Integrated supply chain

Enabling technologies

Power for machines, steel production, and railroad for transportation

U-shaped cells, kanban, and rapid die exchange

Historical company name

Whitney, Colt, and Remington

Moving final assembly line, standardization leading to true interchangeability, and automatic material handling Singer, Ford

Toyota Motor Companyb, Hewlett-Packard, Omark, Harley Davidson, Honda

Virtual reality/simulation, 3D design using low-cost, and very high performance computers, digital technologyc Boeing, Lockheed, Electric Boat, and Mercedes

Economics

Economy of collected technology

Economy of scale, high volume > low unit cost

Economy of scope, wide variety at low unit cost

Economy of global manufacturing

a

MSD (manufacturing system design), including machine tools, material handling equipment, tooling and people System developed by Taiichi Ohno, who called it the Toyota Production System (TPS) c Single-source digital product definition and 3D design, a manufacturing system simulated using virtual reality b

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51

Water sluice

Early factory

Shafts

Milling machines

Grinders River

Waterwheel

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Main power shaft

Lathes

Overhead shafts in plants

Gears in gear house Return

FIGURE 2-16 Early factories used water power to drive the machines.

known as the American Armory System. People from all over the world came to America to observe the American Armory System, and this functionally designed system was duplicated around the factory world as a key driver in the first factory revolution.

FACTORY REVOLUTIONS EVOLVE Revolutions do not happen overnight but require many simultaneous events. An evolution in thought, philosophy, and the mindset of people in the factory, as well as the redesign of the factory, are needed to revolutionize the factory setup. Even though manufacturing is the leading wealth producer in the world, historians have often neglected the technical aspects of history. The model used here is based on Thomas Kuhn’s book, The Structure of Scientific Revolutions. He said there are three phases in any revolution: the crisis stage, the revolution stage, and the normal science stage, as shown in Figure 2-17. Some people have portrayed this last stage as the diffusion of science. The first factory revolution can be considered a crisis due to the problems associated with craft production such as producing products in which filing was used to fit products together. This crisis was highlighted in the United States by a congressional mandate to have interchangeable parts for muskets in the field in the late 18th century. Eli Whitney received a contract from Congress to produce 4000 muskets in 11/2 years with interchangeable parts. In wartime there was a need for part interchangeability so that particular musket parts could be replaced in the field, instead of replacing the entire musket. The goal was to reduce replacement time and cost. At that time, muskets consisted of parts that were all craft-made, filed-to-fit products.

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Evolution of the First Factory Revolution 37 Years of EVOLUTION

Revolutions Don't Happen Overnight 1785

Crisis 24 yrs.

Revolution 13 yrs.

1792 1798 1801 1809 1811 1812 1815 1818 1819 1819 1822

Normal science years

FIGURE 2-17 Evolution of the first factory revolution. (David Cochran)

1825 1834

1839 1845 1860

Thomas Jefferson proposes that Congress mandate interchangeable parts for all musket contracts. Eli Whitney invents cotton gin. Eli Whitney contract for 4000 muskets in 1.5 years. Eli Whitney demonstrates interchangeability to Congress. Eli Whitney delivers, 8.5 years late, non-interchangeable parts. John Hall patents breech-loading rifle. Roswell Lee becomes superintendent of Springfield Armory. Congress orders Ordnance Dept. to require interchangeable parts. Blanchard invents trip hammer for making gun barrels. Blanchard invents lathe for making gunstocks. Lee introduces inspection gages; Springfield Armory. John Hall announces success at Harpers Ferry using system of gages to measure parts. Government report praises effort. Eli Whitney dies. Simeon North at Middletown, CT, adopts Hall's gages, delivers rifles (parts) interchangeable with Harpers Ferry production. Samuel Colt and Eli Whitney Jr. revolver contract. The Armory Practice spreads to private contractors. Pocket micrometer invented.

Part interchangeability was a manufacturing challenge for the armory system from the beginning, and the ultimate success in achieving part interchangeability was the result of John Hall’s work in 1822. His innovation was the ability to measure the product. Instead of attempting to produce parts to a master part, as was the practice at the time, a system of gages was introduced to measure the part. As a result, the engineers and designers had to be able to draw the parts with specifications in terms of their geometric dimensions and tolerances. The entire need for drawings, tolerances, and specifications was the result of part measurement to achieve part interchangeability. Many people viewed part interchangeability as adding additional manufacturing cost and concluded that it was unnecessary. However, standard measurements and part interchangeability were key enabling technologies for the development of the moving assembly line during the second factory revolution. After this evolution occurred, the practice of using gages to achieve part interchangeability was then moved to Middletown, Connecticut, from Harper’s Ferry, which had adopted Hall’s system of gages. A gun could then be assembled from parts made from the Harper’s Ferry and Middletown locations. By 1845, this armory practice spread to private contractors. Soon, the ability to do gaging became ubiquitous throughout the country. In 1860, the pocket micrometer was invented, and the science of measurement spread throughout the world. As a system design, the job shop still exists. Modern-day versions of the job shop produce large volumes of goods in batches or lots of 50 to 200 pieces. Processes are still grouped functionally but the walls are removed. Between the machines, filling the aisles, are tote boxes filled with components in various stages of completion. While it may be difficult to actually count the inventory, these designs have thousands of parts on the floor at any one time. However, the production system for the early job shop was minimal and most decisions about how to make the products were made by the operators on the shop floor.

EVOLUTION OF THE SECOND MSD—THE FLOW SHOP The second factory revolution led to the development of mass production, shown in Figure 2-18. Henry Ford defined the concept of economies of scale with mass production.

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53

Evolution of the Second Factory Revolution 50 Years of REVOLUTION

Revolutions Don't Happen Overnight 1878

1893 Crisis 35 yrs.

Revolution 15 yrs. Normal science years

FIGURE 2-18

1898 1898 1901 1903 1905 1906 1907 1908 1909 1913 1922 1923 1928 1945

Wisconsin sponsors 200-mile race, offering $10,000 for “a cheap and practical substitute for the use of horses and other animals on the highway and farm.” First production gasoline car by Charles & Frank Duryea, only 13 made in three years. “It ran no faster than a man could walk but it did run.” Haynes-Apperson Auto Co. produces one car every 3 weeks for $1500. Winton Motor Carriage Co. 22 single cylinders; 100 in 1899 for $1000. Single-cylinder curved dash Olds – weight 700 pounds, cost $650. 2500 made in 1902; 4000 in 1903; 5508 in 1904; 6500 in 1905. Ford Model A, twin horizontally opposed engine, $750 ea., 1708 in 1904. 25 made/day, Ford Mfg. Co. formed to produce engines/transmissions. Model N outsells Oldsmobile with 8,729, a 4 cylinder at $500. Models N, R ($750) and S ($700) sold 14,887 and 10,202 in 1908. Model T introduced, single cast 4 cylinder, 5 body styles: $825–$1000. 100 produced per day. 17,771 Model T‘s sold. Moving assembly line at Highland Park. 308K in 1914, 501K in 1915 at $440. Over 1 Million model T‘s sold yearly to 1926. 1.82 million produced at average of $300 with more options standard. Chevrolet out-sells Ford and produces 1.2 million vehicles. Beginning of the Third Industrial Revolution

Evolution of the second factory revolution. (David Cochran)

His primary ‘‘mass production’’ tools were the extreme division of labor, the moving assembly line, the shortage chaser to control minimum and maximum stock levels, and the overarching reliance of assembly cycle time predictability with interchangeable parts. Ford is credited with many product design innovations, as well as manufacturing innovations. He developed the process for engine blocks. One reason for the Model T’s success was a single cast engine block instead of four cylinders bolted together. The use of this manufacturing process innovation decreased the car’s weight and increased power. However, the enabling technology for mass production was interchangeable parts based on exact standards of measurement. Ford insisted that every product meet specifications. He was a stickler about keeping all gages calibrated in the factory. In other words, the second factory revolution was founded on the first factory revolution concept of part interchangeability. Flow-line manufacturing began in the 1900s for small items and evolved to the moving assembly line at the Ford Motor Company around 1913. This methodology was developed by Ford production engineers led by Charles Sorenson. Today’s moving assembly line for automobile production has hundreds of stations where the car is assembled. This requires the work at each station to be balanced where tasks at each station take about the same amount of time. This is called line balancing. The moving assembly line makes cars one at a time, in what is now called single piece flow. Just as in the 1800s, people throughout the world came to observe how this system worked, and the new design methodology was again spread around the world. For many companies, a hybrid system evolved, which included a mixture of job shop and flow shop, with the components made in the job shop feeding the assembly line. This design permitted companies to manufacture large volumes of identical products at low unit cost. Mass production relied on the first factory revolution for part interchangeability, while producing products in a fixed cycle time with moving assembly lines. To produce at a fixed cycle time, division of labor was used, and unskilled workers replaced the craftsmen in the factory (see Adam Smith’s Wealth of Nations). With the division of labor, instead of assembling an entire transmission, the workers performed the same small set of tasks on each transmission. As a result, labor turnover in the factory increased dramatically, so Ford introduced the ‘‘five-dollar day,’’ salary for all his workers, an exorbitant amount of money in those days. In fact, the five-dollar day created the economic system that enabled the emergence of the middle class. The workers were able to buy the products they produced in high-volume, mass-production factories.

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Between 1928 and 1945, the Ford factory system diffused to other products. During WWII, airplanes were produced at Ford’s Willow Run Factory using fixed cycle time, moving assembly lines, and interchangeable parts. Planes were moved down an assembly line at the pace of demand. All kinds of tooling and jigs were designed and built so that interchangeable parts could be assembled onto the aircraft in a fixed cycle time. Next came a very automated form of the flow line for machining or assembling complex products like engines for cars in large volumes (200,000 to 400,000 per year). It was called the transfer line. The enabling technology here is repeat cycle automatic machines. This system also required interchangeable parts based on precise standards of measurement (gage blocks). The transfer line is designed for large volumes of identical goods. These systems are very expensive and not very flexible. The parts that feed the flow line were made in the job shop in large lots, held in inventory for long periods of time, then brought to the line where a particular product was being assembled. The line would produce one product over a long run and then switch to another product, which could be made for days or weeks. This mass-production system was in place during WWII and was clearly responsible for producing the military equipment and weapons that enabled the allies to win the war. This massive production machine thrived after WWII, which enabled automobile producers and many other companies to make cars in large volumes using the economy of scale. Just when it appeared that nothing could stop this machine, a new player, the Toyota Motor Company, evolved a new manufacturing system that truly changed the manufacturing world. This system is based on a different system design, which ushered in the third factory revolution, characterized by global companies producing for worldwide markets.

THE THIRD MSD—LEAN PRODUCTION The latest MSD is sometimes called lean production, the Toyota Production System (TPS), just-in-time (JIT) manufacturing, or many other names. A new manufacturing system design brings one or more new companies to the forefront of the industrial world (this time, Toyota). See Figure 2-19 for details of this revolution. Each new design employs some unique enabling technologies, in this case, manufacturing and assembly

Third Factory Revolution at Toyota in Japan (Ohno, 1988) 21 Years of REVOLUTION Crisis 3 yrs.

Revolution 18 yrs.

Diffusion & normal science

FIGURE 2-19

Revolutions Don't Happen Overnight 1945 1948 1949 1950 1950 1953 1955 1955 1958 1961 1962 1962 1962 1965 1966 1971 1971 1981 1990

Need to rebuild wide variety of products in low volume after World War II. Only had six presses, requiring frequent and fast changeover. Withdrawal by subsequent processes. Intermediate warehouses abolished. In-line cells. Horseshoe or U-shaped machine layout replaces job shop. Machining and assembly lines balanced. Supermarket system in machine shop. Assembly and body plants linked. Main plant assembly line production system adopts visual control, line stop, and mixed load. Automation to autonomation. Warehouse withdrawal slips abolished. Andon installed, Motomachi assembly plant. 15-minute main plant setups. Kanban adopted company-wide. Full work control of machines pokayoke. Kanban adopted for ordering outside parts for 100% of supply system; began teaching system to affiliates. First autonomated line Kamigo plant. Main office and Motomachi setups reach 3 minutes. Body indication system at Motomachi Crown line. Publication of Toyota Production System in English and infusion in United States. Publication of the Machine That Changed the World.

Third factory revolution at Toyota in Japan. (David Cochran)

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SECTION 2.5

TABLE 2-4 Zero

Summary of Factory Designs

55

Factory Revolutions Are Driven by New Manufacturing System Designs (MSDs) 1700–1850

No

Craft/cottage production–hand powered tools

MSD

First Factory

1840–1910

Job shop

Revolution

First factory revolution (American Armory System) Creation of factories with powered machines

(functional layout)

Mechanization/Interchangeable parts Second Factory

1910-1970

Flow shop

Revolution

Second factory revolution (the Ford system)

(product layout)

Assembly line–flow shop product layout Economy of scale yields mass production era Automation (automatic material handling) Third Factory Revolution

1960–2010 (estimated) Third factory revolution (lean production)

Lean shop (linked-cell layout)

U-shaped manufacturing and assembly cells One-piece flow with mixed-model final assembly Flexibility in customer demand and product design Integrated control functions (kanban)

cells that produce defect-free goods. The new system has now been adopted by more than 60% of American manufacturing companies and has been disseminated around the industrial world. Black’s Theory of Factory Revolutions as outlined in Table 2-4, proposes that we are now 40-plus years into the factory revolution. This factory revolution is not based on computers, hardware, or a particular process, but once again on the design of the manufacturing system—the complex arrangement of physical elements characterized by measurable parameters. Again, people throughout the world went to observe the new design, but this time they went to Japan to try to understand how this tiny nation became a giant in the global manufacturing arena. After World War II, the Japanese were confronted with different requirements of manufacturing than that of the Americans. The United States had almost an infinite capacity to produce. There were rows of stamping presses in the factories, a surplus of resources, pent-up demand, and many people with money, so all the United States had to do was to produce. In Japan, there were very few presses and very little money. One of the first concepts of the Toyota Production System was that all parts had to be good, because there was no excess capacity and no dealers in the United States to fix the defects. The right mix of cars of perfect quality had to be made with limited resources, and they had to be exactly right the first time in spite of any variations or disturbances to the system. The result was a new manufacturing system design, and just as a new MSD pushed Colt and Remington to the forefront in the first factory revolution and Ford and Singer in the second factory revolution, the development of this new manufacturing system design vaulted Toyota into world leadership. Black defines the new physical system design as linked-cell (Black, 1991) or L-CMS for linked-cell manufacturing system. Toyota called it the Toyota Production System (TPS). Schonberger called it the JIT/ TQC system or World Class Manufacturing (WCM) system. In 1990 it was finally given a name that would become universal: lean production. This term was coined by John Krafcik, an engineer in the International Motor Vehicle program at the Massachusetts Institute of Technology (Womack et al., 1991). What was different about this system design was the development of manufacturing and assembly cells linked to final assembly by a unique material control system, producing a functionally integrated system for inventory and production control. In cells, processes are grouped according to the sequence of operations needed to make a product. This design uses one-piece flow like the flow shop, but is designed for flexibility.

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M

M

L

D

L

L

M

Single cycle automatic machine

D

Cell #2

G

M

Floor space available for manufacturing

Cell #1 L

A

G

FI L

G

G

G

VM

VM

A D

HM

Cell #3 L

Leftover process

G SAW

L

L A

Receiving and shipping

Added to cell

FIGURE 2-20 The functionally designed job shop can be restructured into manufacturing cells to process families of components at production rates that match part consumption.

The cell is designed in a U-shape or in parallel rows so that the workers can readily rebalance the line and change the output rate while moving from machine to machine loading and unloading parts. Figure 2-20 shows how the job shop in Figure 2-4 can be rearranged into manned cells. Cell 3 has one worker who can make a walking loop around the cell in 60 seconds. The machines in the cell have been upgraded to singlecycle automatic capability so they can complete the desired processing untended, turning themselves off when done with a machining cycle. The operator comes to a machine, unloads a part, checks the part, loads a new part into the machine, and starts the machining cycle again. The cell usually includes all the processing needed for a complete part or subassembly and may even include assembly steps. To form a linked-cell manufacturing system, the first step is to restructure portions of the job shop, converting it in stages into manned cells. At the same time, the linear flow lines in subassembly are also reconfigured into U-shaped cells, which operate much like the manufacturing cells. The long setup times typical in flow lines must be vigorously attacked and reduced so that the flow lines can be changed quickly from making one product to another. The need to perform line-balancing tasks is eliminated through design. The standing, walking workers are capable of performing multiples of operations. More details are given in Chapter 29.

SUMMARY ON MANUFACTURING CELLS Product designers can easily see how parts are made in the manufacturing cells because all the operations and processes are together. Because quality-control techniques are also integrated into the cells, the designer knows exactly the cell’s process capability. The designer can easily configure the future designs to be made in the cell. This is truly designing for manufacturing. CNC machining centers can do the same sequence of steps but are not as flexible as a cell composed of multiple, simple machines. Cellular layouts facilitate the integration of critical production functions while maintaining flexibility in producing superior-quality families of components. The cells facilitate

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Problems

57

single-piece-flow (SPF) and volume flexibility. SPF is the movement of one part at a time between machines by the multiprocess operators. In the main, each machine executes a step in the sequence of processes or operations. The outcome of that step is checked before the part is advanced to the next step. Volume flexibility is achieved by the separation of man’s activities from the operations that machines do better. Output per hour can be changed by the rapid reallocation of operations to workers.

& KEY WORDS American Armory System assembly line bill of materials (BOM) continuous process critical path method (CPM) decouplers flexible manufacturing system (FMS) flow shop job shop

Ford Production System just-in-time (JIT) manufacturing lean production lean shop line balancing linked-cell system manufacturing system manufacturing system design (MSD)

mass production operations sheet part interchangeability process planning production system program evaluation and review technique (PERT) programmable logic controller (PLC)

project shop route sheet single-piece-flow (SPF) standard work-in-process (SWIP) Toyota Production System (TPS) transfer line volume flexibility

& REVIEW QUESTIONS 1. What are the major functional elements or departmental areas of the production system? (See Chapter 44 on the Web for help.) 2. What is a route sheet? Who uses it? 3. What is the function of a route sheet? 4. Find an example of a route sheet other than the one in the book. 5. What are other names for a route sheet? 6. What is a process flow chart? How is it related to the bill of materials? 7. What is an operations sheet? How is it related to the route sheet? 8. How does the design of the product influence the design of the manufacturing system, including assembly and the production system?

9. Explain the difference between a route sheet and an operations sheet (or a process planning sheet). 10. What does the study of ergonomics entail? (This was not discussed in Chapter 2.) 11. In project shop manufacturing, what is the critical path? (Not discussed in Chapter 2.) 12. How do the job shop, flow shop, and lean shop manufacturing systems perform in terms of quality, cost, delivery, and flexibility? 13. What are the possibilities of incorporating lean manufacturing concepts into a high-volume transfer line for machining? 14. How did the Ford thinking build on the first factory revolution?

& PROBLEMS 1. Discuss this statement: ‘‘Software can be as costly to design and develop as hardware and will require long production runs to recover, even though these costs may be hidden in the overhead costs.’’ 2. Table 2-1 lists some examples of service job shops. Compare your college to a manufacturing/production system, using the definition of a manufacturing system given in the chapter. Who is the internal customer in the academic job shop? What or who are the products in the academic job shop? 3. Outline your critical path through the academic job shop. 4. Explain how function dictates design with respect to the design of footwear. Use examples of different kinds of footwear (shoes, sandals, high heels, boots, etc.) to emphasize your points. For example, cowboy boots have pointed toes so

that they slip into the stirrups easily and high heels to keep the foot in the stirrup. 5. Most companies, when computing or estimating costs for a job, will add in an overhead cost, often tying that cost to some direct cost, such as direct labor, through the academic job shop. How would you calculate the cost per unit of a product to include overhead? 6. What is the impact of minimizing the unit cost of each operation: a. On machine design? b. On the workers? c. On the factory as a system?

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Manufacturing Systems Design

Chapter 2

CASE STUDY

Jury Duty for an Engineer

K

atrin S. is suing the PogiBear Snowmobile Company and an engineer for PippenCat Components for $750,000 over her friend’s death. He was killed while racing his snowmobile through the woods in the upper peninsula of Michigan. Her lawyer, Ken, claims that her friend was killed because a tie-rod broke, causing him to lose control and crash into a tree, breaking his neck. While it was impossible to determine whether the tie-rod broke before the crash or as a result of the crash, the following evidence has been put forth. The tie-rod was originally designed and made entirely out of low-carbon steel (heat treated by case hardening) in three pieces, as shown in Figure CS-2 These tie-rods were subcontracted by PogiBear to PippenCat Components. PippenCat Components changed the material of the sleeves from steel to a heat-treated aluminum having the same ultimate tensile strength (UTS), value as the steel. They did this because aluminum sleeves were easier to thread than steel sleeves. It was further found that threads on one of the tie-rod bolts were not as completely formed as they should have been. The sleeve of the tie-rod in question was split open (fractured) and one of the tie-rod bolts was bent. Katrin’s lawyer further claimed that the tie-rod was not assembled properly. He claimed that one rod was screwed into the sleeve too far and the other not far

enough, thereby giving it insufficient thread engagement. The engineer for PippenCat testified that these tie-rods are hand assembled and checked only for overall length and that such a misassembly was possible. In his summary, Ken, Katrin’s lawyer, stated that the failure was due to a combination of material change, manufacturing error, and bad assembly—all combining to result in a failure of the tie-rod. A design engineer for PogiBear testified that the tierods were ‘‘way overdesigned’’ and would not fail even with slightly small threads or misassembly. PogiBear’s lawyer then claimed that the accident was caused by driver failure and that the tie-rod broke upon impact of the snowmobile with the tree. One of the men racing with Katrin’s friend claimed that her friend’s snowmobile had veered sharply just before he crashed, but under cross examination he admitted that they had all been drinking that night because it was so cold (he guessed –20 to –30 F). Because this accident had taken place more than 5 years ago, he could not remember how much they had had to drink. You are a member of the jury and have now been sequestered to decide if PogiBear and PippenCat are guilty of negligence resulting in death. The rest of the jury, knowing you are an engineer, has asked for your opinion. What do you think? Who is really to blame for this accident? What actually caused the accident?

Tie-rod bolt

Tie-rod bolt

Sleeve

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CHAPTER 3 PROPERTIES OF MATERIALS 3.1 INTRODUCTION Metallic and Nonmetallic Materials Physical and Mechanical Properties Stress and Strain 3.2 STATIC PROPERTIES Tensile Test Compression Tests Hardness Testing

3.3 DYNAMIC PROPERTIES Impact Test Fatigue and the Endurance Limit Fatigue Failures 3.4 TEMPERATURE EFFECTS (BOTH HIGH AND LOW) Creep 3.5 MACHINABILITY, FORMABILITY, AND WELDABILITY

3.6 FRACTURE TOUGHNESS AND THE FRACTURE MECHANICS APPROACH 3.7 PHYSICAL PROPERTIES 3.8 TESTING STANDARDS AND TESTING CONCERNS Case Study: Separation of Mixed Materials

& 3.1 INTRODUCTION The history of man has been intimately linked to the materials that have shaped his world, so much so that we have associated periods of time with the dominant material, such as the Stone Age, Bronze Age, and Iron Age. Stone was used in its natural state, but bronze and iron were made possible by advances in processing. Each contributed to the comfort, productivity, safety, and security of everyday living. One replaced the other when new advantages and capabilities were realized, iron being lighter and stronger than bronze for example. Many refer to the close of the 20th century as the Silicon Age. The multitude of devices that have been made possible by the transistor and computer chip [ultra-fast computers, cell phones, global positioning system (GPS) units, etc.] have revolutionized virtually every aspect of our lives. From the manufacturing perspective, however, the current era lacks a materials designation. We now use an extremely wide array of materials, falling into the categories of metals, ceramics, polymers, and a myriad of combinations known as composites. With each of these materials, the ultimate desire is to convert it into some Structure Properties form of useful product. Manufacturing has been described as the various activities that are performed to convert ‘‘stuff’’ into ‘‘things.’’ Successful products begin with appropriate materials. You wouldn’t build an airplane out of lead, or an automobile out of concrete—you need to start with the right stuff. But ‘‘stuff’’ rarely comes in the right shape, size, and quantity for the desired use. Parts and Processing Performance components must be produced by subjecting materials to one or more processes (often a series of operations) that alter their shape, their properties, or both. FIGURE 3-1 The interdependent Much of a manufacturing education relates to understanding: (1) the structure of relationships between structure, materials, (2) the properties of materials, (3) the processing of materials, and (4) properties, processing, and performance. the performance of materials, as well as the interrelations between these four factors, as illustrated in Figure 3-1. This chapter will begin to address the properties of engineering materials. Chapters 4 and 5 will discuss the subject of structure and begin to provide the whys behind various properties. Chapter 6 introduces the possibility of controlling and modifying structure to produce desired properties. Many engineering materials do not have a single set of properties, offering instead a range or spectrum of possibilities. Taking advantage of this range, we might want to intentionally make a material weak and ductile for easy shaping (making forming loads low, extending tool life, and preventing cracking or fracture), and then, once the shape has been produced, make the material strong for enhanced performance during use.

59

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Properties of Materials

When selecting a material for a product or application, it is important to ensure that its properties will be adequate for the anticipated operating conditions. The various requirements of each part or component must first be estimated or determined. These requirements typically include mechanical characteristics (strength, rigidity, resistance to fracture, the ability to withstand vibrations or impacts) and physical characteristics (weight, electrical properties, appearance), as well as features relating to the service environment (ability to operate under extremes of temperature or resist corrosion). Candidate materials must possess the desired properties within their range of possibilities. To help evaluate the properties of engineering materials, a variety of standard tests have been developed, and data from these tests have been tabulated and made readily available. Proper use of these data, however, requires sound engineering judgment. It is important to consider which of the evaluated properties are significant, under what conditions the test values were determined, and what cautions or restrictions should be placed on their use. Only by being familiar with the various test procedures, their capabilities, and their limitations can one determine if the resulting data are applicable to a particular problem.

METALLIC AND NONMETALLIC MATERIALS While engineering materials are often grouped as metals, ceramics, polymers and composites, a more simplistic distinction might be to separate into metallic and nonmetallic. The common metallic materials include iron, copper, aluminum, magnesium, nickel, titanium, lead, tin, and zinc, as well as the many alloys of these metals, including steel, brass, and bronze. They possess the metallic properties of luster, high thermal conductivity, and high electrical conductivity; they are relatively ductile; and some have good magnetic properties. Some common nonmetals are wood, brick, concrete, glass, rubber, and plastics. Their properties vary widely, but they generally tend to be weaker, less ductile, and less dense than the metals, with poor electrical and thermal conductivities. Although metals have traditionally been the more important of the two groups, the nonmetallic materials have become increasingly important in modern manufacturing. Advanced ceramics, composite materials, and engineered plastics have emerged in a number of applications. In many cases, metals and nonmetals are viewed as competing materials, with selection being based on how well each is capable of providing the required properties. Where both perform adequately, total cost often becomes the deciding factor, where total cost includes both the cost of the material and the cost of fabricating the desired component. Factors such as product lifetime, environmental impact, energy requirements, and recyclability are also considered.

PHYSICAL AND MECHANICAL PROPERTIES A common means of distinguishing one material from another is through their physical properties. These include such features as density (weight); melting point; optical characteristics (transparency, opaqueness, or color); the thermal properties of specific heat, coefficient of thermal expansion, and thermal conductivity; electrical conductivity; and magnetic properties. In some cases, physical properties are of prime importance when selecting a material, and several will be discussed in more detail near the end of this chapter. More often, however, material selection is dominated by the properties that describe how a material responds to applied loads or forces. These mechanical properties are usually determined by subjecting prepared specimens to standard test conditions. When using the obtained results, however, it is important to remember that they apply only to the specific conditions that were employed in the test. The actual service conditions of engineered products rarely duplicate the conditions of laboratory testing, so considerable caution should be exercised.

STRESS AND STRAIN When a force or load is applied to a material, it deforms or distorts (becomes strained), and internal reactive forces (stresses) are transmitted through the solid. For example, if a weight, W, is suspended from a bar of uniform cross section and

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SECTION 3.2

L

L± L

W

FIGURE 3-2 Tension loading and the resultant elongation.

Static Properties

61

length L, as in Figure 3-2, the bar will elongate by an amount DL. For a given weight, the magnitude of the elongation, DL, depends on the original length of the bar. The amount of elongation per unit length, expressed as e ¼ DL/L, is called the unit strain. Although the ratio is that of a length to another length and is therefore dimensionless, strain is usually expressed in terms of millimeters per meter, inches per inch, or simply as a percentage. Application of the force also produces reactive stresses, which serve to transmit the load through the bar and on to its supports. Stress is defined as the force or load being transmitted divided by the cross-sectional area transmitting the load. Thus, in Figure 3-2, the stress is S ¼ W/A, where A is the cross-sectional area of the supporting bar. Stress is normally expressed in megapascals in SI units (where a Pascal is one Newton per square meter) or pounds per square inch in the English system. In Figure 3-2, the weight tends to stretch or lengthen the bar, so the strain is known as a tensile strain and the stress as a tensile stress. Other types of loadings produce other types of stresses and strains (Figure 3-3). Compressive forces tend to shorten the material and produce compressive stresses and strains. Shear stresses and strains result when two opposing forces acting on a body are offset with respect to one another.

& 3.2 STATIC PROPERTIES When the forces that are applied to a material are constant, or nearly so, they are said to be static. Because static or steady loadings are observed in many applications, it is important to characterize the behavior of materials under these conditions. For design engineers, the strength of a material may be of primary concern, along with the amount of elastic stretching or deflection that may be experienced when the product is under load. Manufacturing engineers, wanting to shape products with mechanical forces, need to know the stresses necessary to effect permanent deformation. At the same time, they want to perform this deformation without inducing cracking or fracture. As a result, a number of standardized tests have been developed to evaluate the static properties of engineering materials. Individual test results can be used to determine if a given material or batch of material has the necessary properties to meet specified requirements. The results of multiple tests can provide the materials characterization information that is used when selecting materials for various applications. In all cases, it is important to determine that the conditions for the product being considered are indeed similar to those of the standard testing. Even when the service conditions differ, however, the results of standard tests may still be helpful in qualitatively rating and comparing various materials.

⌬L

⌬L

L

FIGURE 3-3 Examples of tension, compression, and shear loading—and their response.

L

L

⌬L Tension

Compression

Shear

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CHAPTER 3

Properties of Materials 3⬙ – 10 4 1/8⬙ (3.18) 3/4⬙ (19.05)

.505⬙ 12.83 1⬙ (25.4)

2⬙ (50.8) 2-1/4⬙ (57.15)

FIGURE 3-4 Two common types of standard tensile test specimens: (a) round; (b) flat. Dimensions are in inches, with millimeters in parentheses.

3⬙ (76.2)

2⬙ (50.8)

8⬙ (203.2)

t

1–1/2⬙ (38.1)

C03

9⬙ (228.6)

TENSILE TEST The most common of the static tests is the uniaxial tensile test. The test begins with the preparation of a standard specimen with prescribed geometry, like the round and flat specimens described in Figure 3-4. The standard specimens ensure meaningful and reproducible results and have been designed to produce uniform uniaxial tension in the central portion while ensuring reduced stresses in the enlarged ends or shoulders that are placed in moving grips. Strength Properties. The standard specimen is then inserted into a testing machine like the one shown in Figure 3-5. A tensile force or load, W, is applied and measured by the testing machine, while the elongation or stretch (DL) of a specified length (gage length) is simultaneously monitored. A plot of the coordinated load–elongation data produces a curve similar to that of Figure 3-6. Because the loads will differ for different-size

Tension crosshead Screw column

Tension space

Notched column

Adjustable crosshead Compression space Table Load cell Cylinder Piston

(a)

(b)

FIGURE 3-5 (a) Universal (tension and compression) testing machine; (b) schematic of the load frame showing how motion of the darkened yoke can produce tension or compression with respect to the stationary (white) crosspiece. [(a) Courtesy of Instron, Industrial Products Group, Grove City, PA; (b) Courtesy of Satec Systems Inc., Grove City, PA]

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SECTION 3.2

Static Properties

500

70 60

Maximum (ultimate) load Breaking strength

400 Upper yield point

50

300

40 Lower yield point Proportional limit

200

30 20

Yield-point elongation

100

FIGURE 3-6 Engineering stress-strain diagram for a lowcarbon steel.

63

103 Psi

MPa

Engineering stress

C03

10 0

0 0

5

10 15 20 Engineering strain – % (in./in.*10-2)

25

specimens and the amount of elongation will vary with different gage lengths, it is important to remove these geometric or size effects if we are to produce data that are characteristic of a given material and not a particular specimen. If the load is divided by the original cross-sectional area, Ao, and the elongation is divided by the original gage length, Lo, the size effects are eliminated and the resulting plot becomes known as an engineering stressengineering strain curve (see Figure 3-6). This is simply a load–elongation plot with the scales of both axes modified to remove the effects of specimen size. In Figure 3-6 it can be noted that the initial response is linear. Up to a certain point, the stress and strain are directly proportional to one another. The stress at which this proportionality ceases is known as the proportional limit. Below this value, the material obeys Hooke’s law, which states that the strain is directly proportional to the stress. The proportionality constant, or ratio of stress to strain, is known as Young’s modulus or the modulus of elasticity. This is an inherent property of a given material1 and is of considerable engineering importance. As a measure of stiffness, it indicates the ability of a material to resist deflection or stretching when loaded and is commonly designated by the symbol E. Up to a certain stress, if the load is removed, the specimen will return to its original length. The response is elastic or recoverable, like the stretching and relaxation of a rubber band. The uppermost stress for which this behavior is observed is known as the elastic limit. For most materials the elastic limit and proportional limit are almost identical, with the elastic limit being slightly higher. Neither quantity should be assigned great engineering significance, however, because the determined values are often dependent on the sensitivity and precision of the test equipment. The amount of energy that a material can absorb while in the elastic range is called the resilience. The area under a load–elongation curve is the product of a force and a distance, and is therefore a measure of the energy absorbed by the specimen. If the area is determined up to the elastic limit, the absorbed energy will be elastic (or potential) energy and is regained when the specimen is unloaded. If the same determination is performed on an engineering stress-engineering strain diagram, the area beneath the elastic region corresponds to an energy per unit volume, and is known as the modulus of resilience. Elongation beyond the elastic limit becomes unrecoverable and is known as plastic deformation. When the load is removed, only the elastic stretching will be recovered, and the specimen will retain a permanent change in shape (in this case, an increase in length). For most components, the onset of plastic flow represents failure, because the part dimensions will now be outside of allowable tolerances. In manufacturing 1 The modulus of elasticity is determined by the binding forces between the atoms. Since these forces cannot be changed, the elastic modulus is characteristic of a specific material and is not alterable by the structure modifications that can be induced by processing.

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S2 S1 Stress – MPa (lb/in.2)

C03

0.1% 0.2% Strain – % (in./in.)

FIGURE 3-7 Stress-strain diagram for a material not having a well-defined yield point, showing the offset method for determining yield strength. S1 is the 0.1% offset yield strength; S2 is the 0.2% offset yield strength.

processes where plastic deformation is used to produce the desired shape, the applied stresses must be sufficient to induce the required amount of plastic flow. Permanent deformation, therefore, may be either desirable or undesirable, but in either case, it is important to determine the conditions where elastic behavior transitions to plastic flow. Whenever the elastic limit is exceeded, increases in strain no longer require proportionate increases in stress. For some materials, like the low-carbon steel tested in Figure 3-6, a stress value may be reached where additional strain occurs without any further increase in stress. This stress is known as the yield point, or yield-point stress. In Figure 3-6, two distinct points are observed. The highest stress preceding extensive strain is known as the upper yield point, and the lower, relatively constant, ‘‘run-out’’ value is known as the lower yield point. The lower value is the one that usually appears in tabulated data. Most materials, however, do not have a well-defined yield point, and exhibit stress-strain curves more like that shown in Figure 3-7. For these materials, the elasticto-plastic transition is not distinct, and detection of plastic deformation would be dependent upon machine sensitivity or operator interpretation. To solve this dilemma, we simply define a useful and easily determined property known as the offset yield strength. Offset yield strength does not describe the onset of plastic deformation, but instead defines the stress required to produce a specified, acceptable, amount of permanent strain. If this strain, or ‘‘offset,’’ is specified to be 0.2% (a common value), we simply determine the stress required to plastically deform a 1-in. length to a final length of 1.002 in. (a 0.2% strain). If the applied stresses are then kept below this 0.2% offset yield strength value, the user can be guaranteed that any resulting plastic deformation will be less than 0.2% of the original dimension. Offset yield strength is determined by drawing a line parallel to the elastic line, but displaced by the offset strain, and reporting the stress where the constructed line intersects the actual stress-strain curve. Figure 3-7 shows the determination of both 0.1% offset and 0.2% offset yield strength values, S1 and S2, respectively. The intersection values are reproducible and are independent of equipment sensitivity. It should be noted that the offset yield strength values are meaningless unless they are reported in conjunction with the amount of offset strain used in their determination. While 0.2% is a common offset for many mechanical products (and is generally assumed unless another number is specified), applications that cannot tolerate that amount of deformation may specify offset values of 0.1% or even 0.02%. It is important, therefore, to verify that any tabulated data being used was determined under the desired conditions. As shown in Figure 3-6, the load (or engineering stress) required to produce additional plastic deformation continues to increase. Because the material is deforming, this load is the product of the material strength times the cross-sectional area. During tensile deformation, the specimen is continually increasing in length. The cross-sectional area, therefore, must be decreasing, but the overall load-bearing ability of the specimen continues to increase! For this to occur, the material must be getting stronger. The mechanism for this phenomenon will be discussed in Chapter 4, where we will learn that the strength of a metal continues to increase with increased deformation. During the plastic deformation portion of a tensile test, the weakest location of the specimen is continually undergoing deformation and becoming stronger. As each weakest location strengthens, another location assumes that status and deforms. As a consequence, the specimen deforms and strengthens uniformly, maintaining its original cylindrical or rectangular geometry. As plastic deformation progresses, however, the additional increments of strength decrease in magnitude, and a point is reached where the decrease in area cancels the increase in strength. When this occurs, the load-bearing ability peaks, and the force required to continue straining the specimen begins to decrease, as seen in the Figure 3-6. The stress at which the load-bearing ability peaks is known as the ultimate strength, tensile strength, or ultimate tensile strength of the material. The weakest location in the test specimen at that time continues to be the weakest location by virtue of the decrease in area, and further deformation becomes localized. This localized reduction in cross-sectional area is known as necking, and is shown in Figure 3-8.

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Necked region

FIGURE 3-8 A standard 0.505in.-diameter tensile specimen showing a necked region that has developed prior to failure. (E. Paul DeGarmo)

If the straining is continued, necking becomes intensified and the tensile specimen will ultimately fracture. The stress at which fracture occurs is known as the breaking strength or fracture strength. For ductile materials, necking precedes fracture, and the breaking strength is less than the ultimate tensile strength. For a brittle material, fracture usually terminates the stress-strain curve before necking, and often before the onset of plastic flow. Ductility and Brittleness. When evaluating the suitability of a material for certain manufacturing processes or its appropriateness for a given application, the amount of plasticity that precedes fracture, or the ductility, can often be a significant property. For metal deformation processes, the greater the ductility, the more a material can be deformed without fracture. Ductility also plays a key role in toughness, a property that will be described shortly. One of the simplest ways to evaluate ductility is to determine the percent elongation of a tensile test specimen at the time of fracture. As shown in Figure 3-9, ductile materials do not elongate uniformly when loaded beyond necking. If the percent change of the entire 8-in. gage length were computed, the elongation would be 31%. However, if only the center 2-in. segment is considered, the elongation of that portion is 60%. A valid comparison of material behavior, therefore, requires similar specimens with the same standard gage length. In many cases, material ‘‘failure’’ is defined as the onset of localized deformation or necking. Consider a sheet of metal being formed into an automobile body panel. If we are to assure uniform strength and corrosion resistance in the final panel, the operation must be performed in such a way as to maintain uniform sheet thickness. For this application, a more meaningful measure of material ductility would be the uniform elongation or the percent elongation prior to the onset of necking. This value can be determined by constructing a line parallel to the elastic portion of the diagram, passing through the point of highest force or stress. The intercept where the line crosses the strain axis denotes the available uniform elongation. Because the additional deformation that occurs after necking is not considered, uniform elongation is always less than the total elongation at fracture (the generally reported elongation value).

1.00⬙

1.00⬙

1.00⬙

1.00⬙

1.00⬙

1.00⬙

1.00⬙

1.00⬙

8⬙

(a)

(b)

1.11⬙

1.25⬙

1.45⬙

1.68⬙

1.52⬙

1.31⬙

FIGURE 3-9 Final elongation in various segments of a tensile test specimen: (a) original geometry; (b) shape after fracture.

1.14⬙

10.48⬙

1.02⬙

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Another measure of ductility is the percent reduction in area that occurs in the necked region of the specimen. This can be computed as R:A: ¼

Ao Af 100% Ao

where Ao is the original cross-sectional area and Af is the smallest area in the necked region. Percent reduction in area, therefore, can range from 0% (for a brittle glass specimen that breaks with no change in area) to 100% (for extremely plastic soft bubble gum that pinches down to a point before fracture). When materials fail with little or no ductility, they are said to be brittle. Brittleness, however, is simply the lack of ductility, and should not be confused with a lack of strength. Strong materials can be brittle, and brittle materials can be strong. Toughness. Toughness, or modulus of toughness, is the work per unit volume required to fracture a material. The tensile test can provide one measure of this property, because toughness corresponds to the total area under the stress-strain curve from test initiation to fracture, and thereby encompasses both strength and ductility. Caution should be exercised when using toughness data, however, because the work or energy to fracture can vary markedly with different conditions of testing. Variations in the temperature or the speed of loading can significantly alter both the stress-strain curve and the toughness. In most cases, toughness is associated with impact or shock loadings, and the values obtained from high-speed (dynamic) impact tests often fail to correlate with those obtained from the relatively slow-speed (static) tensile test. True Stress–True Strain Curves. The stress-strain curve in Figure 3-6 is a plot of engineering stress, S, versus engineering strain, e, where S is computed as the applied load divided by the original cross-sectional area, Ao, and e is the elongation, DL, divided by the original gage length, Lo. As the test progresses, the cross section of the test specimen changes continually, first in a uniform manner and then nonuniformly after necking begins. The actual stress should be computed based on the instantaneous crosssectional area, A, not the original, Ao. Because the area is decreasing, the actual or true stress will be greater than the engineering stress plotted in Figure 3-6. True stress, s, can be computed by taking simultaneous readings of the load, W, and the minimum specimen diameter. The actual area can then be computed, and true stress can be determined as s¼

W A

The determination of true strain is a bit more complex. In place of the change in length divided by the original length that was used to compute engineering strain, true strain is defined as the summation of the incremental strains that occur throughout the test. For a specimen that has been stretched from length Lo to length L, the true, natural, or logarithmic strain, would be: Uniform deformation True stress

C03

Nonuniform deformation

ZL e¼

Fracture Maximum load Yield point

True strain

FIGURE 3-10 True stress–true strain curve for an engineering metal, showing true stress continually increasing throughout the test.

Lo

d‘ L D2 Do ¼ ln ¼ o2 ¼ 2 ln D ‘ Lo D

The preceding equalities make use of the following relationships for cylindrical specimens that maintain constant volume (i.e., V o ¼ LoAo ¼ V ¼ LA) L Ao D2o ¼ 2 ¼ A Lo D NOTE: Because these relations are based on cylindrical geometry, they apply only up to the onset of necking. Figure 3-10 depicts the type of curve that results when the data from a uniaxial tensile test are converted to the form of true stress versus true strain.

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Because the true stress is a measure of the material strength at any point during the test, it will continue to rise even after necking. Data beyond the onset of necking should be used with extreme caution, however, because the geometry of the neck transforms the stress state from uniaxial tension (stretching in one direction with compensating contractions in the other two) to triaxial tension, in which the material is stretched or restrained in all three directions. Because of the triaxial tension, voids or cracks (Figure 3-11) tend to form in the necked region and serve as a precursor to final fracture. Measurements of the external diameter no longer reflect the true load-bearing area, and the data are further distorted.

FIGURE 3-11 Section of a tensile test specimen stopped just prior to failure, showing a crack already started in the necked region, which is experiencing triaxial tension. (Photo by E. R. Parker, courtesy E. Paul DeGarmo)

B

Stress

A

f

D

E

e

C O

Strain

FIGURE 3-12 Stress-strain diagram obtained by unloading and reloading a specimen.

Large n

Small n True stress – σ

C03

True strain – ε

FIGURE 3-13 True stress–true strain curves for metals with large and small strain hardening. Metals with larger n values experience larger amounts of strengthening for a given strain.

Strain Hardening and the Strain-Hardening Exponent. Figure 3-12 is a true stress–true strain diagram, which has been modified to show how a ductile metal (such as steel) will behave when subjected to slow loading and unloading. Loading and unloading within the elastic region will result in simply cycling up and down the linear portion of the curve between points O and A. However, if the initial loading is carried through point B (in the plastic region), unloading will follow the path BeC, which is approximately parallel to the line OA, and the specimen will exhibit a permanent elongation of the amount OC. Upon reloading from point C, elastic behavior is again observed as the stress follows the line CfD, a slightly different path from that of unloading. Point D is now the yield point or yield stress for the material in its partially deformed state. A comparison of points A and D reveals that plastic deformation has made the material stronger. If the test were again interrupted at point E, we would find a new, even higher-yield stress. Thus, within the region of plastic deformation, each of the points along the true stress–true strain curve represents the yield stress for the material at the corresponding value of strain. When metals are plastically deformed, they become harder and stronger, a phenomenon known as strain hardening. Therefore, if a stress induces plastic flow, an even greater stress will be required to continue the deformation. In Chapter 4 we will discuss the atomic-scale features that are responsible for this phenomenon. Various materials strain harden at different rates; that is, for a given amount of deformation different materials will exhibit different increases in strength. One method of describing this behavior is to mathematically fit the plastic region of the true stress– true strain curve to the equation s ¼ Ken and determine the best-fit value of n, the strain-hardening exponent.2 As shown in Figure 3-13, a material with a high value of n will have a significant increase in strength with a small amount of deformation. A material with a small n value will show little change in strength with plastic deformation. Damping Capacity. In Figure 3-12 the unloading and reloading of the specimen follow slightly different paths. The area between the two curves is proportional to the amount of energy that is converted from mechanical form to heat and is therefore absorbed by the material. When this area is large, the material is said to exhibit good damping capacity and is able to absorb mechanical vibrations or damp them out quickly. This is an important property in applications such as crankshafts and machinery bases. Gray cast iron is used in many applications because of its high damping capacity. Materials with low damping capacity, such as brass and steel, readily transmit both sound and vibrations. Rate Considerations. The rate or speed at which a tensile test is conducted can have a significant effect on the various properties. Strain rate sensitivity varies widely for the engineering materials. Plastics and polymers are very sensitive to testing speed, as are metals with low melting points, such as lead and zinc. Those materials that are sensitive to speed variations exhibit higher strengths and lower ductility when speed is increased. 2 Taking the logarithm of both sides of the equation yields log s ¼ log K þ n log e, which has the same form as the equation y ¼ mx þ b if y is log s and x is log e. This is the equation of a straight line with slope m and intercept b. Therefore, if the true stress–true strain data were plotted on a log-log scale with stress (s) on the y-axis and strain (e) on the x, the slope of the data in the plastic region would be n.

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It is important to recognize that standard testing selects a standard speed, which may or may not correlate with the conditions of product application.

COMPRESSION TESTS When a material is subjected to compressive loadings, the relationships between stress and strain are similar to those for a tension test. Up to a certain value of stress, the material behaves elastically. Beyond this value, plastic flow occurs. In general, however, a compression test is more difficult to conduct than a standard tensile test. Test specimens must have larger cross-sectional areas to resist bending or buckling. As deformation proceeds, the material strengthens by strain hardening and the cross section of the specimen increases, combining to produce a substantial increase in required load. Friction between the testing machine surfaces and the ends of the test specimen will alter the results if not properly considered. The type of service for which the material is intended, however, should be the primary factor in determining whether the testing should be performed in tension or compression.

HARDNESS TESTING The wear resistance and strength of a material can also be evaluated by assessing its ‘‘hardness.’’ Hardness is actually a hard-to-define property of engineering materials, and a number of different tests have been developed using various phenomena. The most common of the hardness tests are based on resistance to permanent deformation in the form of penetration or indentation. Other tests evaluate resistance to scratching, wear resistance, resistance to cutting or drilling, or elastic rebound (energy absorption under impact loading). Because these phenomena are not the same, the results of the various tests often do not correlate with one another. While hardness tests are among the easiest to perform on the shop floor, caution should be exercised to ensure that the selected test clearly evaluates the phenomena of interest. The various ASTM specifications3 provide details regarding sample preparation, selection of loads and penetrators, minimum sample thicknesses, spacing and near-edge considerations, and conversions between scales. Brinell Hardness Test. The Brinell hardness test was one of the earliest accepted methods of measuring hardness. A tungsten carbide or hardened steel ball 10 mm. in diameter is pressed into the flat surface of a material by a standard load of 500 or 3000 kg. The load is maintained for a period of time to permit sufficient plastic deformation to occur to support the applied load (10 to 15 seconds for iron or steel and up to 30 seconds for softer metals), and the load and ball are then removed. The diameter of the resulting spherical indentation (usually in the range of 2 to 5 mm) is then measured to an accuracy of 0.05 mm using a special grid or traveling microscope. The Brinell hardness number (BHN) is equal to the load divided by the surface area of the spherical indentation when the units are expressed as kilograms per square millimeter. In actual practice, the Brinell hardness number is determined from tables that correlate the Brinell number with the diameter of the indentation produced by the specified load. Figure 3-14 shows a typical Brinell tester, along with a schematic of the testing procedure, which is actually a two-step operation—load then measure. The Brinell test measures hardness over a relatively large area and is somewhat indifferent to small-scale variations in the material structure. It is relatively simple and easy to conduct and is used extensively on irons and steels. On the negative side, however, the Brinell test has the following limitations: 1. It cannot be used on very hard or very soft materials. 2. The results may not be valid for thin specimens. It is best if the thickness of material is at least 10 times the depth of the indentation. Some standards specify the minimum hardnesses for which the tests on thin specimens will be considered valid. 3. The test is not valid for case-hardened surfaces. 3

ASTM hardness testing specifications include E3, E10, E18, E103, E140, and E384.

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Force (kgf)

10-mm (0.4-in.) ball

FIGURE 3-14 (a) Brinell hardness tester; (b) Brinell test sequence showing loading and measurement of the indentation under magnification with a scale calibrated in millimeters. (Courtesy of Wilson Hardness, an Instron Company, Norwood, MA)

(a)

(b)

4. The test must be conducted far enough from the edge of the material so that no edge bulging occurs. 5. The substantial indentation may be objectionable on finished parts. 6. The edge or rim of the indentation may not be clearly defined or may be difficult to see. Portable testers are available for use on pieces that are too large to be brought to a benchtop machine. The Rockwell Test. The Rockwell hardness test is the most widely used hardness test, and is similar to the Brinell test, with the hardness value again being determined by an indentation or penetration produced by a static load. Figure 3-15a shows the key

Penetrator

FIGURE 3-15 (a) Operating principle of the Rockwell hardness tester; (b) typical Rockwell hardness tester with digital readout. (Courtesy of Mitutoyo America Corporation, Aurora, IL)

Depth to which penetrator is forced by minor load

Surface of specimen

Depth to which penetrator is forced by major load after recovery

Increment in depth due to increment in load is the linear measurement that forms the basis of Rockwell hardness tester readings

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TABLE 3-1

Some Common Rockwell Hardness Tests

Scale Symbol

Penetrator

A

Brale

60

Cemented carbides, thin steel, shallow case-hardened steel

B

1 16

100

Copper alloys, soft steels, aluminum alloys, malleable iron

C

Brale

150

Steel, hard cast irons, titanium, deep case-hardened steel

D

Brale

100

Thin steel, medium case-hardened steel

E

1 8

-in. ball

100

Cast iron, aluminum, magnesium

F

1 16

-in. ball

60

G H

-in. ball 1 8-in ball

150 60

-in. ball

1 16

Load (kg)

Typical Materials

Annealed coppers, thin soft sheet metals Hard copper alloys, malleable irons Aluminum, zinc, lead

features of the Rockwell test. A small indenter, either a hardened steel ball of 1/16, 1/8, 14 or 12-in. in diameter, or a diamond-tipped cone called a brale, is first seated firmly against the material by the application of a 10-kg ‘‘minor’’ load. This causes a slight elastic penetration into the surface and removes the effects of any surface irregularities. The location of the indenter is noted, and ‘‘major’’ load of 60, 100, or 150 kg. is then applied to the indenter to produce a deeper penetration by inducing plastic deformation. When the indenter ceases to move, the major load is removed. With the minor load still applied to hold the indenter firmly in place, the testing machine, like the one shown in Figure 3-15b, now displays a numerical reading. This Rockwell hardness number is really an indication of the distance of indenter travel or the depth of the plastic or permanent penetration that was produced by the major load, with each unit representing a penetration depth of 2 mm. To accommodate a wide range of materials with a wide range of strength, there are 15 different Rockwell test scales, each having a specified major load and indenter geometry. Table 3-1 provides a partial listing of Rockwell scales, which are designated by letters, and some typical materials for which they are used. Because of the different scales, a Rockwell hardness number must be accompanied by the letter corresponding to the particular combination of load and indenter used in its determination. The notation RC60 (or Rockwell C 60), for example, indicates that a 120-degree diamond-tipped brale indenter was used in combination with a major load of 150 kg, and a reading of 60 was obtained. The B and C scales are the most common, with B being used for copper and aluminum and C for steels.4 Rockwell tests should not be conducted on thin materials (typically less than 1.5 mm or 1/16 in.), on rough surfaces, or on materials that are not homogeneous, such as gray cast iron. Because of the small size of the indentation, variations in roughness, composition, or structure can greatly influence test results. For thin materials, or where a very shallow indentation is desired (as in the evaluation of surface-hardening treatments such as nitriding or carburizing), the Rockwell superficial hardness test is preferred. Operating on the same Rockwell principle, this test employs smaller major and minor loads (15 or 45 kg and 3 kg, respectively), and uses a more sensitive depth-measuring device. Fifteen different test configurations are again available, so test results must be accompanied by the specific test designation. In comparison with the Brinell test, the Rockwell test offers the attractive advantage of direct readings in a single step. Because it requires little (if any) surface preparation and can be conducted quite rapidly (up to 300 tests per hour or 5 per minute), it is often used for quality control purposes, such as determining if an incoming product meets specification, ensuring that a heat treatment was performed properly, or simply monitoring the properties of products at various stages of manufacture. It has the additional advantage of producing a small indentation that can be easily concealed on the finished product or easily removed in a later operation. The Rockwell C number is computed as 100 (Depth of penetration in mm/2 mm), while the Rockwell B number is 130 (Depth of penetration in mm/2 mm).

4

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Vickers Hardness Test (also called Diamond Pyramid Hardness). The Vickers hardness test is similar to the Brinell test, but uses a 136-degree square-based diamondtipped pyramid as the indenter and loads between 1 and 120 kg. Like the Brinell value, the Vickers hardness number is also defined as load divided by the surface area of the indentation expressed in units of kilograms per square millimeter. The advantages of the Vickers approach include the increased accuracy in determining the diagonal of a square impression as opposed to the diameter of a circle, and the assurance that even light loads will produce some plastic deformation. The use of diamond as the indenter material enables the test to evaluate any material and effectively places the hardness of all materials on a single scale. Like the other indentation or penetration methods, the Vickers test has a number of attractive features: (1) it is simple to conduct, (2) little time is involved, (3) little surface preparation is required, (4) the marks are quite small and are easily hidden or removed, (5) the test can be done on location, (6) it is relatively inexpensive, and (7) it provides results that can be used to evaluate material strength or assess product quality. FIGURE 3-16 Microindentation hardness tester. (Image used with permission from LECO Corporation)

Microindentation Hardness. Hardness tests have also been developed for applications where the testing involves a very precise area of material, or where the material or modified surface layer is exceptionally thin. These tests were previously called microhardness tests, but the newer microindentation term is more appropriate since it is the size of the indentation that is extremely small, not the measured value of hardness.5 Special machines, such as the one shown in Figure 3-16, have been constructed for this type of testing, which must be performed on specimens with a polished metallographic surface. The location for the test is selected under high magnification. A predetermined load ranging from 25 to 3600 g is then applied through a small diamondtipped penetrator. In the Knoop test, a diamond-shaped indenter with the long diagonal seven times the short diagonal is used, and the length of the indentation is measured under a magnification of about 200 to 400X. Figure 3-17 compares the indenters for the Vickers and Knoop tests, and shows a series of Knoop indentations progressing left-to-right across a surface-hardened steel specimen, from the hardened surface to the unhardened core. The hardness value, known as the Knoop hardness number, is again obtained by dividing the load in kilograms by the projected area of the indentation, expressed in square millimeters. A light-load Vickers test can also be used to determine microindentation hardness.

Vickers

Knoop

136°

172° 30’

136° 130°

(a)

(b)

FIGURE 3-17 (a) Comparison of the diamond-tipped indenters used in the Vickers and Knoop hardness tests; (b) series of Knoop hardness indentations progressing left-to-right across a surface-hardened steel specimen (hardened surface to unhardened core). (Courtesy of Buehler) 5 The ASTM Standard E384 has been renamed ‘‘Standard Test Method for Microindentation Hardness of Materials’’ to reflect this change in nomenclature.

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FIGURE 3-18 Durometer hardness tester. (Courtesy of Newage Testing Instruments, An AMETEK Co.; www .hardnesstesters.com)

Other Hardness Determinations. When testing soft, elastic materials, such as rubbers and nonrigid plastics, a durometer is often be used. This instrument, shown in Figure 3-18, measures the resistance of a material to elastic penetration by a spring-loaded conical steel indenter. No permanent deformation occurs. A similar test is used to evaluate the strength of molding sands used in the foundry industry, and will be described in Chapter 12. In the scleroscope test, hardness is measured by the rebound of a small diamondtipped ‘‘hammer’’ that is dropped from a fixed height onto the surface of the material to be tested. This test evaluates the resilience of a material, and the surface on which the test is conducted must have a fairly high polish to yield good results. Because the test is based on resilience, scleroscope hardness numbers should only be used to compare similar materials. A comparison between steel and rubber, for example, would not be valid. Another definition of hardness is the ability of a material to resist being scratched. A crude but useful test that employs this principle is the file test, where one determines if a material can be cut by a simple metalworking file. The test can be either a pass–fail test using a single file, or a semiquantitative evaluation using a series of files that have been pretreated to various levels of known hardness. This approach is commonly used by geologists. Ten selected materials are used to create a scale that enables the hardness of rocks and minerals to be classified from 0 to 10. Relationships among the Various Hardness Tests. Because the various hardness tests often evaluate different phenomena, there are no simple relationships between the different types of hardness numbers. Approximate relationships have been developed, however, by testing the same material on a variety of devices. Table 3-2 presents a correlation of hardness values for plain carbon and low-alloy steels. It may be noted that for Rockwell C numbers above 20, the Brinell values are approximately 10 times the Rockwell number. Also, for Brinell values below 320, the Vickers and Brinell values agree quite closely. Because the relationships among the various tests will differ with material, mechanical processing, and heat treatment, correlations such as Table 3-2 should be used with caution.

TABLE 3-2

Brinell Number

Hardness Conversion Table for Steels

Vickers Number

Rockwell Number C

B

Tensile Strength

Scleroscope Number

ksi

MPa

940

68

97

368

2537

757a

860

66

92

352

2427

722a

800

64

88

337

2324

686a

745

62

84

324

2234

660a 615a

700 655

60 58

81 78

311 298

2144 2055

559a

595

55

73

276

1903

500

545

52

69

256

1765

475

510

50

67

247

1703

452

485

48

65

238

1641

431

459

46

62

212

1462

410

435

44

58

204

1407

390 370

412 392

42 40

56 53

196 189

1351 1303

350

370

38

110

51

176

1213

341

350

36

109

48

165

1138

321

327

34

108

45

155

1069

302

305

32

107

43

146

1007

(continued)

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SECTION 3.2

Brinell Number

Rockwell Number

Dynamic Properties

Vickers Number

C

B

Scleroscope Number

285

287

30

105

277

279

28

104

262

263

26

248

248

228

73

Tensile Strength ksi

MPa

40

138

951

39

34

924

103

37

128

883

24

102

36

122

841

240

20

98

34

116

800

210

222

17

96

32

107

738

202

213

14

94

30

99

683

192 183

202 192

12 9

92 90

29 28

95 91

655 627

174

182

7

88

26

87

600

166

175

4

86

25

83

572

159

167

2

84

24

80

552

153

162

82

23

76

524

148

156

80

22

74

510

140

148

78

22

71

490

135 131

142 137

76 74

21 20

68 66

469 455

126

132

72

20

64

441

121

121

70

62

427

112

114

66

58

a

Tungsten, carbide ball; others, standard ball.

Relationship of Hardness to Tensile Strength. Table 3-2 and Figure 3-19 show a definite relationship between tensile strength and hardness. For plain carbon and lowalloy steels, the tensile strength (in pounds per square inch) can be estimated by multiplying the Brinell hardness number by 500. In this way, an inexpensive and quick hardness test can be used to provide a close approximation of the tensile strength of the steel. For other materials, however, the relationship is different and may even exhibit too much variation to be dependable. The multiplying factor for age-hardened aluminum is about 600, while for soft brass it is around 800.

280

Tensile strength, 1000 psi

C03

240

200

160

120

FIGURE 3-19 Relationship of hardness and tensile strength for a group of standard alloy steels. (Courtesy of ASM International, Materials Park, OH)

80 0

200 400 600 Brinell hardness

800

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& 3.3 DYNAMIC PROPERTIES Products or components can also be subjected to a wide variety of dynamic loadings. These may include (1) sudden impacts or loads that change rapidly in magnitude, (2) repeated cycles of loading and unloading, or (3) frequent changes in the mode of loading, such as from tension to compression. To select materials and design for these conditions, we must be able to characterize the mechanical properties of engineering materials under dynamic loadings. Most dynamic tests subject standard specimens to a well-controlled set of test conditions. The conditions experienced by actual parts, however, rarely duplicate the controlled conditions of the standardized tests. While identical tests on different materials can indeed provide a comparison of material behavior, the assumption that similar results can be expected for similar conditions may not always be true. Because dynamic conditions can vary greatly, the quantitative results of standardized tests should be used with extreme caution, and one should always be aware of test limitations.

IMPACT TEST Several tests have been developed to evaluate the toughness or fracture resistance of a material when it is subjected to a rapidly applied load, or impact. Of the tests that have become common, two basic types have emerged: (1) bending impacts, which include the standard Charpy and Izod tests, and (2) tension impacts. The bending impact tests utilize specimens that are supported as beams. In the Charpy test, shown schematically in Figure 3-20, the standard specimen is a square bar containing a V-, keyhole-, or U-shaped notch. The test specimen is positioned horizontally, supported on the ends, and an impact is applied to the center, behind the notch, to complete a three-point bending. The Izod test specimen, while somewhat similar in size and appearance, is supported vertically as a cantilever beam and is impacted on the unsupported end, striking from the side of the notch (Figure 3-21). Impact testers, like 55 (2.165⬙) 27.5 (1.083⬙)

27.5 (1.083⬙)

10 (0.394)

2 (0.079)

Blow

10 (0.394)

5 (0.197⬙)

1.58 (1/16⬙) Saw cut 2 (0.079) Drill

5 (0.197) Charpy test

Blow (a)

(b)

FIGURE 3-20 (a) Standard Charpy impact specimens; illustrated are keyhole and U-notches; dimensions are in millimeters with inches in parentheses; (b) standard V-notch specimen showing the three-point bending type of impact loading. 75 (2.95⬙) Blow

28 (1.10⬙) 10 (0.394⬙)

22 (0.866⬙) Blow 8 (0.315⬙)

10 (0.394⬙)

C03

45° 0.25 (0.01⬙) Rad.

Izod test (a)

(b)

FIGURE 3-21 (a) Izod impact specimen; dimensions are in millimeters with inches in parentheses; (b) cantilever mode of loading in the Izod test.

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SECTION 3.3

FIGURE 3-22 Impact testing machine. (Courtesy of Tinius Olsen, Inc., Horsham, PA)

Dynamic Properties

75

the one shown in Figure 3-22, supply a predetermined impact energy in the form of a pendulum swinging from a starting height. After breaking or deforming the specimen, the pendulum continues its upward swing with an energy equal to its original minus that absorbed by the impacted specimen. The loss of energy is measured by the angle that the pendulum attains during its upward swing. The test specimens for bending impacts must be prepared with geometric precision to ensure consistent and reproducible results. Notch profile is extremely critical, because the test measures the energy required to both initiate and propagate a fracture. The effect of notch profile is shown dramatically in Figure 3-23. Here, two specimens have been made from the same piece of steel with the same reduced cross-sectional area. The one with the keyhole notch fractures and absorbs only 43 ft-lb of energy, while the unnotched specimen resists fracture and absorbs 65 ft-lb during the impact. Caution should also be placed on the use of impact data for design purposes. The test results apply only to standard specimens containing a standard notch that have been subjected to very specific test conditions. Changes in the form of the notch, minor variations in the overall specimen geometry, or faster or slower rates of loading (speed of the pendulum) can all produce significant changes in the results. Under conditions of sharp notches, wide specimens, and rapid loading, many ductile materials lose their energyabsorbing capability and fail in a brittle manner. (For example, the standard impact tests should not be used to evaluate materials for bullet-proof armor, because the velocities of loading are extremely different.)

FIGURE 3-23 Notched and unnotched impact specimens before and after testing. Both specimens had the same cross-sectional area, but the notched specimen fractures while the other doesn’t. (E. Paul DeGarmo)

The results of standard tests, however, can be quite valuable in assessing a material’s sensitivity to notches and the multi-axial stresses that exist around a notch. Materials whose properties vary with notch geometry are termed notch-sensitive. Good surface finish and the absence of scratches, gouges and defects in workmanship will be key to satisfactory performance. Materials that are notch-insensitive can often be used with as-cast or rough-machined surfaces with no risk of premature failure. Impact testing can also be performed at a variety of temperatures. As will be seen in a later section of this chapter, the evaluation of how fracture resistance changes with temperature, such as a ductile-to-brittle transition, can be crucial to success when selecting engineering materials for low-temperature service. The tensile impact test, illustrated schematically in Figure 3-24, eliminates the use of a notched specimen, thereby avoiding many of the objections inherent in the Charpy and Izod tests. Turned specimens are subjected to uniaxial impact loadings applied through drop weights, modified pendulums, or variable-speed flywheels.

FATIGUE AND THE ENDURANCE LIMIT FIGURE 3-24 test.

Tensile impact

Materials can also fail by fracture if they are subjected to repeated applications of stress, even though the peak stresses have magnitudes less than the ultimate tensile strength and usually less than the yield strength. This phenomenon, known as fatigue, can result from either the cyclic repetition of a particular loading cycle or entirely random

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Properties of Materials Specimen

FIGURE 3-25 Schematic diagram of a Moore rotatingbeam fatigue machine. (Adapted from Hayden et al., The Structure and Properties of Materials, Vol. 3, p. 15, Wiley, 1965)

Revolution counter

Motor

Weight

variations in stress. Almost 90% of all metallic fractures are in some degree attributed to fatigue. For experimental simplicity, a periodic, sinusoidal loading is often utilized, and conditions of equal-magnitude tension–compression reversals provide further simplification. These conditions can be achieved by placing a placing a cylindrical specimen in a rotating drive and hanging a weight so as to produce elastic bending along the axis, as shown in Figure 3-25. Material at the bottom of the specimen is stretched, or loaded in tension, while material on the top surface is compressed. As the specimen turns, the surface of the specimen experiences a sinusoidal application of tension and compression with each rotation. By conducting multiple tests, subjecting identical specimens to different levels of maximum loading and recording the number of cycles necessary to achieve fracture, curves such as that in Figure 3-26 can be produced. These curves are known as stress versus number of cycles, or S–N curves. If the material being evaluated in Figure 3-26 were subjected to a standard tensile test, it would require a stress in excess of 480 MPa (70,000 psi) to induce failure by fracture. Under cyclic loading with a peak stress of only 380 MPa (55,000 psi), the specimen will fracture after about 100,000 cycles. If the peak stress were further reduced to 350 MPa (51,000 psi), the fatigue lifetime would be extended by an order of magnitude to approximately 1,000,000 cycles. With a further reduction to any value below 340 MPa (49,000 psi), the specimen would not fail by fatigue, regardless of the number of stress application cycles. The stress below which the material will not fail regardless of the number of load cycles is known as the endurance limit or endurance strength, and may be an important criterion in many designs. Above this value, any point on the curve is the fatigue strength, the maximum stress that can be sustained for a specified number of loading cycles. A different number of loading cycles is generally required to determine the endurance limit for different materials. For steels, 10 million cycles are usually sufficient. For several of the nonferrous metals, 500 million cycles may be required. For aluminum, the curve continues to drop such that if aluminum has an endurance limit, it is at such a low value that a cheaper and much weaker material could be used. In essence, if aluminum is used under realistic stresses and cyclic loading, it will fail by fatigue after a finite lifetime.

psi*103 70

MPa 500

Stress

C03

FIGURE 3-26 Typical S–N curve for steel showing an endurance limit. Specific numbers will vary with the type of steel and treatment.

60

400

50 300 200 104

40

105

106 Cycles

107

30 108

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SECTION 3.3 MPa 600

Dynamic Properties

77

ksi 80

Stress

C03

FIGURE 3-27 Fatigue strength of Inconel alloy 625 at various temperatures. (Courtesy of Huntington Alloy Products Division, The International Nickel Company, Inc., Toronto, Canada)

500

29⬚C

400

649⬚C

60

760⬚C 300

427⬚C 538⬚C

871⬚C

40

200 100 103

20 104

105 106 Cycles to failure

107

108

The fatigue resistance of an actual product is sensitive to a number of additional factors. One of the most important of these is the presence of stress raisers (or stress concentrators), such as sharp corners, small surface cracks, machining marks, or surface gouges. Data for the S–N curves are obtained from polished-surface, ‘‘flaw-free’’ specimens, and the reported lifetime is the cumulative number of cycles required to initiate a fatigue crack and then grow or propagate it to failure. If a part already contains a surface crack or flaw, the number of cycles required for crack initiation can be reduced significantly. In addition, the stress concentrator magnifies the stress experienced at the tip of the crack, accelerating the rate of subsequent crack growth. Great care should be taken to eliminate stress raisers and surface flaws on parts that will be subjected to cyclic loadings. Proper design and good manufacturing practices are often more important than material selection and heat treatment. Operating temperature can also affect the fatigue performance of a material. Figure 3-27 shows S–N curves for Inconel 625 (a high-temperature Ni–Cr–Fe alloy) determined over a range of temperatures. As temperature is increased, the fatigue strength drops significantly. Because most test data are generated at room temperature, caution should be exercised when the product application involves elevated service temperatures. Fatigue lifetime can also be affected by changes in the environment. When metals are subjected to corrosion during the cyclic loadings, the condition is known as corrosion fatigue, and both specimen lifetime and the endurance limit can be significantly reduced. Moreover, the nature of the environmental attack need not be severe. For some materials, tests conducted in air have been shown to have shorter lifetimes than those run in a vacuum, and further lifetime reductions have been observed with increasing levels of humidity. The test results can also be dependent on the frequency of the loading cycles. For slower frequencies, the environment has a longer time to act between loadings. At high frequencies, the environmental effects may be somewhat masked. The application of test data to actual products, therefore, requires considerable caution. Residual stresses can also alter fatigue behavior. If the specimen surface is in a state of compression, such as that produced from shot peening, carburizing, or burnishing, it is more difficult to initiate a fatigue crack, and lifetime is extended. Conversely, processes that produce residual tension on the surface, such as welding or machining, can significantly reduce the fatigue lifetime of a product. If the magnitude of the load varies during service, the fatigue response can be extremely complex. For example, consider the wing of a commercial airplane. As the wing vibrates during flight, the wing-fuselage joint is subjected to a large number of low-stress loadings. While large in number, these in-flight loadings may be far less damaging than a few high-stress loadings, like those that occur when the plane impacts the runway during landing. From a different perspective, however, the heavy loads may be sufficient to stretch and blunt a sharp fatigue crack, requiring many additional smallload cycles to ‘‘reinitiate’’ it. Evaluating how materials respond to complex patterns of loading is an area of great importance to design engineers. Because reliable fatigue data may take a considerable time to generate, we may prefer to estimate fatigue behavior from properties that can be determined more

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TABLE 3-3

Properties of Materials

Ratio of Endurance Limit to Tensile Strength for Various Materials

Material

Ratio

Aluminum

0.38

Beryllium copper (heat-treated)

0.29

Copper, hard Magnesium Steel AISI 1035 Screw stock AISI 4140 normalized Wrought iron

quickly. Table 3-3 shows the approximate ratio of the endurance limit to the ultimate tensile strength for several engineering metals. For many steels the endurance limit can be approximated by 0.5 times the ultimate tensile strength as determined by a standard tensile test. For the nonferrous metals, however, the ratio is significantly lower.

FATIGUE FAILURES

Components that fail as a result of repeated or cyclic loadings are commonly called fatigue failures. These fractures form a major part of a larger 0.38 group known as progressive fractures. Consider the fracture surfaces shown in Figure 3-28. Arrows identify the points of fracture initiation, 0.46 which often correspond to discontinuities in the form of surface cracks, 0.44 sharp corners, machining marks, or even ‘‘metallurgical notches,’’ such as 0.54 an abrupt change in metal structure. With each repeated application of 0.63 load, the stress at the tip of the crack exceeds the strength of the material, and the crack grows a very small amount. Crack growth continues with each successive application of load until the remaining cross section is no longer sufficient to withstand the peak stresses. Sudden overload fracture then occurs through the remainder of the material. The overall fracture surface tends to exhibit two distinct regions: a smooth, relatively flat region where the crack was propagating by cyclic fatigue and a fibrous, irregular, or ragged region that corresponds to the sudden overload tearing. The size of the overload region reflects the area that must be intact in order to support the highest applied load. In Figure 3-28a, the applied load is high and only a small fatigue area is necessary to reduce the specimen to the point of overload fracture. In the b section of the figure, the fatigue fracture propagates about halfway through before sudden overload occurs. The smooth areas of the fracture often contain a series of parallel ridges radiating outward from the origin of the crack. These ridges may not be visible under normal examination, however. They may be extremely fine, they may have been obliterated by a rubbing action during the compressive stage of repeated loading, or they may be very few in number if the failure occurred after only a few cycles of loading (low-cycle fatigue). Electron microscopy may be required to reveal the ridges, or fatigue striations, 0.33

FIGURE 3-28 Fatigue fractures with arrows indicating the points of fracture initiation, the regions of fatigue crack propagation, and the regions of sudden overload or fast fracture. (a) High applied load results in a small fatigue region compared to the area of overload fracture; (b) low applied load results in a large area of fatigue fracture compared to the area of overload fracture. NOTE: The overload area is the minimum area required to carry the applied loads. (From ‘‘Fatigue Crack Propagation,’’ an article publshed in the May 2008 issue of 1Advanced Materials & Processes magazine, Reprinted with permission of ASM International . All rights reserved. www.asminternational.org)

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SECTION 3.4

Temperature Effects (Both High and Low)

79

FIGURE 3-29 Fatigue fracture of AISI type 304 stainless steel viewed in a scanning electron microscope at 810. Welldefined striations are visible. (From ‘‘Interpretation of SEM Fractographs,’’ Metals Handbook Vol. 9, 8th ed. p.70. Reprinted with permission of ASM International 1 . All rights reserved. www .asminternational.org)

that are characteristic of fatigue failure. Figure 3-29 shows an example of these markings at high magnification. Larger marks, known as beach marks may appear on the fatigued surface, lying parallel to the striations. These can be caused by interruptions to the cyclic loadings, changes in the magnitude of the applied load, and isolated overloads (not sufficient to cause ultimate fracture). Ratchet marks, or offset steps, can appear on the fracture surface if multiple fatigue cracks nucleate at different points and grow together. For some fatigue failures, the overload area may exhibit a crystalline appearance, and the failure is sometimes attributed to the metal having ‘‘crystallized.’’ As will be noted in Chapter 4, engineering metals are almost always crystalline materials. The final overload fracture simply propagated along the intercrystalline surfaces (grain boundaries), revealing the already-existing crystalline nature of the material. The conclusion that the material failed because it crystallized is totally erroneous, and the term is a definite misnomer. Another common error is to classify all progressive-type failures as fatigue failures. Other progressive failure mechanisms, such as creep failure and stress–corrosion cracking, will also produce the characteristic two-region fracture. In addition, the same mechanism can produce fractures with different appearances depending on the magnitude of the load, type of loading (torsion, bending, or tension), temperature, and operating environment. Correct interpretation of a metal failure generally requires far more information than that acquired by a visual examination of the fracture surface. A final misconception regarding fatigue failures is to assume that the failure is time dependent. The failure of materials under repeated loads below their static strength is primarily a function of the magnitude and number of loading cycles. If the frequency of loading is increased, the time to failure should decrease proportionately. If the time does not change, the failure is dominated by one or more environmental factors, and fatigue is a secondary component.

& 3.4 TEMPERATURE EFFECTS (BOTH HIGH AND LOW) The test data used in design and engineering decisions should always be obtained under conditions that simulate those of actual service. A number of engineered structures, such as aircraft, space vehicles, gas turbines, and nuclear power plants, are required to operate under temperatures as low as 130 C (200 F) or as high as 1250 C (2300 F). To cover these extremes, the designer must consider both the short- and long-range effects of temperature on the mechanical and physical properties of the material being considered. From a manufacturing viewpoint, the effects of temperature are equally important. Numerous manufacturing processes involve heat, and the elevated temperature and processing may alter the material properties in both favorable and unfavorable ways. A material can often be processed successfully, or economically, only because heating or cooling can be used to change its properties.

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CHAPTER 3

Properties of Materials MPa

psi*103 0

120 600

200

Temperature (°C) 400

100

600

120 100

Tensile strength

Stress

80 400

80

60

60

Yield point

40

200

40

20

FIGURE 3-30 The effects of temperature on the tensile properties of a medium-carbon steel.

20 Elongation

Elongation (%)

05/18/2011

200

400

600

800

1000

1200

0 1400

Temperature (°F)

Elevated temperatures can be quite useful in modifying the strength and ductility of a material. Figure 3-30 summarizes the results of tensile tests conducted over a wide range of temperatures using a medium-carbon steel. Similar effects are presented for magnesium in Figure 3-31. As expected, an increase in temperature will typically induce a decrease in strength and hardness and an increase in elongation. For manufacturing operations such as metal forming, heating to elevated temperature may be extremely attractive because the material is now both weaker and more ductile. Figure 3-32 shows the combined effects of temperature and strain rate (speed of testing) on the ultimate tensile strength of copper. For a given temperature, the rate of deformation can also have a strong influence on mechanical properties. Room-temperature

psi*103 50

Stress

300

200

300

Tensile strength

80

30

Elongation

60 40

20 Yield strength

10

20

100

200

300

400

500

Temperature (°F) 50

FIGURE 3-32 The effects of temperature and strain rate on the tensile strength of copper. (From A. Nadai and M. J. Manjoine, Journal of Applied Mechanics, Vol. 8, 1941, p. A82, courtesy of ASME)

40 Room temperature 30 200°C

20

400°C 600°C 800°C

10

10-6

100

40

100

FIGURE 3-31 The effects of temperature on the tensile properties of magnesium.

Temperature (°C) 100 200

10-4

10-2 Strain rate, sec-1

1000°C 1

102

0 600

Elongation in 2 in. (%)

MPa

Tensile strength, 1000 psi

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SECTION 3.4 - 40

40

80

Temperature Effects (Both High and Low)

°F

81

standard-rate tensile test data will be of little value if the application involves a material being hot-rolled at speeds of 1300 m/min (5000 ft/min). 60 The effect of temperature on impact properties became the subject 40 of intense study in the 1940s when the increased use of welded-steel con50 struction led to catastrophic failures of ships and other structures while 30 40 operating in cold environments. Welding produces a monolithic (singlepiece) product where cracks can propagate through a joint and continue 30 20 on to other sections of the structure! Figure 3-33 shows the effect of 20 decreasing temperature on the impact properties of two low-carbon 10 steels. Although similar in form, the two curves are significantly differ10 -40 - 20 0 20 40° C ent. The steel indicated by the solid line becomes brittle (requires very Temperature little energy to fracture) at temperatures below 4 C (25 F) while the FIGURE 3-33 The effect of temperature on the other steel retains good fracture resistance down to 26 C (15 F). The temperature at which the toughness goes from high-energy absorption to impact properties of two low-carbon steels. low energy absorption is known as the ductile-to-brittle transition temperature. As the temperature changes across this transition, the fracture appearance also changes. At high temperatures, ductile fracture occurs, and the specimen deforms in the region of ultimate fracture. At low temperatures, the fracture is brittle in nature, and the fracture area retains the original square shape. A plot of percent shear in the fracture surface (ductile fracture) versus temperature results in a curve similar to that for toughness, and the 50% point is reported as the fracture appearance transition temperature (FATT). Figure 3-34 shows the ductile-to-brittle transition temperature for steel salvaged from the R.M.S. Titanic along with data from currently used ship plate material. While both are quality materials for their era, the Titanic steel has a much higher transition temperature and is generally more brittle. The Titanic struck an iceberg in salt water. The water temperature at the time of the accident was 2 C, and the results show that the steel would have been quite brittle. All steels tend to exhibit a ductile-to-brittle transition, but the temperature at which it occurs varies with carbon content and alloy. Metals such as aluminum, copper, and some types of stainless steel do not have a ductile-to-brittle transition and can be used at low temperatures with no significant loss of toughness. Two separate curves are provided for each of the steels in Figure 3-34, reflecting test specimens cut in different orientation with respect to the direction of product rolling. Here, we see that processing features can further affect the properties and performance of a material. Because the performance properties can vary widely with the type of material, chemistry variations within the class of material, and prior processing, special cautions should be taken when selecting materials for low-temperature applications. 70

50

ft-lbs

Energy absorbed – joules

350 Longitudinal 300

FIGURE 3-34 Notch toughness impact data: steel from the Titanic versus modern steel plate for both longitudinal and transverse specimens. (Courtesy I&SM, September 1999, p. 33, Iron and Steel Society, Warrendale, PA)

Absorbed energy (J)

C03

250 Modern hull plate 200 Transverse

150

Longitudinal

100

Titanic plate 50 Transverse 0 -100

-50

50 Test temperature (°C)

100

150

200

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CHAPTER 3

Properties of Materials Fracture

⌱⌱

MPa 1000

⌱⌱⌱

ksi

d␧ = creep rate dt

Stress

100

Slope =

10

␧o 0

1 1

10

Time, t

FIGURE 3-35 Creep curve for a single specimen at a fixed elevated temperature, showing the three stages of creep and reported creep rate. Note the nonzero strain at time zero due to the initial application of the load.

100

538°C (1000° F) 649°C (1 200°F) 760°C (1400° F) 871°C (1600 ° F ) 982°C (1800 °F) 1093° C (200 0°F)

A Strain, ␧

1,000 100 Rupture life, hr

10

1

0.1 10,000 100,000

FIGURE 3-36 Stress–rupture diagram of solutionannealed Incoloy alloy 800 (Fe–Ni–Cr alloy). (Courtesy of Huntington Alloy Products Division, The International Nickel Company, Inc., Toronto, Canada)

CREEP Long-term exposure to elevated temperatures can also lead to failure by a phenomenon known as creep. If a tensile-type specimen is subjected to a constant load at elevated temperature, it will elongate continuously until rupture occurs, even though the applied stress is below the yield strength of the material at the temperature of testing. While the rate of elongation is often quite small, creep can be an important consideration when designing equipment such as steam or gas turbines, power plant boilers, and other devices that operate under loads or pressures for long periods of time at high temperature. If a test specimen is subjected to conditions of fixed load and fixed elevated temperature, an elongation-versus-time plot can be generated, similar to the one shown in Figure 3-35. The curve contains three distinct stages: a short-lived initial stage, a rather long second stage where the elongation rate is somewhat linear, and a short-lived third stage leading to fracture. Two significant pieces of engineering data are obtained from this curve: the rate of elongation in the second stage, or creep rate, and the total elapsed time to rupture. These results are unique to the material being tested and the specific conditions of the test. Tests conducted at higher temperatures or with higher applied loads would exhibit higher creep rates and shorter rupture times. When creep behavior is a concern, multiple tests are conducted over a range of temperatures and stresses, and the rupture time data are collected into a single stress–rupture diagram, like the one shown in Figure 3-36. This simple engineering tool provides an overall picture of material performance at elevated temperature. In a similar manner, creep rate data can also be plotted to show the effects of temperature and stress. Figure 3-37 presents a creep-rate diagram for the same high-temperature nickel-base alloy. MPa 1000

100 Stress

C03

10

FIGURE 3-37 Creep-rate properties of solution-annealed Incoloy alloy 800. (Courtesy of Huntington Alloy Products Division, The International Nickel Company, Inc., Toronto, Canada)

ksi

538°C (1000°F) 200°F) 649°C (1 F) (1400° 760°C 600°F) 871°C (1 800°F) 982°C (1

100

10

C 1093°

°F)

(2000

1 0.00001 0.0001

0.01 0.001 Creep rate, %/hr

0.1

1

0.1 1.0

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SECTION 3.6

Fracture Toughness and the Fracture Mechanics Approach

83

& 3.5 MACHINABILITY, FORMABILITY, AND WELDABILITY While it is common to assume that the various ‘‘-ability’’ terms also refer to specific material properties, they actually refer to the way a material responds to specific processing techniques. As a result, they can be quite nebulous. Machinability, for example, depends not only on the material being machined but also on the specific machining process, the conditions of that process (such as cutting speed), and the aspects of that process that are of greatest interest. Machinability ratings are frequently based on relative tool life. In certain applications, however, we may be more interested in how easy a metal is to cut, or how it performs under high machining speeds, and less interested in the tool life or the resulting surface finish. For other applications, surface finish or the formation of fine chips may be the most desirable feature. A material with high machinability to one individual may be considered to have poor machinability by a person using a different process or different process conditions. In a similar manner, malleability, workability, and formability all refer to a material’s suitability for plastic deformation processing. Because a material often behaves differently at different temperatures, a material with good ‘‘hot formability’’ may have poor deformation characteristics at room temperature. Furthermore, materials that flow nicely at low deformation speeds may behave in a brittle manner when loaded at rapid rates. Formability, therefore, needs to be evaluated for a specific combination of material, process, and process conditions, and the results should not be extrapolated or transferred to other processes or process conditions. Likewise, the weldability of a material will also depend on the specific welding or joining process and the specific process parameters.

& 3.6 FRACTURE TOUGHNESS AND THE FRACTURE MECHANICS APPROACH A discussion of the mechanical properties of materials would not be complete without mention of the many tests and design concepts based on the fracture mechanics approach. Instead of treating test specimens as flaw-free materials, fracture mechanics begins with the premise that all materials contain flaws or defects of some given size. These may be material defects, such as pores, cracks, or inclusions; manufacturing defects, in the form of machining marks, arc strikes, or contact damage to external surfaces; or design defects, such as abrupt section changes, excessively small fillet radii, and holes. When the specimen is subjected to loads, the applied stresses are amplified or intensified in the vicinity of these defects, potentially causing accelerated failure or failure under unexpected conditions. Fracture mechanics seeks to identify the conditions under which a defect will grow or propagate to failure and, if possible, the rate of crack or defect growth. The methods concentrate on three principal quantities: (1) the size of the largest or most critical flaw, usually denoted as a; (2) the applied stress, denoted by s; and (3) the fracture toughness, a quantity that describes the resistance of a material to fracture or crack growth, which is usually denoted by K with subscripts to signify the conditions of testing. Equations have been developed that relate these three quantities at the onset of crack growth or propagation for various specimen geometries, flaw locations, and flaw orientations. If nondestructive testing or quality control methods have been applied, the size of the largest flaw that could go undetected is often known. By mathematically placing this worst possible flaw in the worst possible location and orientation, and coupling this with the largest applied stress for that location, a designer can determine the value of fracture toughness necessary to prevent that flaw from propagating during service. Specifying any two of the three parameters allows the computation of the third. If the material and stress conditions were defined, the size of the maximum permissible flaw could be computed. Inspection conditions could then be selected to ensure that flaws greater than this magnitude are cause for product rejection. Finally, if a component is found to have a significant flaw and the material is known, the maximum operating stress can be determined that will ensure no further growth of that flaw. In the past, detection of a flaw or defect was usually cause for rejection of the part (Detection ¼ Rejection). With enhanced methods and sensitivities of inspection,

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almost every product can now be shown to contain flaws. Fracture mechanics comes to the rescue. According to the philosophy of fracture mechanics, each of the flaws or defects in a material can be either dormant or dynamic. Dormant defects are those whose size remains unchanged through the lifetime of the part, and are indeed permissible. A major goal of fracture mechanics, therefore, is to define the distinction between dormant and dynamic for the specific conditions of material, part geometry, and applied pffiffiffiffiffiffi loading. The basic equation of fracture mechanics assumes the form of K as pa, where K is the fracture toughness of the material (a material property); s is the maximum applied tensile stress; a is the size of the largest or most critical flaw; and a is a dimensionless factor that considers the flaw location, orientation, and shape. The left side of the equation considers the material and the right side describes the usage condition (a combination of flaw and loading). The relationship is usually described as a greater than or equal. When the material number, K, is greater than the usage condition, the flaw is dormant. When equality is reached, the flaw becomes dynamic, and crack growth or fracture occurs. Alternative efforts to prevent material fracture generally involve overdesign, excessive inspection, or the use of premium-quality materials— all of which increase cost and possibly compromise performance. Fracture mechanics can also be applied to fatigue, which has already been cited as causing as much as 90% of all dynamic failures. The standard method of fatigue testing applies cyclic loads to polished, ‘‘flaw-free’’ specimens, and the reported lifetime includes both crack initiation and crack propagation. In contrast, fracture mechanics focuses on the growth of an already existing flaw. Figure 3-38 shows the crack growth Stress intensity factor range, ⌬K, ksi in. 10

20

50

100

10- 2

10-4

Region 1: slow crack growth

10-5 da = C (⌬K)n dN

10- 4

Region 3: rapid, unstable crack growth

10-6

Region 2: power-law behavior

10-7

10- 5

10- 6

FIGURE 3-38 Plot of the fatigue crack growth rate versus DK for a typical steel—the fracture mechanics approach. Similar shape curves are obtained for most engineering metals. (Courtesy of ASM International, Materials Park, OH)

10-8

10- 7

6

8

10

20

30

40 50 60

Stress intensity factor range, ⌬K, MPa m

80 100

Crack growth rate, da/dN, in./cycle

10- 3 Crack growth rate, da/dN, mm/cycle

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Testing Standards and Testing Concerns

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rate (change in size per loading cycle denoted as da/dN) plotted as a function of the fracture mechanics parameter, DK (where DK increases with an increase in either the flaw size and/or the magnitude of applied stress). Because the fracture mechanics approach begins with an existing flaw, it provides a far more realistic guarantee of minimum service life. Fracture mechanics is a truly integrated blend of design (applied stresses), inspection (flaw-size determination), and materials (fracture toughness). The approach has proven valuable in many areas where fractures could be catastrophic.

& 3.7 PHYSICAL PROPERTIES For certain applications, the physical properties of a material may be even more important than the mechanical. These include the thermal, electrical, magnetic, and optical characteristics. We have already seen several ways in which the mechanical properties of materials change with variations in temperature. In addition to these effects, there are some truly thermal properties that should be considered. The heat capacity or specific heat of a material is the amount of energy that must be added to or removed from a given mass of material to produce a 1-degree change in temperature. This property is extremely important in processes such as casting, where heat must be extracted rapidly to promote solidification, or heat treatment, where large quantities of material are heated and cooled. Thermal conductivity measures the rate at which heat can be transported through a material. While this may be tabulated separately in reference texts, it is helpful to remember that for metals, thermal conductivity is directly proportional to electrical conductivity. Metals such as copper, gold, and aluminum that possess good electrical conductivity are also good transporters of thermal energy. Thermal expansion is another important thermal property. Most materials expand upon heating and contract upon cooling, but the amount of expansion or contraction will vary with the material. For components that are machined at room temperature but put in service at elevated temperatures, or castings that solidify at elevated temperatures and then cool to room temperature, the as-manufactured dimensions must be adjusted to compensate for the subsequent changes. Electrical conductivity or electrical resistivity may also be an important design consideration. These properties will vary not only with the material, but also with the temperature and the way the material has been processed. The magnetic response of materials can be classified as diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, or ferrimagnetic. These terms refer to the way in which the material responds to an applied magnetic field. Material properties, such as saturation strength, remanence, and magnetic hardness or softness, describe the strength, duration, and nature of this response. Still other physical properties that may assume importance include weight or density, melting and boiling points, and the various optical properties, such as the ability to transmit, absorb, or reflect light or other electromagnetic radiation.

& 3.8 TESTING STANDARDS AND TESTING CONCERNS When evaluating the mechanical and physical properties of materials, it is important that testing be conducted in a consistent and reproducible manner. ASTM International, formerly the American Society of Testing and Materials, maintains and updates many testing standards, and it is important to become familiar with their contents. For example, ASTM specification E370 describes the ‘‘Standard Test Methods and Definitions for Mechanical Testing of Steel Products.’’ Tensile testing is described in specifications E8 and E83, impact testing in E23, creep in E139, and penetration hardness in E10. Other specifications describe fracture mechanics testing, as well as procedures to evaluate corrosion resistance, compressive strength, shear strength, torsional properties, and corrosion-fatigue. In addition, it is important to note not only the material being tested, but also the location from which the specimen was taken and its orientation. Rolled sheet, rolled

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plate and rolled bars, for example, will have different properties when tested parallel to the direction of rolling (longitudinal) and perpendicular to the rolling direction (transverse). See Figure 3-34 for an example. This variation of properties with direction is known as anisotropy and may be crucial to the success or failure of a product.

& KEY WORDS ASTM International anisotropy brale breaking strength Brinell hardness number (BHN) Brinell hardness test brittle Charpy test compression crack growth rate (da/dN) creep creep rate damping design defects dormant flaw ductile-to-brittle transition temperature (DBTT) ductility durometer dynamic flaw dynamic properties elastic limit electrical conductivity electrical resistivity elongation endurance limit engineering strain

engineering stress– engineering strain curve engineering stress fatigue fatigue failure fatigue strength fatigue striations file test formability fracture appearance transition temperature (FATT) fracture strength fracture toughness gage length hardness heat capacity Hooke’s Law Izod test impact test Knoop hardness Knoop test machinability magnetic response malleability manufacturing manufacturing defects material defects mechanical properties

metal microhardness tests microindentation hardness testing modulus of elasticity modulus of resilience necking nonmetal notch-sensitive notch-insensitive offset yield strength percent elongation percent reduction in area performance physical properties plastic deformation processing properties proportional limit rate of deformation resilience Rockwell hardness test S–N curve scleroscope test specific heat static properties stiffness strain

strain hardening strain-hardening exponent strain rate stress stress–rupture diagram structure tensile strain tensile strength tensile test tensile impact test thermal conductivity thermal expansion time to rupture (rupture time) toughness transition temperature true strain true stress ultimate tensile strength uniaxial tensile test uniform elongation unit strain Vickers hardness test weldability workability yield point Young’s modulus

& REVIEW QUESTIONS 1. What eras in the history of man have been linked to materials? 2. Knowledge of what four aspects is critical to the successful application of a material in an engineering design? 3. Give an example of how we might take advantage of a material that has a range of properties. 4. What are some properties commonly associated with metallic materials? 5. What are some of the more common nonmetallic engineering materials? 6. What are some of the important physical properties of materials? 7. Why should caution be exercised when applying the results from any of the standard mechanical property tests? 8. What are the standard units used to report stress and strain in the English system? In the metric or SI system? 9. What are static properties? 10. What is the most common static test to determine mechanical properties? 11. Why might Young’s modulus or stiffness be an important material property? 12. What are some of the tensile test properties that are used to describe or define the elastic-to-plastic transition in a material?

13. Why is it important to specify the ‘‘offset’’ when providing yield strength data? 14. During the plastic deformation portion of a tensile test, a cylindrical specimen first maintains its cylindrical shape (increasing in length and decreasing in diameter) then transitions into a state called ‘‘necking.’’ What is the explanation for this behavior? 15. What are two tensile test properties that can be used to describe the ductility of a material? 16. Is a brittle material a weak material? What does brittleness mean? 17. What is the toughness of a material? 18. What is the difference between true stress and engineering stress? True strain and engineering strain? 19. Explain how the plastic portion of a true stress–true strain curve can be viewed as a continuous series of yield strength values. 20. What is strain hardening or work hardening? How might this phenomenon be measured or reported? How might it be used in manufacturing? 21. How might tensile test data be misleading for a ‘‘strain rate sensitive’’ material?

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Problems 22. What are some of the different material characteristics or responses that have been associated with the term hardness? 23. What are the similarities and differences between the Brinell and Rockwell hardness tests? 24. Why are there different Rockwell hardness scales? 25. When might a microhardness test be preferred over the more-standard Brinell or Rockwell tests? 26. Why might the various types of hardness tests fail to agree with one another? 27. What is the relationship between penetration hardness and the ultimate tensile strength for steel? 28. Describe several types of dynamic loading. 29. Why should the results of standardized dynamic tests be applied with considerable caution? 30. What are the two most common types of bending impact tests? How are the specimens supported and loaded in each? 31. What aspects or features can significantly alter impact data? 32. What is ‘‘notch sensitivity,’’ and how might it be important in the manufacture and performance of a product? 33. Which type of failure accounts for almost 90% of metal failures? 34. What is the endurance limit? What occurs when stresses are above it? Below it? 35. Are the stresses applied during a fatigue test above or below the yield strength (as determined in a tensile test)? 36. What features may significantly alter the fatigue lifetime or fatigue behavior of a material? 37. What relationship can be used to estimate the endurance limit of a steel? 38. What material, design, or manufacturing features can contribute to the initiation of a fatigue crack? 39. How might the relative sizes of the fatigue region and the overload region provide useful information about the design of the product?

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40. What are fatigue striations, and why do they form? 41. Why is it important for a designer or engineer to know a material’s properties at all possible temperatures of operation? 42. Why should one use caution when using steel at low (below zero Fahrenheit) temperature? 43. How might the orientation of a piece of metal (with respect to its rolling direction) affect properties such as fracture resistance? 44. How might we evaluate the long-term effect of elevated temperature on an engineering material? 45. What is a stress–rupture diagram, and how is one developed? 46. Why are terms such as machinability, formability, and weldability considered to be poorly defined and therefore quite nebulous? 47. What is the basic premise of the fracture mechanics approach to testing and design? 48. What are some of the types of flaws or defects that might be present in a material? 49. What three principal quantities does fracture mechanics attempt to relate? 50. What is a dormant flaw? A dynamic flaw? How do these features relate to the former ‘‘Detection ¼ Rejection’’ criteria for product inspection? 51. What are the three most common thermal properties of a material, and what do they measure? 52. Describe an engineering application where the density of the selected material would be an important material consideration. 53. Why is it important that property testing be performed in a standardized and reproducible manner? 54. Why is it important to consider the orientation of a test specimen with respect to the overall piece of material?

& PROBLEMS 1. Select a product or component for which physical properties are more important than mechanical properties. a. Describe the product or component and its function. b. What are the most important properties or characteristics? c. What are the secondary properties or characteristics that would also be desirable? 2. Repeat Problem 1 for a product or component whose dominant required properties are of a static mechanical nature. 3. Repeat Problem 1 for a product or component whose dominant requirements are dynamic mechanical properties. 4. One of the important considerations when selecting a material for an application is to determine the highest and lowest operating temperature along with the companion properties that must be present at each extreme. The ductile-to-brittle transition temperature, discussed in Section 3.4, has been an important factor in a number of failures. An article that summarized the features of 56 catastrophic brittle fractures that made headline news between 1888 and 1956 noted that low temperatures were present in nearly every case. The water temperature at the time of the sinking of the Titanic was above the freezing point for salt water but below the

transition point for the steel used in construction of the hull of the ship. a. Which of the common engineering materials exhibits a ductile-to-brittle transition? b. For plain carbon and low-alloy steels, what is a typical value (or range of values) for the transition temperature? c. What type of material would you recommend for construction of a small vessel to transport liquid nitrogen within a building or laboratory? d. Figure 3-34 summarizes the results of impact testing performed on hull plate from the R.M.S. Titanic and similar material produced for modern steel-hulled ships. Why should there be a difference between specimens cut longitudinally (along the rolling direction) and transversely (across the rolling direction)? What advances in steel making have led to the significant improvement in low-temperature impact properties? 5. Several of the property tests described in this chapter produce results that are quite sensitive to the presence or absence of notches or other flaws. The fracture mechanics approach to materials testing incorporates flaws into the tests

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and evaluates their performance. The review article mentioned in Problem 4 cites the key role of a flaw or defect in nearly all of the headline-news fractures. a. What are some of the various ‘‘flaws or defects’’ that might be present in a product? Consider flaws that might be present in the starting material; flaws that might be introduced during manufacture; and flaws that might occur due to shipping, handling, use, maintenance, or repair.

Chapter 3

b. What particular properties might be most sensitive to flaws or defects? c. Discuss the relationship of flaws to the various types of loading (tension versus compression, torsion, shear). d. Fracture mechanics considers both surface and interior flaws and assigns terms such as ‘‘crack initiator,’’ ‘‘crack propagator,’’ and ‘‘crack arrestor.’’ Briefly discuss why location and orientation may be as important as the physical size of a flaw.

CASE STUDY

Separation of Mixed Materials

B

ecause of the amount of handling that occurs during material production, within warehouses, and during manufacturing operations, along with loading, shipping, and unloading, material mix-ups and mixed materials are not an uncommon occurrence. Mixed materials also occur when industrial scrap is collected, or when discarded products are used as new raw materials through recycling. Assume that you have equipment to perform each of the tests described in this chapter (as well as access to the full spectrum of household and department store items and even a small machine shop). For each of the following material combinations, determine one or more procedures that would permit separation of the mixed materials. Use standard datasource references to help identify distinguishable properties.

1. Steel and aluminum cans that have been submitted for recycling 2. Stainless steel sheets of Type 430 ferritic stainless and Type 316 austenitic stainless. 3. 6061-T6 aluminum and AZ91 magnesium that have become mixed in a batch of machine shop scrap. 4. Transparent bottles of polyethylene and polypropylene (both thermoplastic polymers) that have been collected for recycling. 5. Hot-rolled bars of AISI 1008 and 1040 steel. 6. Hot-rolled bars of AISI 1040 (plain-carbon) steel and 4140 steel (a molybdenum-containing alloy) 7. Mixed plastic consisting of recyclable thermoplastic polyvinylchloride (PVC) and nonrecyclable polyester—as might occur from automotive dashboards, consoles, and other interior components.

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CHAPTER 4 NATURE OF METALS AND ALLOYS 4.1 STRUCTURE–PROPERTY– PROCESSING–PERFORMANCE RELATIONSHIPS 4.2 THE STRUCTURE OF ATOMS 4.3 ATOMIC BONDING 4.4 SECONDARY BONDS 4.5 ATOM ARRANGEMENTS IN MATERIALS 4.6 CRYSTAL STRUCTURES OF METALS

4.7 DEVELOPMENT OF A GRAIN STRUCTURE 4.8 ELASTIC DEFORMATION 4.9 PLASTIC DEFORMATION 4.10 DISLOCATION THEORY OF SLIPPAGE 4.11 STRAIN HARDENING OR WORK HARDENING 4.12 PLASTIC DEFORMATION IN POLYCRYSTALLINE METALS

4.13 GRAIN SHAPE AND ANISOTROPIC PROPERTIES 4.14 FRACTURE OF METALS 4.15 COLD WORKING, RECRYSTALLIZATION, AND HOT WORKING 4.16 GRAIN GROWTH 4.17 ALLOYS AND ALLOY TYPES 4.18 ATOMIC STRUCTURE AND ELECTRICAL PROPERTIES

& 4.1 STRUCTURE–PROPERTY–PROCESSING–PERFORMANCE RELATIONSHIPS The success of many manufactured products depends on the selection of materials whose properties meet the requirements of the application. Primitive cultures were often limited to the naturally occurring materials in their environment. As civilizations developed, the spectrum of construction materials expanded. Materials could now be processed and their properties altered and improved. The alloying or heat treatment of metals, and the firing of ceramics are examples of techniques that can substantially alter the properties of a material. Fewer compromises were required and enhanced design possibilities emerged. Products, in turn, became more sophisticated. While the early successes in altering materials were largely the result of trial and error, we now recognize that the properties and performance of a material are a direct result of its structure and processing. If we want to change the properties, we will most likely have to induce changes in the material structure. Because all materials are composed of the same basic components—particles that include protons, neutrons, and electrons—it is amazing that so many different materials exist with such widely varying properties. This variation can be explained, however, by the many possible combinations these units can assume in a macroscopic assembly. The subatomic particles combine in different arrangements to form the various elemental atoms, each having a nucleus of protons and neutrons surrounded by the proper number of electrons to maintain charge neutrality. The specific arrangement of the electrons surrounding the nucleus affects the electrical, magnetic, thermal, and optical properties as well as the way the atoms bond to one another. Atomic bonding then produces a higher level of structure, which may be in the form of a molecule, crystal, or amorphous aggregate. This structure, along with the imperfections that may be present, has a profound effect on the mechanical properties. The size, shape, and arrangement of multiple crystals, or the mixture of two or more different structures within a material, produce a higher level of structure, known as microstructure. Variations in microstructure further affect the material properties. Because of the ability to control structures through processing, and the ability to develop new structures through techniques such as composite materials, engineers now have at their disposal a wide variety of materials with an almost unlimited range of properties. The specific properties of these materials depend on all levels of structure, from subatomic to macroscopic (Figure 4-1). This chapter will attempt to develop an understanding of the basic structure of engineering materials and how changes in that structure affect their properties and performance.

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Structure Levels Subatomic

Atomic

Molecule

Micro

Macro

Nuclear Electrical Chemical

FIGURE 4-1 General relationships between structural level and the various types of engineering properties.

Thermal Mechanical

Properties

& 4.2 THE STRUCTURE OF ATOMS Experiments have revealed that atoms consist of a relatively dense nucleus composed of positively charged protons and neutral particles of nearly identical mass, known as neutrons. Surrounding the nucleus are the negatively charged electrons, which appear in numbers equal to the protons so as to maintain a neutral charge balance. Distinct groupings of these basic particles produce the known elements, ranging from the relatively simple hydrogen atom to the unstable transuranium atoms more than 250 times as heavy. Except for density and specific heat, however, the weight of atoms has very little influence on their engineering properties. The light electrons that surround the nucleus play an extremely significant role in determining material properties. These electrons are arranged in a characteristic structure consisting of shells and subshells, each of which can contain only a limited number of electrons. The first shell, nearest the nucleus, can contain only 2. The second shell can contain 8, the third, 18, and the fourth, 32. Each shell and subshell is most stable when it is completely filled. For atoms containing electrons in the third shell and beyond, however, relative stability is achieved with eight electrons in the outermost layer or subshell. If an atom has slightly less than the number of outer-layer electrons required for stability, it will readily accept electrons from another source. It will then have more electrons than protons and becomes a negatively charged atom, or negative ion. Depending on the number of additional electrons, ions can have negative charges of 1, 2, 3, or more. Conversely, if an atom has a slight excess of electrons beyond the number required for stability (such as sodium, with one electron in the third shell), it will readily give up the excess electron and become a positive ion. The remaining electrons become more strongly attached, so further removal of electrons becomes progressively more difficult. The number of electrons surrounding the nucleus of a neutral atom is called the atomic number. More important, however, are those electrons in the outermost shell or subshell, which are known as valence electrons. These are influential in determining chemical properties, electrical conductivity, some mechanical properties, the nature of interatomic bonding, the atom size, and optical characteristics. Elements with similar electron configurations in their outer shells tend to have similar properties.

& 4.3 ATOMIC BONDING Atoms are rarely found as free and independent units, but are usually linked or bonded to other atoms in some manner as a result of interatomic attraction. The electron structure of the atoms plays the dominant role in determining the nature of the bond. Three types of primary bonds are generally recognized, the simplest of which is the ionic bond. If more than one type of atom is present, the outermost electrons can break free from atoms with excesses in their valence shell, transforming them into positive ions. These electrons then transfer to atoms with deficiencies in their outer shell, converting them into negative ions. The positive and negative ions have an electrostatic attraction for each other, resulting in a strong bonding force. Figure 4-2 presents a crude schematic of the ionic bonding process for sodium and chlorine. Ionized atoms do not

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Na

Na+

Cl

Cl–

FIGURE 4-2 Ionization of sodium and chlorine, producing stable outer shells by electron transfer.

Na+

Cl–

Atomic Bonding

91

FIGURE 4-3 Three-dimensional structure of the sodium chloride crystal. Note how the various ions are surrounded by ions of the opposite charge.

usually unite in simple pairs, however. All positively charged atoms attract all negatively charged atoms. Therefore, each sodium ion will attempt to surround itself with negative chlorine ions, and each chlorine ion will attempt to surround itself with positive sodium ions. Because the attraction is equal in all directions, the result will be a three-dimensional structure, like the one shown in Figure 4-3. Because charge neutrality must be maintained within the structure, equal numbers of positive and negative charges must be present in each neighborhood. General characteristics of materials joined by ionic bonds include high density, moderate to high strength, high hardness, brittleness, high melting point, and low electrical and thermal conductivities (because all electrons are captive to specific atoms, movement of electrical charge would require movement of entire atoms or ions). A second type of primary bond is the covalent bond. Here, the atoms in the assembly find it impossible to produce completed shells by electron transfer but achieve the same goal through electron sharing. Adjacent atoms share outer-shell electrons so that each achieves a stable electron configuration. The shared (negatively charged) electrons locate between the positive nuclei, forming a positive–negative–positive bonding link. Figure 4-4 illustrates this type of bond for a pair of chlorine atoms, each of which FIGURE 4-4 Formation of a contains seven electrons in the valence shell. The result is a stable two-atom molecule, chlorine molecule by the electron Cl2. Stable molecules can also form from the sharing of more than one electron from sharing of a covalent bond. each atom, as in the case of nitrogen (Figure 4-5a). The atoms in the assembly need not be identical (as in HF, Figure 4-5b), the sharing does not have to be equal, and a single atom can share electrons with more than one other atom. For atoms such as carbon and silicon, with four electrons in the valence shell, one atom may share its valence electrons with each of four neighboring atoms. The resulting structure is a three-dimensional network of bonded atoms, like the one shown in Figure 4-5c, where each atom is the center of a four-atom tetrahedron formed by its four neighbors as shown in Figure 4-5d. Because each atom only wants four neighbors, carN N H F bon and silicon materials tend to be light in weight. The covalent bond tends to produce materials with high strength and high melt(a) (b) ing point. Because atom movement within the three-dimensional structure (plastic deformation) requires the breaking of discrete bonds, covalent materials are characteristically brittle. Electrical conductivity depends on bond strength, ranging from conductive tin (weak covalent bonding), through semiconductive silicon and germanium, to insulating diamond (carbon). Ionic or covalent bonds are commonly found in ceramic and polymeric materials. A third type of primary bond is possible when a complete outer shell cannot be formed by either electron transfer or electron sharing. This bond is known as the metallic bond (Figure 4-6). If each of the atoms in an aggregate contains only a few valence electrons (one, two, or three), these electrons can be easily (c) (d) removed to produce ‘‘stable’’ ions. The positive ions (nucleus and inner, nonvalence electrons) then arrange in a three-dimensional FIGURE 4-5 Examples of covalent bonding in periodic array, and are surrounded by wandering, universally (a) nitrogen molecule, (b) HF, and (c) silicon. Part shared, valence electrons, sometimes referred to as an electron (d) shows the tetrahedron formed by a silicon atom and its four neighbors. cloud or electron gas. These highly mobile, free electrons account

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FIGURE 4-6 Schematic of the metallic bond showing the positive ions and free electrons.

for the high electrical and thermal conductivity values as well as the opaque (nontransparent) characteristic observed in metals (the free electrons are able to absorb the various discrete energies of light radiation). They also provide the ‘‘cement’’ required for the positive–negative–positive attractions that result in bonding. Bond strength, and therefore material strength and melting temperature, varies over a wide range. More significant, however, is the observation that the positive ions can now move within the structure without the breaking of discrete bonds. Materials bonded by metallic bonds can be deformed by atom-movement mechanisms and produce an altered-shape that is every bit as strong as the original. This phenomenon is the basis of metal plasticity, enabling the wide variety of forming processes used in the fabrication of metal products.

& 4.4 SECONDARY BONDS Weak or secondary bonds, known as van der Waals forces, can form between molecules that possess a nonsymmetrical distribution of electrical charge. Some molecules, such as hydrogen fluoride and water,1 can be viewed as electric dipoles. Certain portions of the molecule tend to be more positive or more negative than others (an effect referred to as polarization). The negative part of one molecule tends to attract the positive region of another, forming a weak bond. Van der Waals forces contribute to the mechanical properties of a number of molecular polymers, such as polyethylene and polyvinyl chloride (PVC).

& 4.5 ATOM ARRANGEMENTS IN MATERIALS As atoms bond together to form aggregates, we find that the particular arrangement of the atoms has a significant effect on the material properties. Depending on the manner of atomic grouping, materials are classified as having molecular structures, crystal structures, or amorphous structures. Molecular structures have a distinct number of atoms that are held together by primary bonds. There is only a weak attraction, however, between a given molecule and other similar groupings. Typical examples of molecules include O2, H2O, and C2H4 (ethylene). Each molecule is free to act more or less independently, so these materials exhibit relatively low melting and boiling points. Molecular materials tend to be weak, because the molecules can move easily with respect to one another. Upon changes of state from solid to liquid or liquid to gas, the molecules remain as distinct entities. Solid metals and most minerals have a crystalline structure. Here, the atoms are arranged in a three-dimensional geometric array known as a lattice. Lattices are describable through a unit building block, or unit cell, that is essentially repeated 1 The H2O molecule can be viewed as a 109-degree boomerang or elbow with oxygen in the middle and the two hydrogens on the extending arms. The eight valence electrons (six from oxygen and two from hydrogen) associate with oxygen, giving it a negative charge. The hydrogen arms are positive. Therefore, when two or more water molecules are present, the positive hydrogen locations of one molecule are attracted to the negative oxygen location of an adjacent molecule.

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SECTION 4.6

Crystal Structures of Metals

93

throughout space. Crystalline structures will be discussed more fully in the following section. In an amorphous structure, such as glass, the atoms have a certain degree of local order (arrangement with respect to neighboring atoms), but when viewed as an aggregate, they lack the periodically ordered arrangement that is characteristic of a crystalline solid.

& 4.6 CRYSTAL STRUCTURES OF METALS

TABLE 4-1 Metal Aluminum Copper Gold Iron Lead Magnesium Silver Tin Titanium

From a manufacturing viewpoint, metals are an extremely important class of materials. They are frequently the materials being processed and often form both the tool and the machinery performing the processing. They are characterized by the metallic bond and possess the distinguishing characteristics of strength, good electrical and thermal conductivity, luster, the ability to be plastically deformed to a fair degree without fracturing, and a relatively high specific gravity or density compared to nonmetals. The fact that some metals possess properties different from the general pattern simply expands their engineering utility. When metals solidify, the atoms assume a crystalline structure; that is, they arrange themselves in a geometric lattice. Many metals exist in only one lattice form. Some, however, can exist in the solid state in two or more lattice forms, with the particular form depending on the conditions of temperature and pressure. These metals are said to be allotropic or polymorphic (poly means ‘‘more than one’’; morph means ‘‘structure’’), and the change from one lattice form to another is called an allotropic transformation. The most notable example of such a metal is iron, where the allotropic change makes it possible for heat-treating procedures that yield a wide range of final properties. It is largely because of its allotropy that iron has become the basis of our most important alloys. There are 14 basic types of crystal structures or lattices. Fortunately, however, nearly all of the commercially imporThe Type of Crystal Lattice for Common tant metals solidify into one of three lattice types: body-centered cubic, face-centered cubic, or hexagonal close-packed. Metals at Room Temperature Table 4-1 lists the room temperature structure for a number Lattice Type of common metals. Figure 4-7 compares these three structures to one another, along with the easily visualized, but Face-centered cubic rarely observed, simple cubic structure. Face-centered cubic To begin our study of crystals, consider the simple cubic Face-centered cubic structure illustrated in Figure 4-7a. This crystal can be conBody-centered cubic structed by placing single atoms on all corners of a cube, and Face-centered cubic then linking identical cube units together. If we assume that Hexagonal the atoms are rigid spheres with atomic radii touching one Face-centered cubic another, computation reveals that only 52% of available Body-centered tetragonal space is occupied. Each atom is in direct contact with only six Hexagonal neighbors (plus and minus in each of the x, y, and z cube edge directions). Both of these observations are unfavorable to the metallic bond, where atoms desire both a high number of nearest neighbors and high-efficiency packing. The largest region of unoccupied space is in the geometric center of the cube, where a sphere of 0.732 times the atom diameter could be inserted.2 If the cube is expanded to permit the insertion of an entire atom, the body-centered cubic (BCC) structure results (Figure 4-7b). Each atom now has eight nearest neighbors, and 68% of the space is occupied. This structure is more favorable to metals and is observed in room temperature iron, chromium, manganese, and the other metals listed in Figure 4-7b. 2 The diagonal of a cube is equal to the square root of three times the length of the cube edge, and the cube edge is here equal to two atomic radii or one atomic diameter. Thus, the diagonal is equal to 1.732 times the atom diameter and is made up of an atomic radius, open space, and another atomic radius. Because two radii equals one diameter, the open space must be equal in size to 0.732 times the atomic diameter.

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Lattice structure

FIGURE 4-7 Comparison of crystal structures: simple cubic, body-centered cubic, facecentered cubic, and hexagonal close-packed.

FIGURE 4-8 Close-packed atomic plane showing three directions of atom touching or close-packing. (Courtesy Ronald Kohser)

Unit cell schematic

Unit cells

Number of nearest neighbors

Packing efficiency

Typical metals

a

Simple cubic

6

52%

None

b

Body-centered cubic

8

68%

Fe, Cr, Mn, Cb, W, Ta, Ti, V, Na, K

c

Face-centered cubic

12

74%

Fe, Al, Cu, Ni, Ca, Au, Ag, Pb, Pt

d

Hexagonal close-packed

12

74%

Be, Cd, Mg, Zn, Zr

Compared to materials with other structures, body-centered-cubic metals tend to be high strength. In seeking efficient packing and a large number of adjacent neighbors, consider maximizing the number of spheres in a single layer and then stacking those layers. The layer of maximized packing is known as a close-packed plane and exhibits the hexagonal symmetry shown in Figure 4-8. The next layer is positioned with its spheres occupying either the ‘‘point-up’’ or ‘‘point-down’’ triangular recesses in the original layer. Depending on the sequence in which the various layers are stacked, two distinctly different structures can be produced. Both have 12 nearest neighbors (six within the original plane and three from each of the layers above and below) and a 74% efficiency of occupying space. If the layers are stacked in sets of three (original location, point-up recess of the original layer, and point-down recess of the original layer), rotation of the resulting structure reveals cubic symmetry where an atom has been inserted into the center of each of the six cube faces, like a dice with the number five on each of its sides. This is the face-centered cubic (FCC) structure shown in Figure 4-7c. It is the preferred structure for many engineering metals and tends to provide the exceptionally high ductility (ability to be plastically deformed without fracture) that is characteristic of aluminum, copper, silver, gold, and elevated temperature iron. A stacking sequence of any two alternating layers results in a structure known as hexagonal close-packed (HCP), where the individual close-packed planes can be clearly identified (Figure 4-7d). Metals having this structure, such as magnesium and zinc, tend to have poor ductility, fail in a brittle manner, and often require special processing procedures.

& 4.7 DEVELOPMENT OF A GRAIN STRUCTURE When a metal solidifies, a small particle of solid forms from the liquid with a lattice structure characteristic of the given material. This particle then acts like a seed or nucleus and grows as other atoms attach themselves. The basic crystalline unit, or unit cell, is repeated, as illustrated in Figure 4-9.

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Elastic Deformation

95

(a)

FIGURE 4-10 Schematic representation of the growth of crystals to produce a polycrystalline material.

(b)

FIGURE 4-9 Growth of crystals to produce an extended lattice: (a) unit cell; (b) multi-cell aggregate. (Courtesy Ronald Kohser)

FIGURE 4-11 Photomicrograph of alpha ferrite. (Courtesy Ronald Kohser)

In actual solidification, many nuclei form independently throughout the liquid and have random orientations with respect to one another. Each then grows until it encounters its neighbors. Because the adjacent lattice structures have different alignments or orientations, growth cannot produce a single continuous structure, and a polycrystalline solid is produced. Figure 4-10 provides a two-dimensional illustration of this phenomenon. The small, continuous regions of solid are known as crystals or grains, and the surfaces that divide them (i.e., the surfaces of crystalline discontinuity) are known as grain boundaries. The process of solidification is one of crystal nucleation and growth. Grains are the smallest unit of structure in a metal that can be observed with an ordinary light microscope. If a piece of metal is polished to mirror finish with a series of abrasives and then exposed to an attacking chemical for a short time (etched), the grain structure can be revealed. The atoms along the grain boundaries are more loosely bonded and tend to react with the chemical more readily than those that are part of the grain interior. When viewed under reflected light, the attacked boundaries scatter light and appear dark compared to the relatively unaffected (still flat) grains (Figure 4-11). In some cases, the individual grains may be large enough to be seen by the unaided eye, as with some galvanized steels, but usually magnification is required. The number and size of the grains in a metal vary with the rate of nucleation and the rate of growth. The greater the nucleation rate, the smaller the resulting grains. Conversely, the greater the rate of growth, the larger the grains. Because the resulting grain structure will influence certain mechanical and physical properties, it is an important property to control and specify. One means of specification is through the ASTM grain size number, defined in ASTM specification E112 as N ¼ 2n1 where N is the number of grains per square inch visible in a prepared specimen at 100magnification, and n is the ASTM grain size number. Low ASTM numbers mean a few massive grains, while high numbers refer to materials with many small grains.

& 4.8 ELASTIC DEFORMATION The mechanical properties of a material are strongly dependent on its crystal structure. An understanding of mechanical behavior, therefore, begins with an understanding of the way crystals react to mechanical loads. Most studies begin with carefully prepared single crystals. Through them, we learn that the mechanical behavior depends on (1) the type of lattice, (2) the interatomic forces (i.e., bond strength), (3) the spacing between adjacent planes of atoms, and (4) the density of the atoms on the various planes. If the applied loads are relatively low, the crystals respond by simply stretching or compressing the distance between adjacent atoms (Figure 4-12). The basic lattice does not change, and all of the atoms remain in their original positions relative to one another. The applied load serves only to alter the force balance of the atomic bonds, and the atoms assume new equilibrium positions with the applied load as an additional

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FIGURE 4-12 Distortion of a crystal lattice in response to various elastic loadings.

Unloaded

Tension

Compression

Shear

component of force. If the load is removed, the atoms return to their original positions and the crystal resumes its original size and shape. The mechanical response is elastic in nature, and the amount of stretch or compression is directly proportional to the applied load or stress. Elongation or compression in the direction of loading results in an opposite change of dimensions at right angles to that direction. The ratio of lateral contraction to axial stretching is known as Poisson’s ratio. This value is always less than 0.5 and is usually about 0.3.

& 4.9 PLASTIC DEFORMATION

(a) Ridges

Valleys

(b)

FIGURE 4-13 (a) Close-packed atomic plane viewed from above; (b) view from the side (across the surface) showing the ridges and valleys that lie in directions of close-packing.

As the magnitude of applied load becomes greater, distortion (or elastic strain) continues to increase, and a point is reached where the atoms either (1) break bonds to produce a fracture or (2) slide over one another in a way that would reduce the load. For metallic materials, the second phenomenon generally requires lower loads and occurs preferentially. The atomic planes shear over one another to produce a net displacement or permanent shift of atom positions, known as plastic deformation. Conceptually, this is similar to the distortion of a deck of playing cards when one card slides over another. Because of the metallic bond, where the freely moving electrons cement the structure together, the result is a permanent change in shape that occurs without a concurrent deterioration in properties. Consider the close-packed plane of Figure 4-8, with the three directions of atom touching (the close-packed directions) identified by bold lines. If we were to look across the top surface of this plane along one of the three close-packed directions, as in Figure 4-13, we see a series of parallel ridges. The point-up or point-down depressions, which become the sites for the next layer of atoms, lie along valleys that parallel the ridges. If the upper layer were to slide in one of the ridge directions, its atoms would simply traverse the valleys and would encounter little resistance. Movement in any other direction would require atoms to climb over the ridges, requiring a greater applied force. Hence, the preference for deformation to occur by movement along close-packed planes in directions of atom touching. If close-packed planes are not available within the crystal structure, plastic deformation tends to occur along planes having the highest atomic density and greatest separation. The rationale for this can be seen in the simplified two-dimensional array of Figure 4-14. Planes A and A’ have higher density and greater separation than planes B and B’. In visualizing relative motion, the atoms of B and B’ would interfere significantly with one another, whereas planes A and A’ do not experience this difficulty. Plastic deformation, therefore, tends to occur by the preferential sliding of maximum-density planes (close-packed planes if present) in directions of closest packing. A specific combination of plane and direction is called a slip system, and the resulting shear deformation or sliding is known as slip. The ability of a metal to deform along a given slip system depends on the ease of shearing along that system and the orientation of the plane with respect to the applied load. Consider a deck of playing cards. The deck will not ‘‘deform’’ when laid flat on the table and pressed from the top, or when stacked on edge and pressed uniformly. The cards will slide over one another, however,

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Dislocation Theory of Slippage

97

B⬘ B A⬘ A

BCC

FIGURE 4-15 FIGURE 4-14 Simple schematic illustrating the lower deformation resistance of planes with higher atomic density and larger interplanar spacing.

FCC

HCP

Slip planes within the BCC, FCC, and HCP crystal structures.

if the deck is skewed with respect to the applied load so as to induce a shear stress along the plane of sliding. With this understanding, consider the deformation properties of the three most common crystal structures: 1. Body-centered cubic. In the BCC structure, there are no close-packed planes. Slip occurs on the most favorable alternatives, which are those planes with the greatest interplanar spacing (six of which are illustrated in Figure 4-15). Within these planes, slip occurs along the directions of closest packing, which are the diagonals through the body of the cube. If each specific combination of plane and direction is considered as a separate slip system, we find that the BCC materials contain 48 attractive ways to slip (plastically deform). The probability that one or more of these systems will be oriented in a favorable manner is great, but the force required to produce deformation is extremely large since there are no close-packed planes. As a result, materials with this structure generally possess high strength with moderate ductility. (Refer to the typical BCC metals in Figure 4-7.) 2. Face-centered cubic. In the FCC structure, each unit cell contains four close-packed planes, as illustrated in Figure 4-15. Each of those planes contains three close-packed directions, the diagonals along the cube faces, giving 12 possible means of slip. The probability that one or more of these will be favorably oriented is great, and for this structure, the force required to induce slip is quite low. Metals with the FCC structure are relatively weak and possess excellent ductility, as can be confirmed by a check of the metals listed in Figure 4-7. 3. Hexagonal close-packed. The hexagonal lattice also contains close-packed planes, but only one such plane exists within the lattice. Although this plane contains three close-packed directions and the force required to produce slip is again rather low, the probability of favorable orientation to the applied load is small (especially if one considers a polycrystalline aggregate). As a result, metals with the HCP structure tend to have low ductility and are often classified as brittle.

& 4.10 DISLOCATION THEORY OF SLIPPAGE A theoretical calculation of the strength of metals based on the sliding of entire atomic planes over one another predicts yield strengths on the order of 3 million pounds per square inch or 20,000 MPa. The observed strengths in actual testing are typically 100 to 150 times lower than this value. Extremely small laboratory-grown crystals, however, have been shown to exhibit the full theoretical strength. An explanation can be provided by the fact that plastic deformation does not occur by all of the atoms in one plane slipping simultaneously over all the atoms of an adjacent plane. Instead, deformation is the result of the progressive slippage of a localized disruption known as a dislocation. Consider a simple analogy. A carpet has been rolled onto a floor, and we now want to move it a short distance in a given direction. One approach would be to pull on one end and try to ‘‘shear the carpet across the floor,’’ simultaneously overcoming the frictional resistance of the entire area of contact. This would require a large force acting over a small distance. An alternative approach might

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Edge dislocation

T

C04

(a)

(b)

FIGURE 4-16 Schematic representation of (a) edge and (b) screw dislocations. [(a) From Elements of Physical Metallurgy, by A. G. Guy, Addison-Wesley Publishing Co., Inc., Reading, MA, 1959; (b) Adapted from Materials Science and Engineering, 7th ed., by William D. Callister Jr., John Wiley & Sons, Inc., 2007]

be to form a wrinkle at one end of the carpet and walk the wrinkle across the floor to produce a net shift in the carpet as a whole—a low-force-over-large distance approach to the same task. In the region of the wrinkle, there is an excess of carpet with respect to the floor beneath it, and the movement of this excess is relatively easy. Electron microscopes have revealed that metal crystals do not have all of their atoms in perfect arrangement, but rather contain a variety of localized imperfections. Two such imperfections are the edge dislocation and screw dislocation (Figure 4-16). Edge dislocations are the edges of extra half-planes of atoms. Screw dislocations correspond to partial tearing of the crystal plane. In each case, the dislocation is a disruption to the regular, periodic arrangement of atoms and can be moved about with a rather low applied force. It is the motion of these atomic-scale dislocations under applied load that is responsible for the observed macroscopic plastic deformation. All metals contain dislocations, usually in abundant quantities. The ease of deformation depends on the ease of making them move. Barriers to dislocation motion, therefore, would tend to increase the overall strength of a metal. These barriers take the form of other crystal imperfections and may be of the point type (missing atoms or vacancies, extra atoms or interstitials, or substitution atoms of a different variety, as shown in Figure 4-17), line type (another dislocation), or surface type (crystal grain boundary or free surface). To increase the strength of a material, we can either remove all defects to create a perfect crystal (nearly impossible) or work to impede the movement of existing dislocations by adding other crystalline defects (the basis of a variety of strengthening mechanisms).

& 4.11 STRAIN HARDENING OR WORK HARDENING As noted in our discussion of the tensile test in Chapter 3, most metals become stronger when they are plastically deformed, a phenomenon known as strain hardening or work hardening. Understanding of this phenomenon can now come from our knowledge of dislocations and a further extension of the carpet analogy. Suppose that this time our goal is to move the carpet diagonally. The best way would be to move a wrinkle in one direction, and then move a second one perpendicular to the first. But suppose that both wrinkles were started simultaneously. We would find that wrinkle 1 would impede the motion of wrinkle 2, and vice versa. In essence, the feature that makes deformation easy can also serve to impede the motion of other, similar dislocations. In metals, plastic deformation occurs through dislocation movement. As dislocations move, they are more likely to encounter and interact with other dislocations or other crystalline defects, thereby producing resistance to further motion. In addition, mechanisms exist that markedly increase the number of dislocations in a metal during

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SECTION 4.12

FIGURE 4-17 Twodimensional schematic showing the various point defects: (a) vacancy; (b) interstitial; (c) smaller-than-host substitutional; (d) larger-than-host substitutional. (Adapted from Essentials of Materials Science and Engineering, 2nd ed., by Donald R. Askeland and Pradeep P. Fulay, Cengage Learning, 2009)

Plastic Deformation in Polycrystalline Metals

(a)

(b)

(c)

(d)

99

deformation (usually by several orders of magnitude), thereby enhancing the probability of interaction. The effects of strain hardening become attractive when one considers that mechanical deformation (metal-forming) is frequently used in the shaping of metal products. As the product shape is being formed, the material is simultaneously becoming stronger. Because strength can be increased substantially during deformation, a strain-hardened (deformed), inexpensive metal can often be substituted for a more costly, stronger one that is machined or cast to shape. Transmission electron microscope studies have confirmed the existence of dislocations and the slippage theory of deformation. By observing the images of individual dislocations in a thin metal section, we can see the increase in the number of dislocations and their interactions during deformation. Macroscopic observations also lend support. When a load is applied to a single metal crystal, deformation begins on the slip system that is most favorably oriented. The net result is often an observable slip and rotation, like that of a skewed deck of cards (Figure 4-18). Dislocation motion becomes more difficult as strain hardening produces increased resistance, and rotation makes the slip system orientation less favorable. Further deformation may then occur on alternative systems that now offer less resistance, a phenomenon known as cross slip.

& 4.12 PLASTIC DEFORMATION IN POLYCRYSTALLINE METALS Commercial metals are not single crystals, but usually take the form of polycrystalline aggregates. Within each crystal, deformation proceeds in the manner previously described. Because the various grains have different orientations, an applied load will produce different deformations within each of the crystals. This can be seen in

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FIGURE 4-19 Slip lines in a polycrystalline material. (From Richard Hertzberg, Deformation and Fracture Mechanics of Engineering Materials. Reprinted with permission of John Wiley & Sons, Inc.)

FIGURE 4-18 Schematic representation of slip and crystal rotation resulting from deformation. (From Richard Hertzberg, Deformation and Fracture Mechanics of Engineering Materials. Reprinted with permission of John Wiley & Sons, Inc.)

Figure 4-19, where a metal has been polished and then deformed. The relief of the polished surface reveals the different slip planes for each of the grains. One should note that the slip lines do not cross from one grain to another. The grain boundaries act as barriers to the dislocation motion (i.e., the defect is confined to the crystal in which it occurs). As a result, metals with a finer grain structure—more grains per unit area—tend to exhibit greater strength and hardness, coupled with increased impact resistance. This near-universal enhancement of properties is an attractive motivation for grain size control during processing.

& 4.13 GRAIN SHAPE AND ANISOTROPIC PROPERTIES When a metal is deformed, the grains tend to elongate in the direction of metal flow (Figure 4-20). Accompanying the nonsymmetric structure are directionally varying properties. Mechanical properties (such as strength and ductility), as well as physical properties (such as electrical and magnetic characteristics), may all exhibit directional differences. Properties that vary with direction are said to be anisotropic. Properties that are uniform in all directions are isotropic. The directional variation of properties can be harmful or beneficial. By controlling the metal flow in processes such as forging, enhanced strength or fracture resistance can be imparted to certain locations or directions. Caution should be exercised, however, because an improvement in one direction is generally accompanied by a decline in another. Moreover, directional variation in properties may create problems during subsequent processing operations, such as the further forming of rolled metal sheets. For

FIGURE 4-20 Deformed grains in a coldworked 1008 steel after 50% reduction by rolling; (From Metals Handbook, 8th ed., 1972. Reprinted with permission of ASM 1 International . All rights reserved. www .asminternational.org)

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Cold Working, Recrystallization, and Hot Working

101

these and other reasons, both the part designer and the part manufacturer should consider the effects of directional property variations.

& 4.14 FRACTURE OF METALS When metals are deformed, strength and hardness increase, while ductility decreases. If too much plastic deformation is attempted, the metal may respond by fracture. If plastic deformation precedes the break, the fracture is known as a ductile fracture. Fractures can also occur before the onset of plastic deformation. These sudden, catastrophic failures, known as brittle fractures, are more common in metals having the BCC or HCP crystal structures. Whether the fracture is ductile or brittle, however, often depends on the specific conditions of material, temperature, state of stress, and rate of loading.

& 4.15 COLD WORKING, RECRYSTALLIZATION, AND HOT WORKING During plastic deformation, a portion of the deformation energy is stored within the material in the form of additional dislocations and increased grain boundary surface area.3 If a deformed polycrystalline metal is subsequently heated to a high enough temperature, the material will seek to lower its energy. New crystals nucleate and grow to consume and replace the original structure (Figure 4-21). This process of reducing the internal energy through the formation of new crystals is known as recrystallization.

FIGURE 4-21 Recrystallization of 70–30 cartridge brass: (a) coldworked 33%; (b) heated at 580 C (1075 F) for 3 seconds; (c) 4 seconds; (d) 8 seconds; 45. (Courtesy J. E. Burke, General Electric Company, Fairfield, CT) 3 A sphere has the least amount of surface area of any shape to contain a given volume of material. When the shape becomes altered from that of a sphere, the surface area must increase. Consider a round balloon filled with air. If the balloon is stretched or flattened into another shape, the rubber balloon is stretched further. When the applied load is removed, the balloon snaps back to its original shape, the one involving the least surface energy. Metals behave in an analogous manner. During deformation, the distortion of the crystals increases the energy of the material. Given the opportunity, the material will try to lower its energy by returning to spherical grains.

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TABLE 4-2

Metal Aluminum Copper Gold Iron Lead Magnesium Nickel Silver Tin Zinc

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The temperature at which recrystallization occurs is different for each metal and also varies with the amount of prior deformation. The greater the amount of prior deformation, the more stored energy, and the lower the recrystallization temperature. There is a lower limit, however, below Temperature [ F( C)] which recrystallization will not take place in a reasonable amount of time. Table 4-2 gives the lowest practical recrystallization temperatures 300 (150) for several materials. This is the temperature at which atomic diffusion 390 (200) (atom movement within the solid) becomes significant, and can often 390 (200) be estimated by taking 0.4 times the melting point of the metal when 840 (450) the melting point is expressed as an absolute temperature (Kelvin or Below room temperature Rankine). 300 (150) When metals are plastically deformed at temperatures below their 1100 (590) recrystallization temperature, the process is called cold working. The 390 (200) metal strengthens by strain hardening, and the resultant structure consists Below room temperature of distorted grains. As deformation continues, the metal decreases in ducRoom temperature tility and may ultimately fracture. It is a common practice, therefore, to recrystallize the material after a certain amount of cold work. Through this recrystallization anneal, the structure is replaced by one of new crystals that have never experienced deformation. All strain hardening is lost, but ductility is restored, and the material is now capable of further deformation without the danger of fracture. If the temperature of deformation is sufficiently above the recrystallization temperature, the deformation process becomes hot working. Recrystallization begins as soon as sufficient driving energy is created, (i.e. deformation and recrystallization take place simultaneously), and extremely large deformations are now possible. Because a recrystallized grain structure is constantly forming, the final product will not exhibit the increased strength of strain hardening. Recrystallization can also be used to control or improve the grain structure of a material. A coarse grain structure can be converted to a more attractive fine grain structure through recrystallization. The material must first be plastically deformed to store sufficient energy to provide the driving force. Subsequent control of the recrystallization process then establishes the more desirable final grain size.

The Lowest Recrystallization Temperature of Common Metals

& 4.16 GRAIN GROWTH Recrystallization is a continuous process in which a material seeks to lower its overall energy. Ideally, recrystallization will result in a structure of uniform crystals with a comparatively small grain size. If a metal is held at or above its recrystallization temperature for any appreciable time, however, the grains in the recrystallized structure can continue to increase in size. In effect, some of the grains become larger at the expense of their smaller neighbors as the material seeks to further lower its energy by decreasing the amount of grain boundary surface area. Because engineering properties tend to diminish as the size of the grains increase, control of recrystallization is of prime importance. A deformed material should be held at elevated temperature just long enough to complete the recrystallization process. The temperature should then be decreased to stop the process and avoid the property changes that accompany grain growth.

& 4.17 ALLOYS AND ALLOY TYPES Our discussion thus far has been directed toward the nature and behavior of pure metals. For most manufacturing applications, however, metals are not used in their pure form. Instead, engineering metals tend to be alloys, materials composed of two or more different elements, and they tend to exhibit their own characteristic properties. There are three ways in which a metal might respond to the addition of another element. The first, and probably the simplest, response occurs when the two materials are insoluble in one another in the solid state. In this case the base metal and the alloying addition each maintain their individual identities, structures, and properties. The alloy in effect becomes a composite structure, consisting of two types of building blocks in an intimate mechanical mixture.

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Atomic Structure and Electrical Properties

103

The second possibility occurs when the two elements exhibit some degree of solubility in the solid state. The two materials can form a solid solution, where the alloy element dissolves in the base metal. The solutions can be: substitutional or interstitial. In the substitutional solution, atoms of the alloy element occupy lattice sites normally filled by atoms of the base metal. In an interstitial solution, the alloy element atoms squeeze into the open spaces between the atoms of the base metal lattice. A third possibility exists where the elements combine to form intermetallic compounds. In this case, the atoms of the alloying element interact with the atoms of the base metal in definite proportions and in definite geometric relationships. The bonding is primarily of the nonmetallic variety (i.e., ionic or covalent), and the lattice structures are often quite complex. Because of the type of bonding, intermetallic compounds tend to be hard, but brittle, high-strength materials. Even though alloys are composed of more than one type of atom, their structure is still one of crystalline lattices and grains. Their behavior in response to applied loadings is similar to that of pure metals, with some features reflecting the increased level of structural complexity. Dislocation movement can be further impeded by the presence of unlike atoms. If neighboring grains have different chemistries and/or structures, they may respond differently to the same type and magnitude of load.

& 4.18 ATOMIC STRUCTURE AND ELECTRICAL PROPERTIES In addition to mechanical properties, the structure of a material also influences its physical properties, such as its electrical behavior. Electrical conductivity refers to the net movement of charge through a material. In metals, the charge carriers are the valence electrons. The more perfect the atomic arrangement, the greater the freedom of electron movement, and the higher the electrical conductivity. Lattice imperfections or irregularities provide impediments to electron transport, and lower conductivity. The electrical resistance of a metal, therefore, depends largely on two factors: (1) lattice imperfections and (2) temperature. Vacant atomic sites, interstitial atoms, substitutional atoms, dislocations, and grain boundaries all act as disruptions to the regularity of a crystalline lattice. Thermal energy causes the atoms to vibrate about their equilibrium position. These vibrations cause the atoms to be out of position, which further interferes with electron travel. For a metal, electrical conductivity will decrease with an increase in temperature. As the temperature drops, the number and type of crystalline imperfections becomes more of a factor. The best metallic conductors, therefore, are those with fewer defects (such as pure metals with large grain size) at low temperature. The electrical conductivity of a metal is due to the movement of the free electrons in the metallic bond. For covalently bonded materials, however, bonds must be broken to provide the electrons required for charge transport. Therefore, the electrical properties of these materials are a function of bond strength. Diamond, for instance, has strong bonds and is a strong insulator. Silicon and germanium have weaker bonds that are more easily broken by thermal energy. These materials are known as intrinsic semiconductors, because moderate amounts of thermal energy enable them to conduct small amounts of electricity. Continuing down Group IV of the periodic table of elements, we find that tin has such weak bonding that a high number of bonds are broken at room temperature, and the electrical behavior resembles that of a metal. The electrical conductivity of intrinsic semiconductors can be substantially improved by a process known as doping. Silicon and germanium each have four valence electrons and form four covalent bonds. If one of the bonding atoms is replaced with an atom containing five valence electrons, such as phosphorus or arsenic, the four covalent bonds would form, leaving an additional valence electron that is not involved in the bonding process. This extra electron would be free to move about and provide additional conductivity. Materials doped in this manner are known as n-type semiconductors. A similar effect can be created by inserting an atom with only three valence electrons, such as aluminum. An electron will be missing from one of the bonds, creating an electron hole. When a voltage is applied, a nearby electron can jump into this hole,

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creating a hole in the location that it vacated. Movement of electron holes is equivalent to a countermovement of electrons and thus provides additional conductivity. Materials containing dopants with three valence electrons are known as p-type semiconductors. The ability to control the electrical conductivity of semiconductor material is the functional basis of solid-state electronics and circuitry. In ionically bonded materials, all electrons are captive to atoms (ions). Charge transport, therefore, requires the movement of entire atoms, not electrons. Consider a large block of salt (sodium chloride). It is a good electrical insulator, until it becomes wet, whereupon the ions are free to move in the liquid solution and conductivity is observed.

& KEY WORDS allotropic alloy amorphous structure anisotropic ASTM grain size number atomic number body-centered cubic (BCC) brittle fracture close-packed planes cold work covalent bond cross slip crystal structure dislocation doping ductile fracture edge dislocation

elastic deformation electrical conductivity electron hole extrinsic semiconductor face-centered cubic (FCC) grain grain boundary grain growth grain structure hexagonal close-packed (HCP) hot work intermetallic compound interstitial intrinsic semiconductor ionic bond

isotropic lattice metallic bond microstructure molecular structure n-type semiconductor negative ion nucleation and growth p-type semiconductor performance plastic deformation Poisson’s ratio polarization polymorphic positive ion primary bond processing

properties recrystallization recrystallization anneal screw dislocation secondary bonds simple cubic structure slip slip system solid solution strain hardening structure substitutional atom unit cell vacancy valence electrons van der Waals forces work hardening

& REVIEW QUESTIONS 1. What enables us to control the properties and performance of engineering materials? 2. What are the next levels of structure that are greater than the atom? 3. What is meant by the term microstructure? 4. What is the most stable configuration for an electron shell or subshell? 5. What is an ion and what are the two varieties? 6. What properties or characteristics of a material are influenced by the valence electrons? 7. What are the three types of primary bonds, and what types of atoms do they unite? 8. What are some general characteristics of ionically bonded materials? 9. Where are the bonding electrons located in a covalent bond? 10. What are some general properties and characteristics of covalently bonded materials? 11. Why are the covalently bonded hydrocarbon polymers light in weight? 12. What are some unique property features of materials bonded by metallic bonds? 13. For what common engineering materials are van der Waals forces important? 14. What is the difference between a crystalline material and one with an amorphous structure? 15. What is a lattice? A unit cell?

16. What are some of the general characteristics of metallic materials? 17. What is an allotropic or polymorphic material? 18. Why did we elect to focus on only three of the fourteen basic crystal structures or lattices? What are those three structures? 19. Why is the simple cubic crystal structure not observed in the engineering metals? 20. What is the efficiency of filling space with spheres in the simple cubic structure? Body-centered-cubic structure? Face-centered cubic structure? Hexagonal close-packed structure? 21. What is the dominant characteristic of body-centered-cubic metals? Face-centered-cubic metals? Hexagonal-closepacked metals? 22. What is a grain? A grain boundary? 23. What is the most common means of describing or quantifying the grain size of a solid metal? 24. What is implied by a low ASTM grain size number? A large ASTM grain size number? 25. How does a metallic crystal respond to low applied loads? 26. What is plastic deformation? 27. What is a slip system in a material? What types of planes and directions tend to be preferred? 28. What structural features account for each of the dominant properties cited in Question 21?

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Problems 29. What is a dislocation? What is the difference between an edge dislocation and a screw dislocation? 30. What role do dislocations play in determining the mechanical properties of a metal? 31. What are some of the common barriers to dislocation movement that can be used to strengthen metals? 32. What are the three major types of point defects in crystalline materials? 33. What is the mechanism (or mechanisms) responsible for the observed deformation strengthening or strain hardening of a metal? 34. Why is a fine grain size often desired in an engineering metal? 35. What is an anisotropic property? Why might anisotropy be a concern? 36. What is the difference between brittle fracture and ductile fracture? 37. How does a metal increase its internal energy during plastic deformation? 38. What is required in order to drive the recrystallization of a cold-worked or deformed material?

105

39. In what ways can recrystallization be used to enable large amounts of deformation without fear of fracture? 40. How might the lowest recrystallization temperature of a metal be estimated? 41. What is the major distinguishing feature between hot and cold working? 42. How can deformation and recrystallization improve the grain structure of a metal? 43. Why is grain growth usually undesirable? 44. What types of structures can be produced when an alloy element is added to a base metal? 45. As a result of their ionic or covalent bonding, what types of mechanical properties are characteristic of intermetallic compounds? 46. How is electrical charge transported in a metal (electrical conductivity)? 47. What features in a metal structure tend to impede or reduce electrical conductivity? 48. What is the difference between an intrinsic semiconductor and an extrinsic semiconductor?

& PROBLEMS 1. A prepared sample of metal reveals a structure with 64 grains per square inch at 100 magnification. a. What is its ASTM grain size number? b. Would this material be weaker or stronger than the same metal with an ASTM grain size number of 4? Why? 2. Brass is an alloy of copper with a certain amount of zinc dissolved and dispersed throughout the structure. Based on the material presented in this chapter: a. Would you expect brass to be stronger or weaker than pure copper? Why? b. Low brass (copper alloy 240) contains 20% zinc. Cartridge brass (copper alloy 260) contains 30% dissolved zinc. Which would you expect to be stronger? Why? 3. It is not uncommon for subsequent processing to expose manufactured products to extreme elevated temperature. Zinc coatings can be applied by immersion into a bath of molten zinc (hot-dip galvanizing). Welding actually melts and resolidifies the crystalline metals. Brazing deposits molten filler metal. How might each of the following structural

features, and their associated properties, be altered by an exposure to elevated temperature? a. A recrystallized polycrystalline metal. b. A cold-worked metal. c. A solid-solution alloy, such as brass where zinc atoms dissolve and disperse throughout copper. 4. Polyethylene consists of fibrous molecules of covalently bonded atoms tangled and interacting like the fibers of a cotton ball. Weaker van der Waals forces act between the molecules with a strength that is inversely related to separation distance. a. What properties of polyethylene can be attributed to the covalent bonding? b. What properties are most likely the result of the weaker van der Waals forces? c. If we pull on the ends of a cotton ball, the cotton fibers go from a random arrangement to an array of somewhat aligned fibers. Assuming we get a similar response from deformed polyethylene, how might properties change? Why?

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CHAPTER 5 EQUILIBRIUM PHASE DIAGRAMS AND THE IRON–CARBON SYSTEM 5.1 INTRODUCTION 5.2 PHASES 5.3 EQUILIBRIUM PHASE DIAGRAMS Temperature–Composition Diagrams Cooling Curves Solubility Studies

Complete Solubility in Both Liquid and Solid States Partial Solid Solubility Insolubility Utilization of Diagrams Solidification of Alloy X Three-Phase Reactions

Intermetallic Compounds Complex Diagrams 5.4 IRON–CARBON EQUILIBRIUM DIAGRAM 5.5 STEELS AND THE SIMPLIFIED IRON–CARBON DIAGRAM 5.6 CAST IRONS Case Study: Fish Hooks

& 5.1 INTRODUCTION As our study of engineering materials becomes more focused on specific metals and alloys, it is increasingly important that we acquire an understanding of their natural characteristics and properties. What is the basic structure of the material? Is the material uniform throughout, or is it a mixture of two or more distinct components? If there are multiple components, how much of each is present, and what are the different chemistries? Is there a component that may impart undesired properties or characteristics? What will happen if temperature is increased or decreased, pressure is changed, or chemistry is varied? The answers to these and other important questions can be obtained through the use of equilibrium phase diagrams.

& 5.2 PHASES Before we move to a discussion of equilibrium phase diagrams, it is important that we first develop a working definition of the term phase. As a starting definition, a phase is simply a form of material possessing a characteristic structure and characteristic properties. Uniformity of chemistry, structure, and properties is assumed throughout a phase. More rigorously, a phase has a definable structure, a uniform and identifiable chemistry (also known as composition), and distinct boundaries or interfaces that separate it from other different phases. A phase can be continuous (like the air in a room) or discontinuous (like grains of salt in a shaker). A phase can be solid, liquid, or gas. In addition, a phase can be a pure substance or a solution, provided that the structure and composition are uniform throughout. Alcohol and water mix in all proportions and will therefore form a single phase when combined. There are no boundaries across which structure and/or chemistry changes. Oil and water, on the other hand, tend to separate into regions with distinct boundaries and must be regarded as two distinct phases. Ice cubes in water are another two-phase system, since there are two distinct structures with interfaces between them.

& 5.3 EQUILIBRIUM PHASE DIAGRAMS An equilibrium phase diagram is a graphic mapping of the natural tendencies of a material or a material system, assuming that equilibrium has been attained for all possible conditions. There are three primary variables to be considered: temperature, pressure, and composition. The simplest phase diagram is a pressure–temperature (P–T) diagram

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Pressure – atmospheres (not to scale)

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FIGURE 5-1 Pressure– temperature equilibrium phase diagram for water.

Equilibrium Phase Diagrams

107

Liquid (water) Solid (ice) 1 Gas (vapor) Triple point 0

100 Temperature – °C (not to scale)

for a fixed-composition material. Areas of the diagram are assigned to the various phases, with the boundaries indicating the equilibrium conditions of transition. As an introduction, consider the pressure–temperature diagram for water, presented as Figure 5-1. With the composition fixed as H2O, the diagram maps the stable form of water for various conditions of temperature and pressure. If the pressure is held constant and temperature is varied, the region boundaries denote the melting and boiling points. For example, at 1 atmosphere pressure, the diagram shows that water melts at 0 C and boils at 100 C. Still other uses are possible. Locate a temperature where the stable phase is liquid at atmospheric pressure. Maintaining the pressure at one atmosphere, drop the temperature until the material goes from liquid to solid (i.e., ice). Now, maintain that new temperature and begin to decrease the pressure. A transition will be encountered where solid goes directly to gas without melting (sublimation). The combined process just described, known as freeze drying, is employed in the manufacture of numerous dehydrated products. With an appropriate phase diagram, process conditions can be determined that might reduce the amount of required cooling and the magnitude of pressure drop required for sublimation. A process operating about the triple point would be most efficient.

TEMPERATURE–COMPOSITION DIAGRAMS While the P–T diagram for water is an excellent introduction to phase diagrams, P–T phase diagrams are rarely used for engineering applications. Most engineering processes are conducted at atmospheric pressure, and variations are more likely to occur in temperature and composition as we consider both alloys and impurities. The most useful mapping, therefore, is usually a temperature–composition phase diagram at 1 atmosphere pressure. For the remainder of the chapter, this will be the form of phase diagram that will be considered. For mapping purposes, temperature is placed on the vertical axis and composition on the horizontal. Figure 5-2 shows the axes for mapping the A–B system, where the left-hand vertical corresponds to pure material A, and the percentage of B (usually expressed in weight percent) increases as we move toward pure material B at the right side of the diagram. The temperature range often includes only solids and liquids, since few processes involve engineering materials in the gaseous state. Experimental investigations that provide the details of the diagram take the form of either vertical or horizontal scans that seek to locate the various phase transitions.

COOLING CURVES Considerable information can be obtained from vertical scans through the diagram where a fixed composition material is heated and slowly cooled. By plotting the cooling

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Constant-composition scan (cooling curve)

Temperature

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FIGURE 5-2 Mapping axes for a temperature–composition equilibrium phase diagram.

Isothermal scan

A

B Composition (weight percent B)

°C (°F)

history in the form of a temperature-versus-time plot, known as a cooling curve, the transitions in structure will appear as characteristic points, such as slope changes or isothermal (constant-temperature) holds. Consider the system composed of sodium chloride (common table salt) and water. Five different cooling curves are presented in Figure 5-3. Curve a is for pure water being cooled from the liquid state. A decreasing-temperature line is observed for the liquid where the removal of heat produces a concurrent drop in temperature. When the freezing point of 0 C is reached (point a), the material begins to change state and releases heat energy as part of the liquid-to-solid transition. Heat is being continuously extracted from the system, but because its source is now the change in state, there is no companion decrease in temperature. An isothermal or constant-temperature hold (a–b) is observed until the solidification is complete. From this point, as heat extraction continues, the newly formed solid experiences a steady drop in temperature. This type of curve is characteristic of pure metals and other substances with a distinct melting point. Curve b in Figure 5-3 presents the cooling curve for a solution of 10% salt in water. The liquid region undergoes continuous cooling down to point c, where the slope abruptly decreases. At this temperature, small particles of ice (i.e., solid) begin to form

0 (32)

a

b - 6.9 (19.5)

Temp.

c

-22 (- 7.6)

d

Time

°C (°F)

FIGURE 5-3 Cooling curves for five different solutions of salt and water: (a) 0% NaCl; (b) 10% NaCl; (c) 23.5% NaCl; (d) 50% NaCl; (e) 100% NaCl.

-22 (-7.6)

e

Time

(a) 0% NaCl

Temp.

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288 (550)

Time

(b) 10% NaCl

850 (1562)

h

- 22 (- 7.6)

j

Time (d) 50% NaCl

f

(c) 23.5% NaCl

l

m

k Time (e) 100% NaCl

g

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l m

l, m

a b

a, b

h

h

c

c

f

d e d, e

FIGURE 5-4 Partial equilibrium diagram for NaCl and H2O derived from cooling-curve information.

0%

109

Equilibrium Phase Diagrams

f, g

10%

23.5% 0 (H2O)

10

23.5

g

j k

j, k 50

50%

100%

100 (NaCl)

% NaCl

and the reduced slope is attributed to the energy released in this transition. The formation of these ice particles leaves the remaining solution richer in salt and imparts a lower freezing temperature. Further cooling results in the formation of additional ice, which continues to enrich the remaining liquid and further lowers its freezing point. Instead of possessing a distinct melting point or freezing point, this material is said to have a freezing range. When the temperature of point d is reached, the remaining liquid undergoes an abrupt reaction and solidifies into an intimate mixture of solid salt and solid water (discussed later), and an isothermal hold is observed. Further extraction of heat produces a drop in the temperature of the fully solidified material. For a solution of 23.5% salt in water, a distinct freezing point is again observed, as shown in curve c. Compositions with richer salt concentration, such as curve d, show phenomena similar to those in curve b, but with salt being the first solid to form from the liquid. Finally, the curve for pure salt, curve e, exhibits behavior similar to that of pure water. If the observed transition points are now transferred to a temperature–composition diagram, such as Figure 5-4, we have the beginnings of a map that summarizes the behavior of the system. Line a–c–f–h–l denotes the lowest temperature at which the material is totally liquid, and is known as the liquidus line. Line d–f–j denotes a particular three-phase reaction and will be discussed later. Between the lines, two phases coexist, one being a liquid and the other a solid. The equilibrium phase diagram, therefore, can be viewed as a collective presentation of cooling curve data for an entire range of alloy compositions. The cooling curve studies have provided some key information regarding the saltwater system, including some insight into the use of salt on highways in the winter. With the addition of salt, the freezing point of water can be lowered from 0 C (32 F) to as low as 22 C (7.6 F).

SOLUBILITY STUDIES The observant reader will note that the ends of the diagram still remain undetermined. Both pure materials have a distinct melting point, below which they appear as a pure solid. Can ice retain some salt as a single-phase solid? Can solid salt hold some water and remain a single phase? If so, how much, and does the amount vary with temperature? Completion of the diagram, therefore, requires several horizontal scans to determine any solubility limits and their possible variation with temperature. These isothermal (constant temperature) scans usually require the preparation of specimens over a range of composition and their subsequent examination by X-ray techniques, microscopy, or other methods to determine whether the structure and chemistry are uniform or if the material is a two-phase mixture. As we move away from a pure material, we often encounter a single-phase solid solution, in which one component is dissolved and dispersed throughout the other. If there is a limit to this solubility, there will be line in the phase diagram, known as a solvus line, denoting the conditions of saturation where the single-phase solid solution becomes a two-phase mixture.

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°F 327°

600

300 Liquid 250 200

500 232°

α+L 183°

α

19.2

150

β+L 97.5

61.9 α+β

100

β

50

FIGURE 5-5 Lead–tin equilibrium phase diagram.

400 300 200 100

0 Pb 10

20

30 40 50 60 70 Weight percent tin

80

90 Sn

Figure 5-5 presents the equilibrium phase diagram for the lead–tin system, using the conventional notation in which Greek letters are used to denote the various singlephase solids. The upper portion of the diagram closely resembles the salt-water diagram, but the partial solubility of one material in the other can be observed on both ends of the diagram.1

COMPLETE SOLUBILITY IN BOTH LIQUID AND SOLID STATES Having developed the basic concepts of equilibrium phase diagrams, we now consider a series of examples in which solubility changes. If two materials are completely soluble in each other in both the liquid and solid states, a rather simple diagram results, like the copper–nickel diagram of Figure 5-6. The upper line is the liquidus line, the lowest temperature for which the material is 100% liquid. Above the liquidus, the two materials form a uniform-chemistry liquid solution. The lower line, denoting the highest temperature at which the material is completely solid, is known as a solidus line. Below the solidus, the materials form a solid-state solution in which the two types of atoms are uniformly distributed throughout a single crystalline lattice. Between the liquidus and solidus is a freezing range, a two-phase region where liquid and solid solutions coexist.

PARTIAL SOLID SOLUBILITY Many materials do not exhibit complete solubility in the solid state. Each is often soluble in the other up to a certain limit or saturation point, which varies with temperature. Such a diagram has already been observed for the lead–tin system in Figure 5-5. At the point of maximum solubility, 183 C, lead can hold up to 19.2 wt% tin in a single-phase solution and tin can hold up to 2.5% lead within its structure and still be °C 1500

°F 1445° 260

1400

Liquid 240

1300 α+L

1200

FIGURE 5-6 Copper–nickel equilibrium phase diagram, showing complete solubility in both liquid and solid states.

220 α

1100 1083° 1000

Ni

10

20

30 40 50 60 70 Weight percent copper

80

200

90 Cu

1 Lead–tin solders have had a long history in joining electronic components. With the miniaturization of components, and the evolution of the circuit board or chip to ever-smaller features, exposure to the potentially damaging temperatures of the soldering operation became an increasing concern. Figure 5-5 reveals why 60– 40 solder (60 wt% tin) became the primary joining material in the lead–tin system. Of all possible alloys, it has the lowest (all liquid) melting temperature.

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Equilibrium Phase Diagrams

a single phase. If the temperature is decreased, however, the amount of solute that can be held in solution decreases in a continuous manner. If a saturated solution of tin in lead (19.2 wt% tin) is cooled from 183 C, the material will go from a single-phase solution to a two-phase mixture as a tin-rich second phase precipitates from solution. This change in structure can be used to alter and control the properties in a number of engineering alloys.

INSOLUBILITY If one or both of the components is totally insoluble in the other, the diagrams will also reflect this phenomenon. Figure 5-7 illustrates the case where component A is completely insoluble in component B in both the liquid and solid states.

UTILIZATION OF DIAGRAMS Before moving to more complex diagrams, let us first return to a simple phase diagram, such as the one in Figure 5-8, and develop several useful tools. For each condition of temperature and composition (i.e., for each point in the diagram), we would like to obtain three pieces of information: 1. The phases present. The stable phases can be determined by simply locating the point of consideration on the temperature–composition mapping and identifying the region of the diagram in which the point appears. 2. The composition of each phase. If the point lies in a single-phase region, there is only one component present, and the composition (or chemistry) of the phase is simply the composition of the alloy being considered. If the point lies in a two-phase region, a tieline must be constructed. A tie-line is simply an isothermal (constant-temperature) line drawn through the point of consideration, terminating at the boundaries of the single-phase regions on either side. The compositions where the tie-line intersects the neighboring single-phase regions will be the compositions of those respective phases in the two-phase mixture. For example, consider point a in Figure 5-8. The tie-line for this temperature runs from S2 to L2. The tie-line intersects the solid phase region at point S2. Therefore, the solid in the two-phase mixture at point a has the composition of point S2. The other end of the tie-line intersects the liquid region at L2, so the liquid phase that is present at point a will have the composition of point L2. 3. The amount of each phase present. If the point lies in a single-phase region, all of the material, or 100%, must be of that phase. If the point lies in a two-phase region, the relative amounts of the two components can be determined by a lever-law calculation using the previously drawn tie-line. Consider the cooling of alloy X in Figure 5-8 in a manner sufficiently slow so as to preserve equilibrium at all temperatures. For

Liquidus t1 Temperature

Liquid A + Liquid B Freezing temp. A

Temperature

C05

Solid A + Liquid B

t2

L1 S1

Liquid solution S2

Liquid solution

L2

+ Solid solution

t3

L3

S3 Point a Solidus

Freezing temp. B

Solid solution

Solid A + Solid B

A

B Composition (weight percent B)

FIGURE 5-7 Equilibrium diagram of two materials that are completely insoluble in each other in both the liquid and solid states.

A

X Composition (weight percent B)

FIGURE 5-8 Equilibrium diagram showing the changes that occur during the cooling of alloy X.

B

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temperatures above t1, the material is a single-phase liquid. Temperature t1 is the lowest temperature for which the alloy is 100% liquid. If we draw a tie-line at this temperature, it runs from S1 to L1 and lies entirely to the left of composition X. At temperature t3, the alloy is completely solid, and the tie-line lies completely to the right of composition X. As the alloy cools from temperature t1 to temperature t3, the amount of solid goes from 0 to 100% while the segment of the tie-line that lies to the right of composition X also goes from 0 to 100%. Similarly, the amount of liquid goes from 100% to 0 as the segment of the tie-line lying to the left of composition X also goes from 100% to 0. Extrapolating these observations to intermediate temperatures, such as temperature t2, we predict that the fraction of the tie-line that lies to the left of point a corresponds to the fraction of the material that is liquid. This fraction can be computed as: %Liquid ¼

a S2 100% L2 S2

where the values of a, S2, and L2 are their composition values in weight percent B read from the bottom scale of the diagram. In a similar manner, the fraction of solid corresponds to the fraction of the tie-line that lies to the right of point a. %Solid ¼

L2 a 100% L2 S2

Each of these mathematical relations could be rigorously derived from the conservation of either A or B atoms, as the material divides into the two different compositions of S2 and L2. Because the calculations consider the tie-line as a lever with the fulcrum at the composition line and the component phases at either end, they are called lever-law calculations. Equilibrium phase diagrams can also be used to provide an overall picture of an alloy system, or to identify the transition points for phase changes in a given alloy. For example, the temperature required to redissolve a second phase or melt an alloy can be easily determined. The various changes that will occur during the slow heating or slow cooling of a material can now be predicted. In fact, most of the questions posed at the beginning of this chapter can now be answered.

SOLIDIFICATION OF ALLOY X Let us now apply the tools that we have just developed—tie-lines and lever-laws—to follow the solidification of alloy X in Figure 5-8. At temperature t1, the first minute amount of solid forms with the chemistry of point S1. As the temperature drops, more solid forms, but the chemistries of both the solid and liquid phases shift to follow the tieline endpoints. The chemistry of the liquid follows the liquidus line, and the chemistry of the solid follows the solidus. Finally, at temperature t3, solidification is complete, and the composition of the single-phase solid is now that of alloy X. The composition of the first solid to form is different from that of the final solid. If the cooling is sufficiently slow, such that equilibrium is maintained or approximated, the composition of the solid changes during cooling and follows the endpoint of the tie-line. These chemistry changes are made possible by diffusion, the process by which atoms migrate through the crystal lattice given sufficient time at elevated temperature. If the cooling rate is too rapid, however, the temperature may drop before sufficient diffusion occurs. The resultant material will have a nonuniform chemistry. The initial solid that formed will retain a chemistry that is different from the solid regions that form later. When these nonequilibrium variations occur on a microscopic level, the resultant structure is referred to as being cored. Variation on a larger scale is called macrosegregation.

THREE-PHASE REACTIONS Several of the phase diagrams that were presented earlier contain a feature in which phase regions are separated by a horizontal (or constant temperature) line. These lines are further characterized by either a V intersecting from above or an inverted-V

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113

intersecting from below. The intersection of the V and the line denotes the location of a three-phase reaction. One common type of three-phase reaction, known as a eutectic, has already been observed in Figures 5-4 and 5-5. It is possible to understand these reactions through use of the tie-line and lever-law concepts that have been developed. Refer to the lead–tin diagram of Figure 5-5 and consider any alloy containing between 19.2 and 97.5 wt% tin at a temperature just above the 183 C horizontal line. Tie-line and lever-law computations reveal that the material contains either a lead-rich or tin-rich solid and remaining liquid. At this temperature, any liquid that is present will have a composition of 61.9 wt% tin, regardless of the overall composition of the alloy. If we now focus on this liquid and allow it to cool to just below 183 C, a transition occurs in which the liquid of composition 61.9% tin transforms to a mixture of lead-rich solid with 19.2% tin and tin-rich solid containing 97.5% tin. The three-phase reaction that occurs upon cooling through 183 C can be written as: 183 C

Liquid61:9% Sn ! a19:2% Sn þ b97:5% Sn Note the similarity to the very simple chemical reaction in which water dissociates or separates into hydrogen and oxygen: H2O ! H2 þ ½O2. Because the two solids in the lead–tin eutectic reaction have chemistries on either side of the original liquid, a similar separation must have occurred. Any chemical separation requires atom movement, but the distances involved in a eutectic reaction cannot be great. The resulting structure, known as eutectic structure, will be an intimate mixture of the two single-phase solids, with a multitude of interphase boundaries. In the preceding example, the eutectic structure always forms from the same chemistry at the same temperature and has its own characteristic set of physical and mechanical properties. Alloys with the eutectic composition have the lowest melting point of all neighboring alloys, and generally possess relatively high strength. For these reasons, they are often used as casting alloys or as filler material in soldering or brazing operations. The eutectic reaction can be written in the general form of: liquid ! solid1 þ solid2 Figure 5-9 summarizes the various types of three-phase reactions that may occur in equilibrium phase diagrams, along with the generic form of the reaction shown below the figures.2 These include the eutectic, peritectic, monotectic, and syntectic reactions, where the suffix -ic denotes that at least one of the three phases in the reaction is a liquid. If the same prefix appears with an -oid suffix, the reaction is of a similar form, but all phases involved are solids. Two all-solid reactions are the eutectoid and the peritectoid. The separation that occurs in the eutectoid reaction produces an extremely fine two-phase mixture. The combination reactions of the peritectic and peritectoid tend to be very sluggish. During actual material processing, these reactions may not reach the completion states predicted by the equilibrium diagram.

INTERMETALLIC COMPOUNDS A final phase diagram feature occurs in alloy systems where the bonding attraction between the component materials is strong enough to form stable compounds. These compounds appear as single-phase solids in the middle of a diagram. If components A and B form such a compound, and the compound cannot tolerate any deviation from its fixed atomic ratio, the product is known as a stoichiometric intermetallic compound and it appears as a single vertical line in the A-B phase diagram. (Note: An example of this is 2

To determine the specific form of a three-phase reaction, locate its horizontal line and the V intersecting from either above or below the line. Go above the point of the V and write the phases that are present. Then go below and identify the equilibrium phase or phases. Write the reaction as the phases above the line transform to those below. Apply this method to the diagrams in Figure 5-9 to identify the specific reactions, and compare them to their generic forms presented below the figures, remembering that the Greek letters denote single-phase solids.

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α

Page 114

Equilibrium Phase Diagrams and the Iron–Carbon System

L+β

α+L

β

α +β

γ α

β

α + L2

Monotectic (L1 → S1 + L2)

L2

L1 + L2

L1 + α

α

α + L2

Syntectic (L1 + L2 → S1)

α+γ

α

α+β

α+β

β

β+γ

Peritectoid (S1 + S2 → S3)

Eutectoid (S1 → S2 + S3)

FIGURE 5-9

β+L

L1 L2

γ

γ+β

α+γ

β

Peritectic (L + S1 → S2)

Eutectic (L → S1 + S2)

L1 + L2

α

α

α+β

L1

L

L+α

Stoichiometric intermetallic compound

Nonstoichiometric intermetallic compound

Schematic summary of three-phase reactions and intermetallic compounds.

the Fe3C or iron carbide that appears at 6.67 wt.% carbon in the upcoming iron-carbon equilibrium diagram.) If some degree of chemical deviation is tolerable, the vertical line expands into a single-phase region, and the compound is known as a nonstoichiometric intermetallic compound. Figure 5-9 shows schematic representations of both stoichiometric and nonstoichiometric compounds. The single-phase intermetallic compounds appear in the middle of equilibrium diagrams, with locations consistent with whole number atomic ratios, such as AB, A2B, AB2, A3B, AB3, etc.3 In general, they tend to be hard, brittle materials, because these properties are a consequence of their ionic or covalent bonding. If they are present in large quantities or lie along grain boundaries in the form of a continuous film, the overall alloy can be extremely brittle. If the same compound is dispersed throughout the alloy in the form of small discrete particles, the result can be a considerable strengthening of the base metal.

COMPLEX DIAGRAMS The equilibrium diagrams for actual alloy systems may be one of the basic types just discussed or some combination of them. In some cases the diagrams appear to be quite complex and formidable. However, by focusing on a particular composition and analyzing specific points using the tie-line and lever-law concepts, even the most complex diagram can be interpreted and understood. If the properties of the various components are known, phase diagrams can then be used to predict the behavior of the resultant structures.

& 5.4 IRON–CARBON EQUILIBRIUM DIAGRAM Steel, composed primarily of iron and carbon, is clearly the most important of the engineering metals. For this reason, the iron–carbon equilibrium diagram assumes special importance. The diagram most frequently encountered, however, is not the full iron–carbon diagram but the iron–iron carbide diagram shown in Figure 5-10. Here, a stoichiometric intermetallic compound, Fe3C, is used to terminate the carbon range at 6.67 wt% carbon. The names of key phases and structures, and the specific notations The use of ‘‘weight percent’’ along the horizontal axis tends to mask the whole number atomic ratio of intermetallic compounds. Many equilibrium phase diagrams now include a second horizontal scale to reflect ‘‘atomic percent.’’ Intermetallic compounds then appear at atomic percents of 25, 33, 50, 67, 75, and similar values that reflect whole number atomic ratios.

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Iron–Carbon Equilibrium Diagram

°C 1600

115

°F 2800

δ

Liquid

1400

δ +γ γ + liquid 1148° (2098°) 2.11

γ

2400

Liquid + Fe3C

1200 Temperature

C05

4.3

2000

γ + Fe3C

1600

1000 912° (1674°) 800 α+γ

α

1200

0.02

600

FIGURE 5-10 The iron–carbon equilibrium phase diagram. Single phases are: a, ferrite; g, austenite; d, d–ferrite; Fe3 C, cementite.

727° (1341°) 0.77 α + Fe3C

400

800 0

1

2

3 4 5 Weight percent carbon

6

7

used on the diagram, have evolved historically, and will be used in their generally accepted form. There are four single-phase solids within the diagram. Three of these occur in pure iron, and the fourth is the iron carbide intermetallic that forms at 6.67% carbon. Upon cooling, pure iron solidifies into a body-centered-cubic solid that is stable down to 1394 C (2541 F). Known as delta-ferrite, this phase is present only at extremely elevated temperatures and has little engineering importance. From 1394 to 912 C (2541 to 1674 F) pure iron assumes a face-centered-cubic structure known as austenite in honor of the famed metallurgist Roberts-Austen of England. Designated by the Greek letter g, austenite exhibits the high formability that is characteristic of the face-centered-cubic structure and is capable of dissolving more than 2% carbon in single-phase solid solution. Hot forming of steel takes advantage of the low strength, high ductility, and chemical uniformity of austenite. Most of the heat treatments of steel begin by forming the high-temperature austenite structure. Alpha ferrite, or more commonly just ferrite, is the stable form of iron at temperatures below 912 C (1674 C). This body-centered-cubic structure can hold only 0.02 wt% carbon in solid solution and forces the creation of a two-phase mixture in most steels. Upon further cooling to 770 C (1418 F), iron undergoes a transition from nonmagnetic-to-magnetic. The temperature of this transition is known as the Curie point, but because it is not associated with any change in phase (but is an atomic-level transition), it does not appear on the equilibrium phase diagram. The fourth single phase is the stoichiometric intermetallic compound, Fe3C, which goes by the name cementite, or iron–carbide. Like most intermetallics, it is quite hard and brittle, and care should be exercised in controlling the structures in which it occurs. Alloys with excessive amounts of cementite, or cementite in undesirable form, tend to have brittle characteristics. Because cementite dissociates prior to melting, its exact melting point is unknown, and the liquidus line remains undetermined in the high carbon region of the diagram. Three distinct three-phase reactions can also be identified. At 1495 C (2723 F), a peritectic reaction occurs for alloys with a low weight percentage of carbon. Because of its high temperature and the extensive single-phase austenite region immediately below it, the peritectic reaction rarely assumes any engineering significance. A eutectic is observed at 1148 C (2098 F), with the eutectic composition of 4.3% carbon. All alloys containing more than 2.11% carbon will experience the eutectic reaction and are classified by the general term cast irons. The final three-phase reaction is a eutectoid at 727 C

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(1341 F) with a eutectoid composition of 0.77 wt% carbon. Alloys with less than 2.11% carbon miss the eutectic reaction and form a two-phase mixture when they cool through the eutectoid. These alloys are known as steels. The point of maximum solubility of carbon in iron, 2.11 wt%, therefore, forms an arbitrary separation between steels and cast irons.

& 5.5 STEELS AND THE SIMPLIFIED IRON–CARBON DIAGRAM If we focus on the materials normally known as steel, the phase diagram of Figure 5-10 can be simplified considerably. Those portions near the delta phase (or peritectic) region are of little significance, and the higher carbon region of the eutectic reaction only applies to cast irons. By deleting these segments and focusing on the eutectoid reaction, we can use the simplified diagram of Figure 5-11 to provide an understanding of the properties and processing of steel. Rather than begin with liquid, our considerations generally begin with hightemperature, face-centered-cubic, single-phase austenite. The key transition will be the conversion of austenite to the two-phase ferrite plus carbide mixture as the temperature drops. Control of this reaction, which arises as a result of the drastically different carbon solubilities of the face-centered and body-centered structures, enables a wide range of properties to be achieved through heat treatment. To begin to understand these processes, consider a steel of the eutectoid composition, 0.77% carbon, being slow cooled along line x–x0 in Figure 5-11. At the upper temperatures, only austenite is present, with the 0.77% carbon being dissolved in solid solution within the face-centered structure. When the steel cools through 727 C (1341 F), several changes occur simultaneously. The iron wants to change crystal structure from the face-centered-cubic austenite to the body-centered-cubic ferrite, but the ferrite can only contain 0.02% carbon in solid solution. The excess carbon is rejected, and forms the carbon-rich intermetallic (Fe3C) known as cementite. The net reaction at the eutectoid composition and temperature is: Austenite0:77% C;FCC ! Ferrite0:02% C;BCC þ Cementite6:67% C

Liquid + austenite Austenite 1148

Temperature °C

C05

y

912

α-ferrite + Austenite

z

x Austenite + cementite

727 α-ferrite Ferrite + cementite

y⬘ 0.020

x⬘ 0.77

z⬘ 1.0

2.0

Weight percent carbon

FIGURE 5-11 Simplified iron–carbon phase diagram with labeled regions. Figure 5-10 shows the more-standard Greek letter notation.

FIGURE 5-12 Pearlite; 1000. (Courtesy United States Steel Corporation)

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FIGURE 5-13 Photomicrograph of a hypoeutectoid steel showing regions of primary ferrite (white) and pearlite; 500. (Courtesy United States Steel Corporation)

FIGURE 5-14 Photomicrograph of a hypereutectoid steel showing primary cementite along grain boundaries; 500. (Courtesy United States Steel Corporation)

Cast Irons

117

Because the chemical separation occurs entirely within crystalline solids, the resultant structure is a fine mixture of ferrite and cementite. Specimens prepared by polishing and etching in a weak solution of nitric acid and alcohol reveal a lamellar structure composed of alternating layers or plates, as shown in Figure 5-12. Because it always forms from a fixed composition at a fixed temperature, this structure has its own set of characteristic properties (even though it is composed of two distinct phases) and goes by the name pearlite because of its metallic luster and resemblance to mother-ofpearl when viewed at low magnification. Steels having less than the eutectoid amount of carbon (less than 0.77%) are called hypoeutectoid steels (hypo means ‘‘less than’’). Consider the cooling of a typical hypoeutectoid alloy along line y–y0 in Figure 5-11. At high temperatures the material is entirely austenite. Upon cooling, however, it enters a region where the stable phases are ferrite and austenite. Tie-line and lever-law calculations show that the low-carbon ferrite nucleates and grows, leaving the remaining austenite richer in carbon. At 727 C (1341 F), the remaining austenite will have assumed the eutectoid composition (0.77% carbon), and further cooling transforms it to pearlite. The resulting structure, therefore, is a mixture of primary or proeutectoid ferrite (ferrite that forms before the eutectoid reaction) and regions of pearlite as shown in Figure 5-13. Hypereutectoid steels (hyper means ‘‘greater than’’) are those that contain more than the eutectoid amount of carbon. When such a steel cools, as along line z–z0 in Figure 5-11, the process is similar to the hypoeutectoid case, except that the primary phase or proeutectoid phase is now cementite instead of ferrite. As the carbon-rich phase nucleates and grows, the remaining austenite decreases in carbon content, again reaching the eutectoid composition at 727 C (1341 F). This austenite then transforms to pearlite upon slow cooling through the eutectoid temperature. Figure 5-14 is a photomicrograph of the resulting structure, which consists of primary cementite and pearlite. In this case the continuous network of primary cementite (a brittle intermetallic compound) will cause the overall material to be extremely brittle. It should be noted that the transitions just described are for equilibrium conditions, which can be approximated by slow cooling. Upon slow heating, the transitions will occur in the reverse manner. When the alloys are cooled rapidly, however, entirely different results may be obtained, since sufficient time may not be provided for the normal phase reactions to occur. In these cases, the equilibrium phase diagram is no longer a valid tool for engineering analysis. Because the rapid-cool processes are important in the heat treatment of steels and other metals, their characteristics will be discussed in Chapter 6, and new tools will be introduced to aid our understanding.

& 5.6 CAST IRONS As shown in Figure 5-10, iron–carbon alloys with more than 2.11% carbon experience the eutectic reaction during cooling. These alloys are known as cast irons. Being relatively inexpensive, with good fluidity and rather low liquidus (full-melting) temperatures, they are readily cast and occupy an important place in engineering applications. While we are exploring the iron–carbon equilibrium phase diagram, we should note that most commercial cast irons also contain a significant amount of silicon. Cast irons typically contain 2.0 to 4.0% carbon, 0.5 to 3.0% silicon, less than 1.0% manganese, and less than 0.2% sulfur. The silicon produces several metallurgical effects. By promoting the formation of a tightly adhering surface oxide, the high silicon enhances the oxidation and corrosion resistance of cast irons. Therefore, cast irons generally exhibit a level of corrosion resistance that is superior to most steels. Because silicon partially substitutes for carbon (both have four valence electrons in their outermost shell), use of the iron-carbon equilibrium phase diagram requires replacing the weight percent carbon scale with a carbon equivalent. Several formulations exist to compute this number, with the simplest being the weight percent of carbon plus one-third of the weight percent of silicon: Carbon EquivalentðCEÞ ¼ ðwt% CarbonÞ þ 1=3ðwt% SiliconÞ

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Equilibrium Phase Diagrams and the Iron–Carbon System 1600 Fe–Fe3 C diagram Fe–graphite diagram

δ

Liquid & graphite

1400 γ + liquid Temperature °C

C05

1200

γ

2.08%

1154°

2.11%

1148°

1000 α + graphite or α + carbide

.68% 800

FIGURE 5-15 An iron–carbon diagram showing two possible high-carbon phases. Solid lines denote the iron–graphite system; dashed lines denote iron– cementite (or iron–carbide).

4.26%

α +γ

4.3% γ + graphite or γ + carbide 738°

.77% 600

1

727° 2

3

4

5

6

Weight percent carbon or carbon equivalent

By using carbon equivalent, the two-component iron-carbon diagram can be used to determine melting points and compute microstructures for the three-component ironcarbon-silicon alloys. Silicon also tends to promote the formation of graphite as the carbon-rich phase instead of the Fe3C intermetallic. The eutectic reaction, therefore, now has two distinct possibilities, as shown in the modified phase diagram of Figure 5-15: Liquid ! Austenite þ Fe3 C Liquid ! Austenite þ Graphite The final microstructure of cast iron, therefore, will contain either the carbon-rich intermetallic compound, Fe3C, or pure carbon in the form of graphite. Which occurs depends on the metal chemistry and various other process variables. Graphite is the more stable of the two, and is the true equilibrium structure. Its formation is promoted by slow cooling, high carbon and silicon contents, heavy or thick section sizes, inoculation practices, and the presence of sulfur, phosphorus, aluminum, magnesium, antimony, tin, copper, nickel, and cobalt. Cementite (Fe3C) formation is favored by fast cooling, low carbon and silicon levels, thin sections, and alloy additions of titanium, vanadium, zirconium, chromium, manganese, and molybdenum. Cast iron is really a generic term that is applied to a variety of metal alloys. Depending on which type of high-carbon phase is present and the form or nature of that phase, the cast iron can be classified as gray, white, malleable, ductile or nodular, or compacted graphite. The ferrous metals, including steels, stainless steels, tool steels, and cast irons, will be presented in more detail in Chapter 6.

& KEY WORDS austenite carbon equivalent cast iron cementite composition cooling curve cored structure Curie point delta-ferrite diffusion equilibrium phase diagram eutectic

eutectic structure eutectoid ferrite freeze drying freezing range graphite hypereutectoid steel hypoeutectoid steel inoculation interfaces lever law liquidus

macrosegregation monotectic nonstoichiometric intermetallic compound pearlite peritectic peritectoid phase pressure–temperature (P–T) diagram primary phase primary phasesolidus

solubility limit solute solvus steel stoichiometric intermetallic compound syntectic temperature–composition phase diagram three-phase reaction tie-line

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Problems

119

& REVIEW QUESTIONS 1. What kind of questions can be answered by equilibrium phase diagrams? 2. What are some features that are useful in defining a phase? 3. Supplement the examples provided in the text with another example of a single phase that is each of the following: continuous, discontinuous, gaseous, and a liquid solution. 4. What is an equilibrium phase diagram? 5. What three primary variables are generally considered in equilibrium phase diagrams? 6. Why is a pressure–temperature phase diagram not that useful for most engineering applications? 7. What is a cooling curve? 8. What features in a cooling curve indicate some form of change in a material’s structure? What causes a constanttemperature hold? A slope change? 9. What is a solubility limit, and how might it be determined? 10. In general, how does the solubility of one material in another change as temperature is increased? 11. Describe the conditions of complete solubility, partial solubility, and insolubility. 12. What types of changes occur upon cooling through a liquidus line? A solidus line? A solvus line? 13. What three pieces of information can be obtained for each point in an equilibrium phase diagram? 14. What is a tie-line? For what types of phase diagram regions would it be useful? 15. What points on a tie-line are used to determine the chemistry (or composition) of the component phases? 16. What tool can be used to compute the relative amounts of the component phases in a two-phase mixture? How does this tool work? 17. What is a cored structure? Under what conditions is it produced? 18. What is the difference between a cored structure and macrosegregation? 19. What features in a phase diagram can be used to identify three-phase reactions? 20. What is the general form of a eutectic reaction?

21. What is the general form of the eutectic structure? 22. Why are alloys of eutectic composition attractive for casting and as filler metals in soldering and brazing? 23. What is a stoichiometric intermetallic compound, and how would it appear in a temperature–composition phase diagram? How would a nonstoichiometric intermetallic compound appear? 24. What type of mechanical properties would be expected for intermetallic compounds? 25. In what form(s) might intermetallic compounds be undesirable in an engineering material? In what form(s) might they be attractive? 26. What are the four single phases in the iron–iron carbide diagram? Provide both the phase diagram notation and the assigned name. 27. What features of austenite make it attractive for forming operations? What features make it attractive as a starting structure for many heat treatments? 28. What feature in the iron–carbon diagram is used to distinguish between cast irons and steels? 29. Which of the three-phase reactions in the iron–carbon diagram is most important in understanding the behavior of steels? Write this reaction in terms of the interacting phases and their composition. 30. Describe the relative ability of iron to dissolve carbon in solution when in the form of austenite (the elevated temperature phase) and when in the form of ferrite at room temperature. 31. What is pearlite? Describe its structure. 32. What is a hypoeutectoid steel, and what structure will it assume upon slow cooling? What is a hypereutectoid steel, and how will its structure differ from that of a hypoeutectoid? 33. In addition to iron and carbon, what other element is present in rather large amounts in cast iron? 34. What is carbon equivalent and how is it computed? 35. What are the two possible high-carbon phases in cast irons? What features tend to favor the formation of each?

& PROBLEMS 1. Obtain a binary (two-component) phase diagram for a system not discussed in this chapter. Identify the following: a. Single phase. b. Three phase reaction. c. Intermetallic compound. 2. Copper and aluminum are both extremely ductile materials, as evidenced by the manufacture of fine copper wire and aluminum foil. Equal weights of copper and aluminum are melted together to produce an alloy and solidified in a pencil-shaped mold to produce short-length rods approximately 1 4 in. or 6.5 mm in diameter. These rods appear extremely

bright and shiny, almost as if they had been chrome-plated. When dropped on a concrete floor from about waist height, however, the rods shatter into a multitude of pieces, a behavior similar to that observed with glass. a. How might you explain this result? [Hint: Use the aluminumcopper phase diagram provided in Figure 6-3 of this text to determine the structure of 50–50 wt% alloy.] b. Several of the high-strength aerospace aluminum alloys are aluminum-copper alloys. Explain how the observations above might be useful in providing the desired properties in these alloys.

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Chapter 5

CASE STUDY

Fish Hooks

T

he fish hook has been identified as one of the most important tools in the history of man and dates back a number of centuries. Early examples were made from wood, bones, animal horn, seashells, and stone. As technology developed, so did the fish hook. Copper hooks were made in the Far East more than 7000 years ago. Bronze and iron soon followed. Steel has now become the material of preference, but hooks are available in an enormous variety—sizes, shapes and materials—with selection depending on the purpose and preference of the fisherman. To be successful, fish hooks must be super strong, but not brittle—balancing both strength and flexibility. Because they may find use in both fresh and salt water, and are often placed in storage while wet, they must possess a certain degree of corrosion resistance. The points and barbs must be sharp and retain their sharpness through use and handling. The simple single hook contains a point (the sharp end that penetrates the fish’s mouth), the barbs (backward projections that secure the bait on the hook and subsequently keeps the fish from unhooking), the bend and shank (wire lengths that form the J-portion of the hook), the gap (the specified distance between the legs of the J portion), and the eye (the rounded closure at the end of the hook where the fishing line is tied). Most hooks are currently made from high-carbon steel, alloy steel, or stainless steel. A wide variety of coatings are used to provide corrosion resistance and a full spectrum of appearances and colors. 1. Consider the manufacture of a fish hook beginning with 1080 carbon steel wire. What forming operations

would you want to perform before subjecting the material to a quench-and-temper hardening operation? Would it be best to sharpen the point before or after hardening? Would your answer be different if you were to sharpen by chemical removal instead of mechanical grinding? 2. If a stainless steel were to be used, what type of stainless would be best? Consider ferritic, austenitic, and martensitic (described in Chapter 7). What are the advantages and disadvantages of the stainless steel option? 3. A wide spectrum of coatings and surface treatments have been applied to fish hooks. These have included clear lacquer; platings of gold, tin, nickel, and other metals; Teflon; and a variety of coloration treatments involving chemical treatments, thermal oxides, and others. Select several possible surface treatments, and discuss how they might be applied while (a) maintaining the sharpness of the point and barbs and (b) preserving the mechanical balance of strength and flexibility. At what stage of manufacture should each of the proposed surface treatments be performed? 4. The geometry of the part may present some significant problems to heat treatment and surface treatment. For example, many small parts are plated by tumbling in plating baths. Masses of fish hooks would emerge as a tangled mass. Quantities of fish hooks going into a quench tank might experience similar problems. Consider the manufacturing sequences proposed earlier, and discuss how they could be specifically adapted or performed on fish hooks.

(Wikimedia Commons and Mike Cline)

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CHAPTER 6 HEAT TREATMENT 6.1 INTRODUCTION 6.2 PROCESSING HEAT TREATMENTS Equilibrium Diagrams as Aids Processing Heat Treatments for Steel Heat Treatments for Nonferrous Metals 6.3 HEAT TREATMENTS USED TO INCREASE STRENGTH 6.4 STRENGTHENING HEAT TREATMENTS FOR NONFERROUS METALS Precipitation or Age Hardening 6.5 STRENGTHENING HEAT TREATMENTS FOR STEEL

Isothermal-Transformation (I-T) or Time-TemperatureTransformation (T-T-T) Diagrams Tempering of Martensite Continuous Cooling Transformations Jominy Test for Hardenability Hardenability Considerations Quenching and Quench Media Design Concerns, Residual Stresses, Distortion, and Cracking Techniques to Reduce Cracking and Distortion

Ausforming Cold Treatment and Cryogenic Processing 6.6 SURFACE HARDENING OF STEEL Selective Heating Techniques Techniques Involving Altered Surface Chemistry 6.7 FURNACES Furnace Types and Furnace Atmospheres Furnace Control 6.8 HEAT TREATMENT AND ENERGY Case Study: A Carpenter’s Claw Hammer

& 6.1 INTRODUCTION In the previous chapters, you were introduced to the interrelationships among the structure, properties, processing, and performance of engineering materials. Chapters 4 and 5 considered aspects of structure, while Chapter 3 focused on properties. In this chapter, we begin to incorporate processing as a means of manipulating and controlling the structure and the companion properties of materials. Many engineering materials can be characterized not by a single set of properties but by an entire spectrum of possibilities that can be selected and varied at will. Heat treatment is the term used to describe the controlled heating and cooling of materials for the purpose of altering their structures and properties. The same material can be made weak and ductile for ease in manufactur, and then retreated to provide high strength and good fracture resistance for use and application. Because both physical and mechanical properties (such as strength, toughness, machinability, wear resistance, and corrosion resistance) can be altered by heat treatment, and these changes can be induced with no concurrent change in product shape, heat treatment is one of the most important and widely used manufacturing processes. Technically, the term heat treatment applies only to processes where the heating and cooling are performed for the specific purpose of altering properties, but heating and cooling often occur as incidental phases of other manufacturing processes, such as hot forming or welding. The structure and properties of the material will be altered, however, just as though an intentional heat treatment had been performed, and the results can be either beneficial or harmful. For this reason, both the individual who selects material and the person who specifies its processing must be fully aware of the possible changes that can occur during heating or cooling activities. Heat treatment should be fully integrated with other manufacturing processes if effective results are to be obtained. To provide a basic understanding, this chapter will present both the theory of heat treatment and a survey of the more common heat-treatment processes. Because more than 90% of all heat treatment is performed on steel and other ferrous metals, these materials will receive the bulk of our attention.

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Heat Treatment

& 6.2 PROCESSING HEAT TREATMENTS The term heat treatment is often associated with those thermal processes that increase the strength of a material, but the broader definition permits inclusion of another set of processes that we will call processing heat treatments. These are often performed as a means of preparing the material for fabrication. Specific objectives may be the improvement of machining characteristics, the reduction of forming forces, or the restoration of ductility to enable further processing.

EQUILIBRIUM DIAGRAMS AS AIDS Most of the processing heat treatments involve rather slow cooling or extended times at elevated temperatures. These conditions tend to approximate equilibrium, and the resulting structures, therefore, can be reasonably predicted through the use of an equilibrium phase diagram (presented in Chapter 4). These diagrams can be used to determine the temperatures that must be attained to produce a desired starting structure and to describe the changes that will then occur upon subsequent cooling. It should be noted, however, that these diagrams are for true equilibrium conditions, and any departure from equilibrium may lead to substantially different results.

PROCESSING HEAT TREATMENTS FOR STEEL Because many of the processing heat treatments are applied to plain-carbon and lowalloy steels, they will be presented here with the simplified iron–carbon equilibrium diagram of Figure 5-11 serving as a reference guide. Figure 6-1 shows this diagram with the key transition lines labeled in standard notation. The eutectoid line is designated by the symbol A1, and A3 designates the boundary between austenite and ferrite þ austenite.1 The transition from austenite to austenite þ cementite is designated as the Acm line. A number of process heat-treating operations have been classified under the general term of annealing. These may be employed to reduce strength or hardness, remove residual stresses, improve machinability, improve toughness, restore ductility, refine grain size, reduce segregation, stabilize dimensions, or alter the electrical or magnetic properties of the material. By producing a certain desired structure, characteristics can be imparted that will be favorable to subsequent operations (such as machining or forming) or applications. Because of the variety of anneals, it is important to designate the specific treatment, which is usually indicated by a preceding adjective. The specific temperatures, cooling rate, and details of the process will depend on the material being treated and the objectives of the treatment.

Ferrite + austenite

1148 Temperature °C

C06

FIGURE 6-1 Simplified iron–carbon phase diagram for steels with transition lines labeled in standard notation as A1, A3, and Acm.

912 727

Liquid + austenite

Austenite Eutectoid

A3

Acm A1

A1 Ferrite

0.02

Austenite + cementite

Ferrite + cementite

0.77

1.0

2.0

% Carbon 1 Historically, an A2 line once appeared between the A1 and A3. This line designated the magnetic property change known as the Curie point. Because this transition was later shown to be an atomic change, and not a change in phase, the line was deleted from the equilibrium phase diagram without a companion relabeling.

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Processing Heat Treatments

123

In the full annealing process, hypoeutectoid steels (less than 0.77% carbon) are heated to 30 to 60 C (50 to 100 F) above the A3 temperature, held for sufficient time to convert the structure to homogeneous single-phase austenite of uniform composition and temperature, and then slowly cooled at a controlled rate through the A1 temperature. Cooling is usually done in the furnace by decreasing the temperature by 10 to 30 C (20 to 50 F) per hour to at least 30 C (50 F) below the A1 temperature. At this point, all structural changes are complete, and the metal can be removed from the furnace and air cooled to room temperature. The resulting structure is one of coarse pearlite (widely spaced layers or lamellae) with excess ferrite in amounts predicted by the equilibrium phase diagram. In this condition, the steel is quite soft and ductile. The procedure to full-anneal a hypereutectoid alloy (greater than 0.77% carbon) is basically the same, except that the original heating is only into the austenite plus cementite region (30 to 60 C above the A1). If the material were to be slow cooled from the all-austenite region, a continuous network of cementite may form on the grain boundaries and make the entire material brittle. When properly annealed, a hypereutectoid steel will have a structure of coarse pearlite with excess cementite in dispersed spheroidal form. While full anneals produce the softest and weakest properties, they are quite time consuming, and considerable amounts of energy must be spent to maintain the elevated temperatures required during soaking and furnace cooling. When maximum softness and ductility are not required and cost savings are desirable, normalizing may be specified. In this process, the steel is heated to 60 C (100 F) above the A3 (hypoeutectoid) or Acm (hypereutectoid) temperature, held at this temperature to produce uniform austenite, and then removed from the furnace and allowed to cool in still air. The resultant structures and properties will depend on the subsequent cooling rate. Wide variations are possible, depending on the size and geometry of the product, but fine pearlite with excess ferrite or cementite is generally produced. One should note a key difference between full annealing and normalizing. In the full anneal, the furnace imposes identical cooling conditions at all locations within the metal, which results in identical structures and properties. With normalizing, the cooling will be different at different locations. Properties will vary between surface and interior, and different thickness regions will also have different properties. When subsequent processing involves a substantial amount of machining that may be automated, the added cost of a full anneal may be justified, because it produces a product with uniform machining characteristics at all locations. If cold working has severely strain hardened a metal, it is often desirable to restore the ductility, either for service or to permit further processing without danger of fracture. This is often achieved through the recrystallization process described in Chapter 4. When the material is a low-carbon steel (0.60% carbon) are to undergo extensive machining or cold-forming, a heat treatment known as spheroidization is often employed. Here, the objective is to produce a structure in which all of the cementite is in the form of small spheroids or globules dispersed throughout a ferrite matrix. This can be accomplished by a variety of techniques, including (1) prolonged heating at a temperature just below

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CHAPTER 6

Heat Treatment °C 1000

°F 1800

900

Temperature

C06

Normalizing Acm Full annealing and hardening

A3

800

A1

1400

700 Spheroidizing anneal

1200

Process anneal

600

FIGURE 6-2 Graphical summary of the process heat treatments for steels on an equilibrium diagram.

1600

1000 500

0.2

0.4 0.6 0.8 1.0 1.2 Weight percent carbon

1.4

1.6

the A1 followed by relatively slow cooling; (2) prolonged cycling between temperatures slightly above and slightly below the A1; or (3) in the case of tool or high-alloy steels, heating to 750 to 800 C (1400 to 1500 F) or higher and holding at this temperature for several hours, followed by slow cooling. Although the selection of a processing heat treatment often depends on the desired objectives, steel composition strongly influences the choice. Process anneals are restricted to low-carbon steels, and spheroidization is a treatment for high-carbon material. Normalizing and full annealing can be applied to all carbon contents, but even here, preferences are noted. Because different cooling rates do not produce a wide variation of properties in low-carbon steels, the air cool of a normalizing treatment often produces acceptable uniformity. For higher carbon contents, different cooling rates can produce wider property variations, and the uniform furnace cooling of a full anneal is often preferred. For plain-carbon steels, the recommended treatments are: 0 to 0.4% carbon—normalize; 0.4 to 0.6% carbon—full anneal; and above 0.6% carbon— spheroidize. Due to the effects of hardenability (to be discussed later in this chapter), the transition between normalizing and full annealing for alloy steels is generally lowered to 0.2% carbon. Figure 6-2 provides a graphical summary of the process heat treatments.

HEAT TREATMENTS FOR NONFERROUS METALS Most of the nonferrous metals do not have the significant phase transitions observed in the iron–carbon system, and for them, the process heat treatments do not play such a significant role. Aside from the strengthening treatment of precipitation hardening, which is discussed later, the nonferrous metals are usually heat-treated for three purposes: (1) to produce a uniform, homogeneous structure; (2) to provide stress relief; or (3) to bring about recrystallization. Castings that have been cooled too rapidly can possess a segregated solidification structure known as coring (discussed more fully in Chapter 5). Homogenization can be achieved by heating to moderate temperatures and then holding for a sufficient time to allow thorough diffusion to take place. Similarly, heating for several hours at relatively low temperatures can reduce the internal stresses that are often produced by forming, welding, or brazing. Recrystallization (discussed in Chapter 4) is a function of the particular metal, the amount of prior deformation, and the desired recrystallization time. In general, the more a metal has been strained, the lower the recrystallization temperature or the shorter the time. Without prior straining, however, recrystallization will not occur and heating will only produce undesirable grain growth.

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Strengthening Heat Treatments for Nonferrous Metals

125

& 6.3 HEAT TREATMENTS USED TO INCREASE STRENGTH Six major mechanisms are available to increase the strength of metals: 1. Solid-solution strengthening 2. Strain hardening 3. Grain-size refinement 4. Precipitation hardening 5. Dispersion hardening 6. Phase transformations While all of these may not be applicable to a given metal or alloy, these heat treatments can often play a significant role in inducing or altering the final properties of a product. In solid-solution strengthening, a base metal dissolves other atoms, either as substitutional solutions, where the new atoms occupy sites in the host crystal lattice, or as interstitial solutions, where the new atoms squeeze into ‘‘holes’’ between the atoms of the base lattice. The amount of strengthening depends on the amount of dissolved solute and the size difference of the atoms involved. Because distortion of the host structure makes dislocation movement more difficult, the greater the size difference, the more effective the addition. Strain hardening (discussed in Chapter 4) produces an increase in strength by means of plastic deformation under cold-working conditions. Because grain boundaries act as barriers to dislocation motion, a metal with small grains tends to be stronger than the same metal with larger grains. Thus, grain-size refinement can be used to increase strength, except at elevated temperatures, where grain growth can occur and grain boundary diffusion contributes to creep and failure. It is important to note that grain-size refinement is one of the few processes that can improve strength without a companion loss of ductility and toughness. In precipitation hardening, or age hardening, strength is obtained from a nonequilibrium structure that is produced by a three-step heat treatment. Details of this method will be provided in Section 6.4. Strength obtained by dispersing second-phase particles throughout a base material is known as dispersion hardening. To be effective, the dispersed particles should be stronger than the matrix, adding strength through both their reinforcing action and the additional interfacial surfaces that present barriers to dislocation movement. Phase transformation strengthening involves those alloys that can be heated to form a single phase at elevated temperature and subsequently transform to one or more low temperature phases upon cooling. When this feature is used to increase strength, the cooling is usually rapid and the phases that are produced are usually of a nonequilibrium nature.

& 6.4 STRENGTHENING HEAT TREATMENTS FOR NONFERROUS METALS All six of the mechanisms just described can be used to increase the strength of nonferrous metals. Solid-solution strengthening can impart strength to single-phase materials. Strain hardening can be quite useful if sufficient ductility is present. Alloys containing eutectic structure exhibit considerable dispersion hardening. Among all of the possibilities, however, the most effective strengthening mechanism for the nonferrous metals tends to be precipitation hardening.

PRECIPITATION OR AGE HARDENING To be a candidate for precipitation hardening, an alloy system must exhibit solubility that decreases with decreasing temperature, such as the aluminum-rich portion of the aluminum–copper system shown in Figure 6-3 and enlarged in Figure 6-4. Consider the alloy with 4% copper, and use the phase diagram to determine its equilibrium structure. Liquid metal cools through the alpha plus liquid region and solidifies into a single-phase solid (a phase). At 1000 F, the full 4% of copper would be dissolved and distributed

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Heat Treatment 1500 1400 1300 Liq

1200 Liq + θ

α

800 α+θ

θ

1000

1018ºF α

5.65%

900 800 700

600

α + liq

1100

Sol vu s

Temperature (ºF)

α + liq 1000

96% Al 4% Cu

1200

Liquid

Temperature (ºF)

C06

α+θ

600 400

500 0

Al

10

20

30

40

50 CuAl2

Weight percent copper 10 20 Atomic percent copper

400

30

FIGURE 6-3 High-aluminum section of the aluminum–copper equilibrium phase diagram.

0 100% Al

2 4 6 8 Weight percent copper

FIGURE 6-4 Enlargement of the solvus-line region of the aluminum–copper equilibrium diagram of Figure 6-3.

throughout the alpha-phase crystals. As the temperature drops, however, the maximum solubility of copper in aluminum decreases from 5.65% at 1018 F to less than 0.2% at room temperature. Upon cooling through the solvus (or solubility limit) line at 930 F, the 4% copper alloy enters a two-phase region, and copper-rich theta-phase precipitates begin to form and grow. (Note: Theta phase is actually a hard, brittle intermetallic compound with the chemical formula of CuAl2). The equilibrium structure at room temperature, therefore, would be an aluminum-rich alpha-phase structure with coarse theta-phase precipitates, generally lying along alpha-phase grain boundaries where the nucleation of second-phase particles can benefit from the existing interfacial surface. Whenever two or more phases are present, the material exhibits dispersion strengthening. Dislocations are confined to their own crystal and cannot cross interfacial boundaries. Therefore, each interface between alpha-phase and the theta-phase precipitate is a strengthening boundary. Take a particle of theta precipitate, cut it into two halves, and separate the segments. Forming the two half-size precipitates has just added two additional interfaces, corresponding to both sides of the cut. If the particle were to be further cut into quarters, eighths, and sixteenths, we would expect strength to increase as we continually add interfacial surface. Ideally, we would like to have millions of ultra-small particles dispersed throughout the alpha-phase structure. When we try to form this more desirable nonequilibrium configuration (nonequilibrium because energy is added each time new interfacial surface is created), we gain an unexpected benefit that adds significant strength. This new nonequilibrium treatment is known as age hardening or precipitation hardening. The process of precipitation hardening is actually a three-step sequence. The first step, known as solution treatment, erases the room-temperature structure and redissolves any existing precipitate. The metal is heated to a temperature above the solvus and held in the single-phase region for sufficient time to redissolve the second phase and uniformly distribute the solute atoms (in this case, copper). If the alloy were slow cooled, the second-phase precipitate would nucleate and the material would revert back to a structure similar to equilibrium. To prevent this from

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FIGURE 6-5 Two-dimensional illustrations depicting (a) a coherent precipitate cluster where the precipitate atoms are larger than those in the host structure and (b) its companion overaged or discrete secondphase precipitate particle. Parts (c) and (d) show equivalent sketches where the precipitate atoms are smaller than the host.

Strengthening Heat Treatments for Nonferrous Metals

(a)

(b)

(c)

(d)

127

happening, age hardening alloys are quenched from their solution treatment temperature. The rapid-cool quenching, usually in water, suppresses diffusion, trapping the dissolved atoms in place. The result is a room-temperature supersaturated solid solution. For the alloy discussed earlier, the alpha phase would now be holding 4% copper in solution at room temperature—far in excess of its equilibrium maximum of 1

Increases hardenability by lowering transformation points and causing transformations to be sluggish

Molybdenum

0.2–5

Stable carbides; inhibits grain growth

Nickel

2–5

Toughener

12–20

Corrosion resistance

0.2–0.7 2

Increases strength Spring steels

Silicon

Higher percentages

Improves magnetic properties

Sulfur

0.08–0.15

Free-machining properties

Titanium

Fixes carbon in inert particles

Tungsten

Hardness at high temperatures

Vanadium

0.15

Stable carbides; increases strength while retaining ductility, Promotes fine grain structure

Reduces martensitic hardness in chromium steels

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however, resulting in the immense variety of alloy steels that are commercially available. To provide some degree of simplification, a classification system has been developed and has achieved general acceptance in a variety of industries.

AISI–SAE CLASSIFICATION SYSTEM The most common classification scheme for alloy steels is the AISI–SAE identification system. This system, which classifies alloys by chemistry, was started by the Society of Automotive Engineers (SAE) to provide some standardization for the steels used in the automotive industry. It was later adopted and expanded by the American Iron and Steel Institute (AISI) and has been incorporated into the Universal Numbering System that was developed to include all engineering metals. Both plain-carbon and low-alloy steels are identified by a four-digit number, where the first number indicates the major alloying elements and the second number designates a subgrouping within the major alloy system. These first two digits can be interpreted by looking them up on a list, such as the one presented in Table 7-4. The last two digits of the number indicate the approximate amount of carbon, expressed as ‘‘points,’’ where one point is equal to 0.01%. Thus, a 1080 steel would be a plain-carbon steel with 0.80% carbon. Similarly, a 4340 steel would be a Mo–Cr–Ni alloy with 0.40% carbon. Because of the meanings associated with the numbers, the designation is not read as a series of single digits, such as

TABLE 7-4

AISI–SAE Standard Steel Designations and Associated Chemistries Alloying Elements (%)

AISI Number 1xxx

Type

Mn

Ni

Cr

Plain carbon

11xx 12xx

Free cutting (S) Free cutting (S) and (P)

13xx

High manganese

15xx

High manganese Nickel steels

3.5–5.0

3xxx

Nockel–chromium

1.0–3.5

0.5–1.75

4xxx

Molybdenum

1.65–2.00

0.40–1.10 0.40–0.90

40xx

Mo

41xx 43xx

Mo, Cr Mo, Cr, Ni

44xx

Mo

46xx

Mo, Ni (low)

0.70–2.00

47xx

Mo, Cr, Ni

0.90–1.20

48xx

Mo, Ni (high)

3.25–3.75

0.15–0.30 0.08–0.35 0.20–0.30 0.35–0.60 0.15–0.30 0.35–0.55

0.15–0.40 0.20–0.30

Chromium

50xx

0.20–0.60 0.70–1.15 Chromum–vanadium

61xx 8xxx

0.50–1.10

0.10–0.15

Ni, Cr,Mo

81xx

0.20–0.40

0.30–0.55

0.08–0.15

86xx

0.40–0.70

0.40–0.60

0.15–0.25

87xx

0.40–0.70

0.40–0.60

0.20–0.30

88xx

0.40–0.70

0.40–0.60

0.30–0.40

9xxx 92xx

Other

1.60–1.90

2xxx

51xx 6xxx

V

Carbon steels

10xx

5xxx

Mo

Other High silicon

1.20–2.20Si

93xx

Ni, Cr,Mo

3.00–3.50

1.00–1.40

0.08–0.15

94xx

Ni, Cr,Mo

0.30–0.60

0.30–0.50

0.08–0.15

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Steel

161

four-three-four-zero, but as a pair of double-digit groupings, such as ten-eighty or fortythree forty. Letters may also be incorporated into the designation. The letter B between the second and third digits indicates that the base metal has been supplemented by the addition of boron. Similarly, an L in this position indicates a lead addition for enhanced machinability. A letter prefix may also be employed to designate the process used to produce the steel, such as E for electric furnace. When hardenability is a major requirement, one might consider the H grades of AISI steels, designated by an H suffix attached to the standard designation. The chemistry specifications are somewhat less stringent, but the steel must now meet a hardenability standard. The hardness values obtained for each location on a Jominy test specimen (see Chapter 6) must fall within a predetermined band for that particular type of steel. When the AISI designation is followed by an RH suffix (restricted hardenability), an even narrower range of hardness values is imposed. Other designation organizations, such as the American Society for Testing and Materials (ASTM) and the U.S. government [military (MIL) and federal], have specification systems based more on specific applications. Acceptance into a given classification is generally determined by physical or mechanical properties rather than the chemistry of the metal. ASTM designations are often used when specifying structural steels.

SELECTING ALLOY STEELS From the previous discussion it is apparent that two or more alloying elements can often produce similar effects. Thus, when properly heat-treated, steels with substantially different chemical compositions can possess almost identical mechanical properties. Figure 7-8 clearly demonstrates this fact, which becomes particularly important when one realizes that some alloying elements can be very costly and others may be in short supply due to emergencies or political constraints. Overspecification has often been employed to guarantee success despite sloppy manufacturing and heat-treatment

260

240

220 Tensile strength, 1000 psi

C07

200

Yield strength

180 Elongation 160

Water quenched

140

SAE 120

FIGURE 7-8 Relationships between the mechanical properties of a variety of properly heat-treated AISI–SAE alloy steels. (Courtesy of ASM International, Materials Park, OH)

100 80

1330 2330 3130 4130 5130 6130

120 160 200 Yield strength, 1000 psi

240

Reduction of area

0 20 40 60 Elongation and reduction of area, %

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practice. The correct steel, however, is usually the least expensive one that can be consistently processed to achieve the desired properties. This usually involves taking advantage of the effects provided by all of the alloy elements. When selecting alloy steels, it is also important to consider both use and fabrication. For one product, it might be permissible to increase the carbon content to obtain greater strength. For another application, such as one involving assembly by welding, it might be best to keep the carbon content low and use a balanced amount of alloy elements, obtaining the desired strength while minimizing the risk of weld cracking. Steel selection involves defining the required properties, determining the best microstructure to provide those properties (strength can be achieved through alloying, cold work, and heat treatment, as well as combinations thereof), determining the method of part or product manufacture (casting, machining, metal forming, etc.), and then selecting the steel with the best carbon content and hardenability characteristics to facilitate those processes and achieve the desired goals.

HIGH-STRENGTH LOW-ALLOY STRUCTURAL STEELS Among the general categories of alloy steels are (1) the constructional alloys, where the desired properties are typically developed by a separate thermal treatment and the specific alloy elements tend to be selected for their effect on hardenability, and (2) the highstrength low-alloy (HSLA) or microalloyed types, which rely largely on chemical composition to develop the desired properties in the as-rolled or normalized condition. The constructional alloys are usually purchased by AISI–SAE identification, which effectively specifies chemistry. The HSLA designations generally focus on product (size and shape) and desired properties. When steels are specified by mechanical properties, the supplier or producer is free to adjust the chemistry (within limits), and substantial cost savings may result. To ensure success, however, it is important that all of the necessary properties be specified. The HSLA materials provide increased strength-to-weight compared to conventional carbon steels for only a modest increase in cost. They are available in a variety of forms, including sheet, strip, plate, structural shapes, and bars. The dominant property requirements generally are high yield strength, good weldability, and acceptable corrosion resistance. Ductility and hardenability may be somewhat limited, however. The increase in strength, and the resistance to martensite formation in a weld zone, is obtained by controlling the amounts of carbon, manganese, and silicon, with the addition of small amounts of niobium, vanadium, titanium or other alloys. About 0.2% copper can be added to improve corrosion resistance. Because of their higher yield strength, weight savings of 20 to 30% can often be achieved with no sacrifice to strength or safety. Rolled and welded HSLA steels are being used in automobiles, trains, bridges, and buildings. Because of their low-alloy content and high-volume application, their cost is often little more than that of the ordinary plain-carbon steels. Table 7-5 presents the chemistries and properties of several of the more common types. TABLE 7-5

Typical Compositions and Strength Properties of Several Groups of High-Strength Low-Alloy (HSLA) Structural Steels Strength Properties Chemical Compositionsa (%)

Group

C

Mn

Si

Cb 0.01

Yield V

ksi

MPa

Tensile ksi

MPa

Elongation in 2 in. (%)

Columbium or vanadium

0.20

1.25

0.30

0.01

55

379

70

483

20

Low manganese–vanadium Manganese–copper

0.10 0.25

0.50 1.20

0.10 0.30

0.02

40 50

276 345

60 75

414 517

35 20

Manganese–vanadium–copper

0.22

1.25

0.30

0.02

50

345

70

483

22

a

All have 0.04% P, 0.05% S, and 0.20% Cu.

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Steel

163

MICROALLOYED STEELS IN MANUFACTURED PRODUCTS In terms of both cost and performance, microalloyed steels occupy a position between carbon steels and the alloy grades, and are being used increasingly as substitutes for heat-treated steels in the manufacture of small- to medium-sized discrete parts. These low- and medium-carbon steels contain small amounts (0.05 to 0.15%) of alloying elements, such as niobium, vanadium, titanium, molybdenum, zirconium, boron, rare earth elements, or combinations thereof. Many of these additions form alloy carbides— nitrides or carbonitrides—whose primary effect is to provide grain refinement and/or precipitation strengthening. Yield strengths between 500 and 750 MPa (70 and 110 ksi) can be obtained without heat treatment. Weldability can be retained or even improved if the carbon content is simultaneously decreased. In essence, these steels offer maximum strength with minimum carbon, while simultaneously preserving weldability, machinability, and formability. Compared to a quenched-and-tempered alternative, however, ductility and toughness are generally somewhat inferior. Cold-formed microalloyed steels require less cold work to achieve a desired level of strength, so they tend to have greater residual ductility. Hot-formed products, such as forgings, can often be used in the air-cooled condition. By means of accurate temperature control and controlled-rate cooling directly from the forming operation, mechanical properties can be produced that approximate those of quenched-and-tempered material. Machinability can be enhanced because of the more uniform hardness and the fact that the ferrite–pearlite structure of the microalloyed steel is often more machinable than the ferrite–carbide structure of the quenched-and-tempered variety. Fatigue life and wear resistance can also be superior to those of the heat-treated counterparts. In applications where the properties are adequate, microalloyed steels can often provide attractive cost savings. Energy savings can be substantial, straightening or stress relieving after heat treatment is no longer necessary, and quench cracking is not a problem. Due to the increase in material strength, the size and weight of finished products can often be reduced. As a result, the cost of a finished forging may be reduced by 5 to 25%. If these materials are to attain their optimum properties, however, certain precautions must be observed. During the elevated-temperature segments of processing, the material must be heated high enough to place all of the alloys into solution. After forming, the products should be rapidly air cooled to 540 to 600 C (1000 to 1100 F) before dropping into collector boxes. In addition, microalloyed steels tend to through-harden upon air cooling, so products fail to exhibit the lower-strength, higher-toughness interiors that are typical of the quenched-and-tempered materials.

BAKE-HARDENABLE STEEL SHEET Bake-hardenable steel has assumed a significant role in automotive sheet applications. These low-carbon steels are processed in such a way that they are resistant to aging during normal storage but begin to age during sheet-metal forming. A subsequent exposure to heat during the paint-baking operation completes the aging process and adds an additional 35 to 70 MPa (5 to 10 ksi), raising the final yield strength to approximately 275 MPa (40 ksi). Because the increase in strength occurs after the forming operation, the material offers good formability coupled with improved dent resistance in the final product. In addition, it allows weight savings to be achieved without compromising the attractive features of steel sheet, which include spot weldability, good crash energy absorption, low cost, and full recyclability.

ADVANCED HIGH-STRENGTH STEELS (AHSS) Traditional methods of producing high-strength steel have included adding carbon and/ or alloy elements followed by heat treatment or cold-working to a high level followed by a partial anneal to restore some ductility. As strength increased, however, ductility and toughness decreased and often became a limiting feature. The high-strength lowalloy (HSLA) and microalloyed steels, introduced about 40 years ago, used thermomechanical processing to further increase strength, but were accompanied by an even further decline in ductility. Beginning in the mid-1990s, enhanced thermomechanical processing capabilities and controls have led to the development of a variety of new

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high-strength steels that go collectively by the name advanced high-strength steel (AHSS). Many were developed for weight savings in automotive applications (higher strength enabling reduced size or thickness) while preserving or enhancing energy absorption. As a result, large amounts of low-carbon and HSLA steels are being replaced by the advanced high-strength steels. One group (including the dual-phase and TRIP steels to be discussed later) provided greater formability for already-existing levels of strength. Another group (including complex-phase and martensitic varieties) provided higher levels of strength while retaining current levels of ductility. Because of the improved formability, the AHSS materials can often be stamped or hydroformed into more complex parts. Parts can often be integrated into single pieces, eliminating the cost and time associated with assembly, and the higher strength can provide weight reduction accompanied by improved fatigue and crash performance. The various types of advanced high-strength steels are primarily ferrite-phase, soft steels with varying amounts of martensite, bainite, or retained austenite that offer high strength with enhanced ductility. Each of the types will be described briefly below: Dual-phase steels form when material is cooled to a temperature that is above the A1 but below the A3 to form a structure that consists of ferrite and high-carbon austenite, and then followed with a rapid-cool quench. During the quench, the ferrite remains unaffected, while the high-carbon austenite transforms to high-carbon martensite. A lowor medium-carbon steel now has a mixed microstructure of a continuous, weak, ductile ferrite matrix combined with islands of high-strength, high-hardness, high-carbon martensite. The dual-phase structure offers strengths that are comparable to HSLA materials, coupled with improved forming characteristics and no loss in weldability. The high workhardening rates and excellent elongation lead to a high ultimate tensile strength coupled with a low initial yield strength. The high strain-rate sensitivity means that the faster the steel is crushed, the more energy it absorbs—a feature that further enhances the crash resistance of automotive structures. Dual-phase steels also exhibit the bake-hardening effect, a precipitation-induced increase in yield strength when stamping or forming is followed by the elevated temperature of a paint-bake oven. While the dual-phase steels have structures of ferrite and martensite, transformationinduced plasticity (TRIP) steels contain a matrix of ferrite combined with hard martensite or bainite and at least 5 vol% of retained austenite. Because of the hard phase dispersed in the soft ferrite, deformation behavior begins much like the dual-phase steels. At higher strains, however, the retained austenite transforms progressively to martensite, enabling the high work-hardening to persist to greater levels of deformation. At lower levels of carbon, the austenite transformation begins at lower levels of strain, and the extended ductility of the lower-carbon TRIP steels offers significant advantages in operations such as stretch-forming and deep-drawing. At higher carbon levels, the retained austenite is more stable and requires greater strains to induce transformation. If the retained austenite can be carried into a finished part, subsequent deformation (such as a crash) can induce transformation. The conversion of retained austenite to martensite, and the companion high rate of work hardening, can then be used to provide excellent energy absorption. Complex-phase (CP) steels and martensitic (Mart) steels offer even higher strengths with useful capacity for deformation and energy absorption. The CP steels have a microstructure of ferrite and bainite, combined with small amounts of martensite, retained austenite and pearlite, and are strengthened further by grain refinement created by a fine precipitate of niobium, titanium or vanadium carbides or nitrides. The Mart steels are almost entirely martensite, and can have tensile strengths up to 1700 MPa (245 ksi). Still other types are under development and are making the transition from research into production. These include the ferritic-bainitic (FB) steels—also known as stretch-flangeable (SF) and high hole expansion (HHE) because of the improved stretch formability of sheared edges—twinning-induced plasticity (TWIP) steels, nano steels, and others. The FB steels have a microstructure of fine ferrite and bainite, coupled with grain refinement. TWIP steels contain between 17 and 24% manganese, making the steel fully austenitic at room temperature. Deformation occurs by twinning inside the grains, with the newly created twin boundaries providing increased strength and a high rate of strain hardening. The result is a high strength (>1000 MPa or 145 ksi)

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Steel

165

70 Low-strength

High-strength steels

60 steels (700 MPa)

50 Conventional HSS

40 MILD

30

AHSS BH

20

DP

HSLA

TRIP

10

MART

0 0

300

600

900

1200

1700

Tensile strength (MPa) FIGURE 7-9 Relative strength and formability (elongation) of conventional, high-strength lowalloy, and advanced high-strength steels. BH ¼ bake hardenable; DP ¼ dual phase; Mart ¼ martensitic.

combined with extremely high ductility (as high as 70% elongation). Nano steels replace the hard phases that are present in the dual-phase (DP) and TRIP steels with an array of ultra-fine nano-sized precipitates (diameters 1000 ft/min) finish machining of steels and some malleable cast irons. Cemented carbide tools are available in insert form in many different shapes: squares, triangles, diamonds, and rounds. They can be either brazed or mechanically clamped onto the tool shank. Mechanical clamping (Figure 21-10) is more popular because when one edge or corner becomes dull, the insert is rotated or turned over to expose a new cutting edge. Mechanical inserts can be purchased in the as-pressed state, or the insert can be ground to closer tolerances. Naturally, precision-ground inserts cost more. Any part tolerance less than 0.003 normally cannot be manufactured without radial adjustment of the cutting tool, even with ground inserts. If no radial adjustment is performed, precision-ground inserts should be used only when the part tolerance is between 0.006 and 0.003. Pressed inserts have an application advantage because the

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Cutting Tools for Machining Insert holder size

C

Insert type and size Insert screw

B

Adapter size

A

D

Groove width Groove angle

FIGURE 21-10 Boring head with carbide insert cutting tools. These inserts have a chip groove that can cause the chips to curl tightly and break into small, easily disposed lengths.

Groove depth

Land width Coating thickness 0.005 mm (0.0002 in.)

cutting edge is unground and thus does not leave grinding marks on the part after machining. Ground inserts can break under heavy cutting loads because the grinding marks on the insert produce stress concentrations that result in brittle fracture. Diamond grinding is used to finish carbide tools. Abusive grinding can lead to thermal cracks and premature (early) failure of the tool. Brazed tools have the carbide insert brazed to the steel tool shank. These tools will have a more accurate geometry than the mechanical insert tools, but they are more expensive. Because cemented carbide tools are relatively brittle, a 90degree corner angle at the cutting edge is desired. To strengthen the edge and prevent edge chipping, it is rounded off by honing, or an appropriate chamfer or a negative land (a T-land) on the rake face is provided. The preparation of the cutting edge can affect tool life. The sharper the edge (smaller edge radius), the more likely the edge is to chip or break. Increasing the edge radius will increase the cutting forces, so a trade-off is required. Typical edge radius values are 0.001 to 0.003 in. A chip groove (see Figure 21-10) with a positive rake angle at the tool tip may also be used to reduce cutting forces without reducing the overall strength of the insert significantly. The groove also breaks up the chips by causing them to curl tightly, thus making disposal easier. For very low-speed cutting operations, the chips tend to weld to the tool face and cause subsequent microchipping of the cutting edge. Cutting speeds for carbides are generally in the range of 150 to 600 ft/min. Higher speeds (>1000 ft/min) are recommended for certain less-difficult-to-machine materials (such as aluminum alloys) and much lower speeds (100 ft/min) for more difficult-to-machine materials (such as titanium alloys). In interrupted cutting applications, it is important to prevent edge chipping by choosing the appropriate cutter geometry and cutter position with respect to the workpiece. For interrupted cutting, finer grain size and higher cobalt content improve toughness in straight WC–Co grades. After use, carbide inserts (called disposable or throwaway inserts) are generally recycled in order to reclaim the tantalum, WC, and cobalt. This recycling not only conserves strategic materials but also reduces costs. A new trend is to regrind these tools for future use where the actual size of the insert is not of critical concern.

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Cutting Tool Materials

581

COATED-CARBIDE TOOLS Beginning in 1969 with TiC-coated WC, coated tools became the norm in the metalworking industry because coating can consistently improve tool life 200 or 300% or more. In cutting tools, material requirements at the surface of the tool need to be abrasion resistant, hard, and chemically inert to prevent the tool and the work material from interacting chemically with each other during cutting. A thin, chemically stable, hard refractory coating accomplishes this objective. The bulk of the tool is a tough, shockresistant carbide that can withstand high-temperature plastic deformation and resist breakage. The result is a composite tool as shown in Figure 21-11. The coatings must be fine grained, free of binders and porosity. Naturally, the coatings must be metallurgically bonded to the substrate. Interface coatings are graded to match the properties of the coating and the substrate. The coatings must be thick enough to prolong tool life but thin enough to prevent brittleness. Coatings should have a low coefficient of friction so the chips do not adhere to the rake face. Coating materials include single coatings of TiC, TiN, Al2O3, HfN, or HfC. Multiple coatings are used, with each layer imparting its own characteristic to the tool. Successful coating combinations include TiN/TiC/TiCN/TiN and TiC/Al2O3/TiN. Chemical vapor deposition is used to obtain coated carbides. The coatings are formed by chemical reactions that take place only on or near the substrate. Like electroplating, CVD is a process in which the deposit is built up atom by atom. It is, therefore, capable of producing deposits of maximum density and of closely reproducing fine detail on the substrate surface. Control of critical variables such as temperature, gas concentration, and flow pattern is required to ensure adhesion of the coating to the substrate. The coating-tosubstrate adhesion must be better for cutting tool inserts than for most other coatings applications to survive the cutting pressure and temperature conditions without flaking off. Grain size and shape are controlled by varying temperature and/or pressure. The purpose of multiple coatings is to tailor the coating thickness for prolonged tool life. Multiple coatings allow a stronger metallurgical bond between the coating and the substrate and provide a variety of protection processes for machining different work materials, thus offering a more general-purpose tool material grade. A very thin final coat of TiN coating (in microns, or mm) can effectively reduce crater formation on the tool face by one to two orders of magnitude relative to uncoated tools. Coated inserts of carbides are finding wide acceptance in many metalcutting applications. Coated tools have two or three times the wear resistance of the best uncoated tools with the same breakage resistance. This results in a 50 to 100% increase in speed for the same tool life. Because most coated inserts cover a broader application range, fewer grades are needed; therefore, inventory costs are lower. Aluminum oxide coatings have demonstrated excellent crater wear resistance by providing a chemical diffusion reaction barrier at the tool–chip interface, permitting a 90% increase in cutting speeds in machining some steels. Coated-carbide tools have progressed to the place where in the United States about 80 to 90% of the carbide tools used in metalworking are coated.

CERAMICS Ceramics are made of pure aluminum oxide, Al2O3, or Al2O3 used as a metallic binder. Using P/M, very fine particles are formed into cutting tips under a pressure of 267 to 386 MPa (20 to 28 ton/in.2) and sintered at about 1000 C (1800 F). Unlike the case with ordinary ceramics, sintering occurs without a vitreous phase. Ceramics are usually in the form of disposable tips. They can be operated at two to three times the cutting speeds of tungsten carbide. They almost completely resist cratering, run with no coolant, and have about the same tool life at their higher speeds as tungsten carbide does at lower speeds. As shown in Table 21-2, ceramics are usually as hard as carbides but are more brittle (lower bend strength) and therefore require more rigid tool holders and machine tools in order to take advantage of their capabilities. Their hardness and chemical inertness make ceramics a good material for high-speed finishing and/or high-removal-rate machining applications of superalloys, hard-chill

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Titanium carbide remains as the basic material covering the substrate for strength and wear resistance. The second layer is aluminium oxide, which has proven chemical stability at high temperatures and resists abrasive wear. The third layer is a thin coating of titanium nitride to give the insert a lower coefficient of friction and to reduce edge build up. Titanium nitride coating—low coefficient of friction

Tungs carbid ten e core

WC

Titanium nitride coating Aluminum oxide–2nd layer Titanium carbide–1st layer

Relative thickness of coatings

Carbide substrate

Aluminum oxide 2nd layer Titanium carbide (TiCN) as first layer—strength and wear resistance Al2O3 Aluminum oxide 2nd layer—chemical stability at high temperature—resists abrasive wear

TiN Al2O3 TiCN

Special carbide substrate

FIGURE 21-11 Triple-coated carbide tools provide resistance to wear and plastic deformation in machining of steel, abrasive wear in cast iron, and built-up edge formation. (Courtesy J T. Black)

cast iron, and high-strength steels. Because ceramics have poor thermal and mechanical shock resistance, interrupted cuts and interrupted application of coolants can lead to premature tool failure. Edge chipping is usually the dominant mode of tool failure. Ceramics are not suitable for aluminum, titanium, and other materials that react chemically with alumina-based ceramics. Recently, whisker-reinforced ceramic materials that have greater transverse rupture strength have been developed. The whiskers are made from silicon carbide.

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TABLE 21-2

583

Cutting Tool Materials

Properties of Cutting Tool Materials Compared for Carbides, Ceramics, HSS, and Cast Cobalta Hardness Rockwell A or C

Transverse Rupture (bend) Strength ( l03 psi)

Compressive Strength ( l03 psi)

90–95 RA 91–93 RA

250–320 100–250

750–860 710–840

High–speed steel

86 RA

600

600–650

30

Ceramic (oxide)

92–94 RA

100–125

400–650

50–60

Cast cobalt

46–62 RC

80–120

220–335

40

Carbide C1–C4 Carbide C5–C8

a

Modulus of Elasticity (e) ( l06 psi) 89–93 66–81

Exact properties depend on materials, grain size, bonder content, volume.

CERMETS Cermets are a class of tool materials often used for finishing processes. Cermets are ceramic TiC, nickel, cobalt, and tantalum nitrides. TiN and other carbides are used for binders. Cermets have superior wear resistance, longer tool life, and can operate at higher cutting speeds with superior wear resistance. Cermets have higher hot hardness and oxidation resistance than cemented carbides. The better finish imparted by a cermet is due to its low level of chemical reaction with iron [less cratering and built-up edge (BUE) formation]. Compared to carbide, the cermet has less toughness, lower thermal conductivity, and greater thermal expansion, so thermal cracking can be a problem during interrupted cuts. Cermets are usually cold pressed, and proper processing techniques are required to prevent insert cracking. New cermets are designed to resist thermal shocking during milling by using high nitrogen content in the titanium carbonitride phase (produces finer grain size) and adding WC and TaC to improve shock resistance. PVD-coated cermets have the wear resistance of cermets and the toughness range of a coated carbide, and they perform well with a coolant. Figure 21-12 shows a comparison of speed feed coverage of typical cermets compared to ceramics, carbides, and coated carbides. The values illustrate that cermets can clearly cover a wide range of important metal-cutting applications.

DIAMONDS Diamond is the hardest material known. Industrial diamonds are now available in the form of polycrystalline compacts, which are finding industrial application in the machining of aluminum, bronze, and plastics, greatly reducing the cutting forces as compared to carbides. Diamond machining is done at high speeds, with fine feeds for finishing, and produces excellent finishes. Recently, single-crystal diamonds, with a cutting-edge radius of 100 A or less, have been used for precision machining of large mirrors. However, single-crystal diamonds have been used for years to machine brass watch faces, thus eliminating polishing. They have also been used to slice biological materials into thin films for viewing in transmission electron microscopes. (This process, known as ultramicrotomy, is one of the few industrial versions of orthogonal machining in common practice.) The salient features of diamond tools include high hardness; good thermal conductivity; the ability to form a sharp edge of cleavage (single-crystal, natural diamond); very low friction; nonadherence to most materials; the ability to maintain a sharp edge for a long period of time, especially in machining soft materials such as copper and aluminum; and good wear resistance. To be weighed against these advantages are some shortcomings, which include a tendency to interact chemically with elements of Group IVB to Group VIII of the periodic table. In addition, diamond wears rapidly when machining or grinding mild steel. It wears less rapidly with high-carbon alloy steels than with low-carbon steel and has occasionally machined gray cast iron (which has high carbon content) with long life.

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PCD or CBN Ceramics

1500 Speed (sfpm)

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Cermets Coated cermets Coated carbides

1000 Tungsten carbide 500

High-speed steel

0.005

0.01

0.015

0.020

0.025

ipr

Feedrate and toughness increase

Wear resistance increase

Tool Material Group

General Applications

Versus Cermet

PCD (polycrystal diamond)

High-speed machining of aluminum alloys, nonferrous metals, and nonmetals.

Cermets can machine same materials, but at lower speeds and significantly less cost per corner.

CBN (cubic boron nitride)

Hard workpieces and high-speed machining on cast irons.

Cermets cannot machine the harder workpieces that CBN can. Cermets cannot machine cast iron at the speeds CBN can. The cost per corner of cermets is significantly less.

Ceramics (cold press)

High-speed turning and grooving of steels and cast iron.

Cermets are more versatile and less expensive than cold press ceramics but cannot run at the higher speeds.

Ceramics (hot press)

Turning and grooving of hard workpieces; high-speed finish machining of steels and irons.

Cermets cannot machine the harder workpieces or run at the same speeds on steels and irons but are more versatile and less expensive.

Ceramics (silicon nitride)

Rough and semirough machining of cast irons in turning and milling applications at high speeds and under unfavorable conditions.

Cermets cannot machine cast iron at the high speeds of silicon nitride ceramics, but in moderate-speed applications cermets may be more cost effective.

Coated carbide

General-purpose machining of steels, stainless steels, cast iron, etc.

Cermets can run at higher cutting speeds and provide better tool life at less cost for semiroughing to finishing applications.

Carbides

Tough material for lower-speed applications on various materials.

Cermets can run at higher speeds, provide better surface finishes and longer tool life for semiroughing to finishing applications.

FIGURE 21-12

Comparison of cermets with various cutting tool materials.

Diamond has a tendency to revert at high temperatures (700 C) to graphite and/or to oxidize in air. Diamond is very brittle and is difficult and costly to shape into cutting tools—the process for doing the latter being a tightly held industry practice.

POLYCRYSTALLINE DIAMONDS The limited supply of, increasing demand for, and high cost of natural diamonds led to the ultra-high-pressure (50 Kbar), high-temperature (1500 C) synthesis of diamond

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Cutting Tool Materials

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from graphite at the General Electric Company in the mid-1950s and the subsequent development of polycrystalline sintered diamond tools in the late 1960s. Polycrystalline diamond (PCD) tools consist of a thin layer (0.5 to 1.5 mm) of finegrain-size diamond particles sintered together and metallurgically bonded to a cemented carbide substrate. A high-temperature/high-pressure process, using conditions close to those used for the initial synthesis of diamond, is needed. Fine diamond powder (1 to 30 mm) is first packed on a support base of cemented carbide in the press. At the appropriate sintering conditions of pressure and temperature in the diamond stable region, complete consolidation and extensive diamond-to-diamond bonding take place. Laser cutting followed by grinding is used to shape, size, and accuracy finish PCD tools, as outlined in Figure 21-13. The cemented carbide provides the necessary elastic support for the hard and brittle diamond layer above it. The main advantages of sintered polycrystalline tools over natural single-crystal tools are better quality, greater toughness, and improved wear resistance, resulting from the random orientation of the diamond grains and the lack of large cleavage planes. Diamond tools offer dramatic

Polycrystal diamond Sintered carbide substrate Raw material from sintering and compacting Laser dicing disk into segments

Carbide inserts with precision pockets to accept the segment

Standard tungsten carbide insert

Segment

After the segment is brazed to the carbide insert, the insert is ready for use

FIGURE 21-13 The process to make polycrystalline diamond tools uses carbides and diamond inserts and are restricted to simple geometries.

Compax blank (0.020 in. [0.51 mm] thick diamond layer with carbide substrate)

Chamfer Braze line

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performance improvements over carbides. Tool life is often greatly improved, as is control over part size, finish, and surface integrity. Positive rake tooling is recommended for the vast majority of diamond tooling applications. If BUE formation is a problem, increasing cutting speed and using more positive rake angles may eliminate it. If edge breakage and chipping are problems, the feed rate can be reduced. Coolants are not generally used in diamond machining unless, as in the machining of plastics, it is necessary to reduce airborne dust particles. Diamond tools can be reground. There is much commercial interest in being able to coat HSS and carbides directly with diamond, but getting the diamond coating to adhere reliably has been difficult. Diamond-coated inserts would deliver roughly the same performance as PCD tooling when cutting nonferrous materials but could be given more complex geometries and chip breakers while reducing the cost per cutting edge.

POLYCRYSTALLINE CUBIC BORON NITRIDES Polycrystalline cubic boron nitride (PCBN) is a man-made tool material widely used in the automotive industry for machining hardened steels and superalloys. It is made in a compact form for tools by a process quite similar to that used for sintered polycrystalline diamonds. It retains its hardness at elevated temperatures (Knoop 4700 at 20 C, 4000 at 1000 C) and has low chemical reactivity at the tool–chip interface. This material can be used to machine hard aerospace materials like Inconel 718 and Ren e 95 as well as chilled cast iron. Although not as hard as diamond, PCBN is less reactive with such materials as hardened steels, hard-chill cast iron, and nickel- and cobalt-based superalloys. PCBN can be used efficiently and economically to machine these difficult-to-machine materials at higher speeds (fivefold) and with a higher removal rate (fivefold) than cemented carbide, and with superior accuracy, finish, and surface integrity. PCBN tools are available in basically the same sizes and shapes as sintered diamond and are made by the same process. The cost of an insert is somewhat higher than either cemented carbide or ceramic tools, but the tool life may be five to seven times that of a ceramic tool. Therefore, to see the economy of using PCBN tools, it is necessary to consider all the factors. Here is an industrial example of analysis of tooling economics, where a comparison is being made between two tool materials (insert tools). A manufacturer of diesel engines is producing an in-line six-cylinder engine block that is machined on a transfer line. Each cylinder hole must be bored to accept a sleeve liner. This operation has a depth of cut of 0.062 in. per side, for a total of 0.125 in. stock removal. The tolerance on this bore is 0.001 in. and the spindle is operating at 2000 sfpm. Ceramic inserts are used on this operation, but with these inserts, wear was severe enough to require indexing after only 35 pieces. The ceramic insert was replaced with PCBN inserts made of a highcontent BCN. Both inserts had a 0.001- to 0.002-in. radius hone for edge preparation. Table 21-3 is a cost comparison between the ceramic and PCBN insert. The PCBN insert used in the application is a full-top PCBN insert, meaning that the entire top of the insert is a layer of PCBN material. At first glance the PCBN tool appears to be extremely expensive. Each insert costs $208.00 and provides only three usable edges, whereas the ceramic insert costs $14.90 and provides six usable edges. However, the ceramic tool must be indexed every 35 pieces. The PCBN tool is indexed every 500 pieces. The cost per bore, including insert cost and the cost of labor to perform indexing, comes to $0.125 per bore for the ceramic tool and $0.142 per bore for the PCBN tool. This appears to make the ceramic tool more cost-effective, but downtime for indexing has not been accounted for. In this application, the ceramic insert required 10.75 hours of downtime for indexing, whereas the PCBN tool required only 0.75 hour of downtime for indexing. Use of the PCBN cutting tool significantly reduces the total cost per piece by eliminating 10 hours of downtime of the machine. Later in this chapter the economics of machining will be addressed again. The two predominant wear modes of PCBN tools are notching at the depth-of-cut line (DCL) and microchipping. In some cases, the tool will exhibit flank wear of the cutting edge. These tools have been used successfully for heavy interrupted cutting and

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SECTION 21.3

TABLE 21-3

587

Tool Geometry

Cost Comparison for Machining Liner Bores in 1500 Engine Blocksa Ceramic TNG-433

PCBN BTNG-433

Cost per insert

$14.90

$208.00

Edges per insert

6

3

Cost per edge

$2.48

$69.33

Time per index (6 tools)

0.25 hr

0.25 hr

Cost per index at $45 per hour

$11.25

$11.25

Indexes per 1500 blocks

43

3

Indexing cost (indexes $11.25) Insert cost for 6 spindles

$483.75 $638.34

$33.75 $1248.00

Labor and tool cost

$1122.09

$1281.00

Cost per bore

$0.125

$0.142

Total number of tool changes

43

3

Downtime for 1500 blocks

0:25 hr 10:75 hr

0:25 hr 0:75 hr

a To see the economy of using PCBN cutting tools, it is important to consider all factors of the operation, especially downtime for tool changing.

TABLE 21-4

Suggested Application of Four (4) Cutting Tool Materials to Workpiece Materials Applicable Tool Material

Workplace Material

Carbide-Coated Carbide

Cast irons carbon steels

Ceramic, Cermet

Cubic Boron Nitride

Diamond Compacts

uninterrupted finishing cuts X

X

Alloy steels alloy cast iron

X

X

Aluminium, brass

X

X

High-silicon aluminium

X

Nickel-based

X

Titanium Plastic composites

X X

X X X

X

X X

for milling white cast iron and hardened steels using negative lands and honed cutting edges. See Table 21-4 for suggested applications of CBN and diamonds along with carbides and ceramics. Because diamond and PCBN are extremely hard but brittle materials, new demands are being placed on the machine tools and on machining practice in order to take full advantage of the potential of these tool materials. These demands include: Use of more rigid machine tools and machining practices involving gentle entry and exit of the cut in order to prevent microchipping. Use of high-precision machine tools, because these tools are capable of producing high finish and accuracy. Use of machine tools with higher power, because these tools are capable of higher metal removal rates and faster spindle speeds.

& 21.3 TOOL GEOMETRY Selecting tool geometry is a critical part of selecting the cutting tool. Tool geometry can be very complex. Figure 21-14 shows the cutting-tool geometry for a single-point tool (HSS) used in turning in oblique (three forces) machining. The back rake angle affects

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End cutting-edge angle (ECEA)

Face Nose angle

Nose radius

Side cutting-edge angle (SCEA) Back rake angle (BRA) Side rake angle (RA) Wedge angle θ Front Flank end view Heel γ

Side relief angle (SRA)

ECEA End cutting-edge angle

Work Top view

Location of side rake 90° 90° Location of resultant rake

Reference plane Side view shank

End relief angle (ERA)

SCEA Side cutting-edge angle Location of back rake

Location of inclination

(c)

s A xi Y

Sh

End cutting-edge Side angle (ECEA) rake angle, + (SR)

an

k

(a)

Tool point

Work

n plane

Inclinatio

Cutting edge

N ra os di e us

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Back rake angle (BRA)

X Axis Side relief angle (SRA) Side cutting-edge angle (SCEA)

Refe re

SC

nce

EA

Inclination

Too l

Too l plan e

face

Side rake

plan

e

Back rake Z Axis Clearance or end relief angle Resultant (b)

(d)

FIGURE 21-14 Standard terminology to describe the geometry of single-point tools: (a) three dimensional views of tool, (b) oblique view of tool from cutting edge, (c) top view of turning with single-point tool, (d) oblique view from shank end of singlepoint turning tool.

the ability of the tool to shear the work material and form the chip. It can be positive or negative. Positive rake angles reduce the cutting forces, resulting in smaller deflections of the workpiece, tool holder, and machine. In machining hard work materials, the back rake angle must be small, even negative for carbide and diamond tools. Generally speaking, the higher the hardness of the workpiece, the smaller the back rake angle. For high-speed steels, back rake angle is normally chosen in the positive range, depending on the type of tool (turning, planing, end milling, face milling, drilling, etc.) and the work material. For carbide tools, inserts for different work materials and tool holders can be supplied with several standard values of back rake angle: 6 to þ6 degrees. The side rake angle and the back rake angle combine to form the effective rake angle. This is also called the true rake angle or resultant rake angle of the tool. True rake inclination of a cutting tool has a major effect in determining the amount of chip compression and the onset of shear angle, f. A small rake angle causes high compression, tool forces, and friction, resulting in a thick, highly deformed, hot chip. Increasing the back or side rake angles reduces the compression, the forces, and the friction, yielding a thinner, less deformed, and cooler chip. In general, the power consumption is reduced by approximately 1% for each 1-degree change in alpha (a). The end relief angle is called gamma (g). Unfortunately, it is difficult to take much advantage of the desirable effects of larger positive rake angles because they are offset by the reduced strength of the cutting tool, due to the reduced tool section, and by its greatly reduced capacity to conduct heat away from the cutting edge.

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Tool-Coating Processes

589

To provide greater strength at the cutting edge and better heat conductivity, zero or negative rake angles are commonly employed on carbide, ceramic, polydiamond, and PCBN cutting tools. These materials tend to be brittle, but their ability to hold their superior hardness at high temperatures results in their selection for high-speed and continuous machining operations. While the negative rake angle increases tool forces, it keeps the tool in compression and provides added support to the cutting edge. This is particularly important in making intermittent cuts, as in milling, and in absorbing the impact during the initial engagement of the tool and work. The wedge angle, Q, determines the strength of the tool and its capacity to conduct heat and depends on the values of a and g. The relief angles mainly affect the tool life and the surface quality of the workpiece. To reduce the deflections of the tool and the workpiece and to provide good surface quality, larger relief values are required. For high-speed steel, relief angles in the range of 5 to 10 degrees are normal, with smaller values being for the harder work materials. For carbides, the relief angles are lower to give added strength to the tool. The side and end cutting-edge angles define the nose angle and characterize the tool design. The nose radius has a major influence on surface finish. Increasing the nose radius usually decreases tool wear and improves surface finish. Tool nomenclature varies with different cutting tools, manufacturers, and users. Many terms are still not standard because of this variety. The most common tool terms will be used in later chapters to describe specific cutting tools. The introduction of coated tools has spurred the development of improved tool geometries. Specifically, low-force groove (LFG) geometries have been developed that reduce the total energy consumed and break up the chips into shorter segments. These grooves effectively increase the rake angle, which increases the shear angle and lowers the cutting force and power. This means that higher cutting speeds or lower cutting temperatures (and better tool lives) are possible. As a chip breaker, the groove deflects the chip at a sharp angle and causes it to break into short pieces that are easier to remove and are not as likely to become tangled in the machine and possibly cause injury to personnel. This is particularly important on high-speed, mass-production machines. The shapes of cutting tools used for various operations and materials are compromises, resulting from experience and research so as to provide good overall performance. For coated tools, edge strength is an important consideration. A thin coat enables the edge to retain high strength, but a thicker coat exhibits better wear resistance. Normally, tools for turning have a coating thickness of 6 to 12 mm. Edge strength is higher for multilayer coated tools. The radius of the edge should be 0.0005 to 0.005 in.

& 21.4 TOOL-COATING PROCESSES The two most effective coating processes for improving the life and performance of tools are the chemical vapor deposition and physical vapor deposition of titanium nitride (TiN) and titanium carbide (TiC). The selection of the cutting materials for cutting tools depends on what property you are seeking. If you want Oxidation and corrosion resistance; high-temperature stability

select

Al2O3, TiN, TiC

Crater resistance Hardness and edge retention

select select

Al2O3, TiN, TiC TiC, TiN, Al2O3

Abrasion resistance and flank wear

select

Al2O3, TiN, TiC

Low coefficient of friction and high lubricity

select

TiN, Al2O3, TiC

Fine grain size

select

TiN, TiC, Al2O3

The CVD process, used to deposit a protective coating onto carbide inserts, has been benefiting the metal removal industry for many years and is now being applied with equal success to steel. The PVD processes have quickly become the preferred TiN coating processes for high-speed steel and carbide-tipped cutting tools.

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Graphite shelves

TiCL4 H2 + CH4

FIGURE 21-15 Chemical vapor deposition (CVD) is used to apply layers (TiC, TiN, etc.) to carbide cutting tools.

Gas mixing chamber

Gas outlet To scrubber

CHEMICAL VAPOR DEPOSITION Chemical vapor deposition is an atmosphere-controlled process carried out at temperatures in the range of 950 to 1050 C (1740 to 1920 F). Figure 21-15 shows a schematic of the CVD process. Cleaned tools ready to be coated are staged on precoated graphite work trays (shelves) and loaded onto a central gas distribution column (tree). The tree loaded with parts to be coated is placed inside the retort of the CVD reactor. The tools are heated under an inert atmosphere until the coating temperature is reached. The coating cycle is initiated by the introduction of titanium tetrachloride (TiCL4), hydrogen, and methane (CH4) into the reactor. TiCL4 is a vapor and is transported into the reactor via a hydrogen carrier gas; CH4 is introduced directly. The chemical reaction for the formation of TiC is: TiCl4 þ CH4 ! TiC þ 4HCl

ð21-1Þ

To form titanium nitride, a nitrogen–hydrogen gas mixture is substituted for methane. The chemical reaction for TiN is: 2TiCl4 þ 2H2 þ N2 ! 2TiN þ 4HCl

ð21-2Þ

PHYSICAL VAPOR DEPOSITION The simplest form of PVD is evaporation, where the substrate is coated by condensation of a metal vapor. The vapor is formed from a source material called the charge, which is heated to a temperature less than 1000 C. PVD methods currently being used include reactive sputtering, reactive ion plating, low-voltage electron-beam evaporation, triode high-voltage electron-beam evaporation, cathodic evaporation, and arc evaporation. In each of the methods, the TiN coating is formed by reacting free titanium ions with nitrogen away from the surface of the tool and relying on a physical means to transport the coating onto the tool surface. All of these PVD processes share the following common features: 1. The coating takes place inside a vacuum chamber under a hard vacuum with the workpiece heated to 200 to 405 C (400 to 900 F). 2. Before coating, all parts are given a final cleaning inside the chamber to remove oxides and improve coating adhesion. 3. The coating temperature is relatively low (for cutting and forming tools), typically about 450 C (842 F). 4. The metal source is vaporized in an inert gas atmosphere (usually argon), and the metal atoms react with gas to form the coating. Nitrogen is the reactive gas for nitrides, and methane or acetylene (along with nitrogen) is used for carbides. 5. All four are ion-assisted deposition processes. The ion bombardment compresses the atoms on the growing film, yielding a dense, well-adhered coating.

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Tool-Coating Processes

591

Plasma

Neutral gas

Reactive gas Substrate

+1 Vacuum pump

Evaporator

Coating material

Vacuum chamber

Evaporated material

FIGURE 21-16 Schematic of physical vapor deposition (PVC) arc evaporation process.

Power supply

A typical cycle time for the coating of functional tools, including heat-up and cool down, is about 6 hr. In Figure 21-16, PVD arc evaporation is shown. The plasma sources are from several arc evaporators located on the sides and top of the vacuum chamber. Each evaporator generates plasma from multiple arc spots. In this way, a highly localized electrical arc discharge causes minute evaporation of the material of the cathode, and a self-sustaining arc is produced that generates a high-energy and concentrated plasma. The kinetic energy of deposition is much greater than that found in any other PVD method. During coating, this energy is of the order of 150 eV and more. Therefore, the plasma is highly reactive and the greater percentage of the vapor is atomic and ionized. Coating temperatures can be selected and controlled so that metallurgy is preserved. This enables a coating of a wide variety of sintered carbide tools—for example, brazed tools and solid carbide tools such as drills, end mills, form tools, and inserts. The PVD arc evaporation process will preserve substrate metallurgy, surface finish, edge sharpness, geometrical straightness, and dimensions.

CVD AND PVD—COMPLEMENTARY PROCESSES CVD and PVD are complementary coating processes. The differences between the two processes and resultant coatings dictate which coating process to use on different tools. Because CVD is done at higher temperatures, the adhesion of these coatings tends to be superior to a PVD–CVD-deposited coating. CVD coatings are normally deposited thicker than PVD coatings (6 to 9 mm for CVD, 1 to 3 mm for PVD). See Table 21-4 With CVD multiple coatings, layers may be readily deposited, but the tooling materials are restricted. CVD coated tools must be heat treated after coating. This limits the application to loosely toleranced tools. However, the CVD process, being a gaseous process, results in a tool that is coated uniformly all over; this includes blind slots and blind holes. Because PVD is mainly a line-of-sight process, all surfaces of the part to be coated may be masked. PVD also requires fixturing of each part in order to affect the substrate bias.

APPLICATIONS Applications for the two different processes are as follows: CVD Loosely toleranced tooling. Piercing and blanking punches, trim dies, phillips punches, upsetting punches.

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Cutting Tools for Machining

AISI A, D, H, M, and air-hardening and tool steel parts. Solid carbide tooling. PVD All HSS, solid carbide, and carbide-tipped cutting tools. Fine blanking punches, dies (0.001 in. tolerance or less). Non-composition-dependent process; virtually all tooling materials, including mold steels and bronze.

& 21.5 TOOL FAILURE AND TOOL LIFE In metal cutting, the failure of the cutting tool can be classified into two broad categories, according to the failure mechanisms that caused the tool to die (or fail): 1. Physical failures mainly include gradual tool wear on the flank(s) of the tool below the cutting edge (called flank wear) or wear on the rake face of the tool (called crater wear) or both. 2. Chemical failures, which include wear on the rake face of the tool (crater wear) are rapid, usually unpredictable, and often catastrophic failures resulting from abrupt, premature death of a tool. Other modes of failure are outlined in Figure 21-17. The selection of failure criteria is also widely varied. Figure 21-17 also shows a sketch of a ‘‘worn’’ tool, showing crater wear and flank wear, along with wear of the tool nose radius and an outer-diameter groove (the DCL groove). Tools also fail by edge chipping and edge fracture. Chip Chips in edge of tool

t

A Depth-ofcut line

Depth of cut Nose radius

Work

Wear on flank

Crater depth, KT

V

Crater wear in rake face

wf Contact length ᐉ

A Groove at DCL or outer edge of cut

Cutting edge

Tool

Cross section AA

8

5 7 1

2 3

6

10 4

No.

Failure

Cause

1-3

Flank wear

Due to the abrasive effect of hard grains contained in the work material

4-5

Groove

Due to wear at the DCL or outer edge of the cut

6

Chipping

Physical

Fine chips caused by high-pressure cutting, chatter, vibration, etc.

7

Partial fracture

Due to the mechanical impact when an excessive force is applied to the cutting edge

8

Crater wear

Carbide particles are removed due to degradation of tool performances and chemical reactions at high temperature

9

Deformation

10

Thermal crack

Thermal fatigue in the heating and cooling cycle with interrupted cutting

1

Built-up edge

A portion of the workpiece material adheres to the insert cutting edge

Chemical The cutting edge is deformed due to its softening at high temperature

FIGURE 21-17 Tools can fail in many ways. Tool wear during oblique cutting can occur on the flank or the rake face; t ¼ uncut chip thickness; kt ¼ crater depth; wf ¼ flank wear land length; DCL ¼ depth-of-cut line.

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SECTION 21.6

593

Flank Wear

As the tool wears, its geometry changes. This geometry change will influence the cutting forces, the power being consumed, the surface finish obtained, the dimensional accuracy, and even the dynamic stability of the process. Worn tools are duller, creating greater cutting forces and often resulting in chatter in processes that otherwise are usually relatively free of vibration. The actual wear mechanisms active in this high-temperature environment are abrasion, adhesion, diffusion, or chemical interactions. It appears that in metal cutting, any or all of these mechanisms may be operative at a given time in a given process. Tool failure by plastic deformation, brittle fracture, fatigue fracture, or edge chipping can be unpredictable. Moreover, it is difficult to predict which mechanism will dominate and result in a tool failure in a particular situation. What can be said is that tools, like people, die (or fail) from a great variety of causes under widely varying conditions. Therefore, tool life should be treated as a random variable, or probabilistically, not as a deterministic quantity.

& 21.6 FLANK WEAR During machining, the tool is performing in a hostile environment in which high-contact stresses and high temperatures are commonplace; therefore, tool wear is always an unavoidable consequence. At lower speeds and temperatures, the tool most commonly wears on the flank. Suppose that the tool wear experiment were to be repeated 15 times without changing any of the input parameters. The result would look like Figure 21-18, which depicts the variable nature of tool wear and shows why tool wear must be treated as a random variable. In Figure 21-18 the average time is denoted as mT and the standard deviation as s T, where the wear limit criterion was 0.025 in. At a given time during the test, 35 min, the tool displayed flank wear ranging from 0.013 to 0.021 in, with an average of mw ¼ 0.00175 in. with standard deviation s w 0.001 in. In Figure 21-19 four characteristic tool wear curves (average values) are shown for four different cutting speeds, V1 through V4; V1 is the fastest cutting speed and therefore generates the fastest wear rates. Such curves often have three general regions, as shown in the figure. The central region is a steady-state region (or the region of secondary wear). This is the normal operating region for the tool. Such curves are typical for both

Tool life distribution at wf = 0.025 in. Flank wear

0.030

0.025

wf values for general life determination (for cemented carbides) Width of Wear (in.)

Applications

0.008

Finish cutting of nonferrous alloys, fine and light cut, etc.

0.016

Cutting of special steels

0.028

Normal cutting of cast irons, steels, etc.

0.040–0.050

Flank tool wear (in.)

C21

wf

μT σT

0.021

0.020 μw = 0.0175 0.015

Tool wear distribution 0.013 μw , σw at 35 min.

0.010

0.005

Rough cutting of common cast irons 0 10

FIGURE 21-18

20

30 35 40 50 Cutting time, T (min)

Tool wear on the flank displays a random nature, as does tool life. wf ¼ flank wear limit value.

60

70

Page 594

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Flank wear (in.)

594

15:13:24

FIGURE 21-19 Typical tool wear curves for flank wear at different velocities. The initial wear is very fast, then it evens out to a more gradual pattern until the limit is reached; after that, the wear substantially increases.

V4 V3 V2 V1

Wear limit .025 inch

06/30/2011

V4

V2

V3

T4 T3 T2 T1

V1

wf

T4 0

T3

T2

Primary or Initial wear zone

T1

Secondary wear zone Steady state region

Tertiary or Accelerated wear zone Cutting time, T (min)

flank wear and crater wear. When the amount of wear reaches the value wf, the permissible tool wear on the flank, the tool is said to be ‘‘worn out.’’ The value wf is typically set at 0.025 to 0.030 in. for flank wear for high-speed steels and 0.008 to 0.050 for carbides, depending on the application. For crater wear, the depth of the crater, kt,, is used to determine tool failure. Using the empirical tool wear data shown in Figure 21-19, which used the values of T (time in minutes) associated with V (cutting speed) for a given amount of tool wear, wf (see the dashed-line construction), Figure 21-20 was developed. When V and T are plotted on log-log scales, a linear relationship appears, described by the equation VT n ¼ Constant ¼ C

ð21-3Þ

This equation is called the Taylor tool life equation because in 1907, F. W Taylor published his now-famous paper, ‘‘On the Art of Cutting Metals,’’ in ASME Transactions, wherein tool life (T) was related to cutting speed (V) and feed (f). This equation had the form1 T¼

Constant f xVy

ð21-4Þ

Over the years, the equation took the more widely published form VT n ¼ C

1

Width of flank wear (in.)

Width of flank wear (in.)

V2

V1

wf

V4 V3 KT

LOG V4 V3 V2 V1

V2

V1

T4 T3 T2 T1 LOG Cutting time (mm) LOG V4 V3 V2 V1

T4

T3 T2 T1 Tool life (min)

LOG

Cutting speed (sfm)

FIGURE 21-20 Construction of the Taylor tool life curve using data from deterministic tool wear plots like those of Figure 21-19. Curves like this can be developed for both flank and crater wear.

V4 V3

Crater Wear

T4 T3 T2 T1 LOG Cutting time (mm) Cutting speed (sfm)

Tool Wear Plots

Flank Wear

Life Curve

C21

T4

T3 T2 T1 Tool life (min)

LOG

Carl Barth, who was Taylor’s mathematical genius, is generally thought to be the author of these formulations along with early versions of slide rules.

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Page 595

SECTION 21.6

TABLE 21-5

595

Flank Wear

Tool Life Information for Various Materials and Conditions VT n ¼ C

Size of Cut (in.) Source

Tool Material

Geometry

1

High-carbon steel

8.14, 6.6, 6.15, 3/64

Workpiece Material

Depth

Feed

Cutting Fluid

n

C

Yellow brass (.60 Cu,

.050

.0255

Dry

.081

40 Zn, 85 Ni, .006 Pb)

.100

.0127

Dry

.096

242 299

.0255

Dry

.086

190

1

High-carbon steel

8.14, 6.6, 6.15, 3/64

Bronze (.9 Cu, .1.5n)

.050 .100

.0127

Dry

.111

232

1

HSS-18-4-1

8.14, 6.6, 6.15, 3/64

Cast iron 160 Bhn

.050

.0255

Dry

.101

172

Cast iron. Nickel, 164 Bhn

.050

.0255

Dry

.111

186

Cast iron. Ni-Cr, 207 Bhn Stell, SAE B1113 C.D.

.050 .050

.0255 .0127

Dry Dry

.088 .080

102 260

Stell, SAE B1112 C.D.

.050

.0127

Dry

.105

225

Stell, SAE B1120 C.D.

.050

.0127

Dry

.100

270

Stell, SAE B1120 þ Pb C.D.

.050

.0127

Dry

.060

290

Stell, SAE B1035 C.D.

.050

.0127

Dry

.110

130

Stell, SAE B1035 þ Pb C.D.

.050

.0127

Dry

.110

147

8.14, 6.6, 6.15, 3/64

Stell, SAE 1045 CD.

.100

.0127

Dry

.110

192

8.14, 6.6, 6.13, 3/64 8.14, 6.6, 6.15, 3/64

Stell, SAE 2340 185 Bhn Stell, SAE 2345 198 Bhn

.100 .050

.0125 .0255

Dry Dry

.147 .105

143 126

1

1

1

HSS-18-4-1

HSS-18-4-1

HSS-18-4-1

8.14, 6.6, 6.15, 3/64

8.14, 6.6, 6.15, 3/64

Stell, SAE 3140 190 Bhn

.100

.0125

Dry

.160

178

8.14, 6.6, 6.15, 3/64

Stell, SAE 4350 363 Bhn

.0125

.0127

Dry

.080

181

Stell, SAE 4350 363 Bhn

.0125

.0255

Dry

.125

146

Stell, SAE 4350 363 Bhn

.0250

.0255

Dry

.125

95

Stell, SAE 4350 353 Bhn

.100

.0127

Dry

.110

78

Stell, SAE 4350 363 Bhn

.100

.0255

Dry

.110

46

.050 .050

.0127 .0127

Dry Dry

.180 .180

190 159

1

HSS-18-4-1

8.14, 6.6, 6.15, 3/64

Stell, SAE 4140 230 Bhn Stell, SAE 4140 271 Bhn Stell, SAE 6140 240 Bhn

.050

.0127

Dry

.150

197

1

HSS-18-4-1

8.22, 6.6, 6.15, 3/64

Monel metal 215 Bhn

.100

.0127

Dry

.084

170

.150

.0255

Dry

.074

127

.100

.0127

Em

.080

185

.100

.0127

SMO

.105

189

.187

.031

Dry

.190

215

.125 .062

.031 .031

Dry Dry

.190 .190

240 270

.031

.031

Dry

.190

310

.062

0.31

Dry

.150

205

1

Stellite 2400

0.0, 6.6, 6.0, 3/32

1

Stellite No. 3

0.0, 6.6, 6.0, 3/32

1

Carbide (T 64)

6.12, 5.5, 10.45

2

Ceramic

not available

Steel. SAE 3240 annealed

Cast iron 200 Bhn Steel. SAE 1040 annealed

.062

.025

Dry

.156

800

Steel. SAE 1060 annealed

.125

.025

Dry

.167

660

Steel. SAE 1060 annealed

.187

.025

Dry

.167

615

Steel. SAE 1060 annealed

.250

.025

Dry

.167

560

Steel. SAE 1060 annealed Steel. SAE 1060 annealed

.062 .062

.021 .042

Dry Dry

.167 .164

880 510

Steel. SAE 1060 annealed

.062

.062

Dry

.162

400

Steel. SAE 2340 annealed

.062

.025

Dry

.162

630

AISI 4150

.160

.016

Dry

.400

2000

AISI 4150

.160

.016

Dry

.200

620

Sources: 1- Fundamentals of Tool Design. ASTME. A. R. Koneeny, W. J. Potthoff; 2 - Theory of Metal Cutting, P. N. Black

where n is an exponent that depends mostly on tool material but is affected by work material, cutting conditions, and environment and C is a constant that depends on all the input parameters, including feed. Table 21-5 provides some data on Taylor tool life constants. Figure 21-21 shows typical tool life curves for one tool material and three work materials. Notice that all three plots have about the same slope, n. Typical values for n

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Cutting Tools for Machining 500

300 Cutting speed, V (fpm)

C21

FIGURE 21-21 Log-log tool life plots for three steel work materials cut with HSS tool material.

Low-carbon st

eel

Low-alloy med

ium-carbon st

100

eel

High-alloy med

ium-carbon st

50

eel

VTn = C

Decreasing C with n ⬵ 0.14 to 0.16

20 1

10

100

Tool life, T (min)

are 0.14 to 0.16 for HSS, 0.21 to 0.25 for uncoated carbides, 0.30 for TiC inserts, 0.33 for poly-diamonds, 0.35 for TiN inserts, and 0.40 for ceramic-coated inserts. It takes a great deal of experimental effort to obtain the constants for the Taylor equation because each combination of tool and work material will have different constants. Note that for a tool life of 1 min, C ¼ V, or the cutting speed that yields about 1 min of tool life for this tool. A great deal of research has gone into developing more sophisticated versions of the Taylor equation, wherein constants for other input parameters (typically feed, depth of cut, and work material hardness) are experimentally determined. For example, VT n F m dp ¼ K 0

ð21-5Þ

where n, m, and p are exponents and K0 is a constant. Equations of this form are also deterministic and determined empirically. The problem has been approached probabilistically in the following way. Because T depends on speed, feed, materials, and so on, one writes T¼

K 1=n V

1=n

¼

K Vm

ð21-6Þ

where K is now a random variable that represents the effects of all unmeasured factors and is an input variable. The sources of tool life variability include factors such as: 1. Variation in work material hardness (from part to part and within a part). 2. Variability in cutting tool materials, geometry, and preparation. 3. Vibrations in machine tool, including rigidity of work and tool-holding devices. 4. Changing surface characteristics of workpieces. The examination of the data from a large number of tool life studies in which a variety of steels were machined shows that regardless of the tool material or process, tool life distributions are usually log normal and typically have a large standard deviation. As shown in Figure 21-22, tool life distributions have a large coefficient of variation, which means that tool life is not very predictable. Other criteria can be used to define tool death in addition to wear limits: When surface finish deteriorates unacceptably. When workpiece dimension is out of tolerance.

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Page 597

Frequency of failure

SECTION 21.6

Coefficient of variation σ CV = T ⬵ 0.3 to 0.4 TAVE

Flank Wear

597

Mean failure time (min)

σ T = standard deviation

Log normal distribution of failures

FIGURE 21-22 Tool life viewed as a random variable has a log normal distribution with a large coefficient of variation.

σT

TAVE Tool life or time to failure

When power consumption or cutting forces increase to a limit. Sparking or chip discoloration and disfigurement. Cutting time or component quantity. In automated processes, it is very beneficial to be able to monitor the tool wear online so that the tool can be replaced prior to failure, wherein defective products may also result. The feed force has been shown to be a good, indirect measure of tool wear. That is, as the tool wears and dulls, the feed force increases more than the cutting force increases. Once criteria for failure have been established, tool life is that time elapsed between start and finish of the cut, in minutes. Other ways to express tool life, other than time, include: 1. Volume of metal removed between regrinds or replacement of tool. 2. Number of pieces machined per tool. 3. Number of holes drilled with a given tool (see Figure 21-23).

99.9 99 90

1000 rpm 65.5 sfpm 0.006 ipr

70 Cumulative % failure

C21

50 30

5 3

FIGURE 21-23 Tool life test data for various coated drills. TiNcoated HSS drills outperform uncoated drills. Life based on the number of holes drilled before drill failure.

2 10

Bright finished twist drills–uncoated CTD black–oxide drill CTD drills with TiN coat by company A

20

30

40

60

80 100

TiN coating by company D TiN coating by company B TiN coating by company E

200

300 400

600

1000

No. of holes drilled Drill performance based on the number of holes drilled with 1/4-in.-diameter drills in T–1 structural steel.

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Drilling tool failure is discussed more in Chapter 23 and is very complex because of the varied and complex geometry of the tools and as shown here in Figure 21-23, the tool material.

MACHINABILITY Machinability is a much-maligned term that has many different meanings but generally refers to the ease with which a metal can be machined to an acceptable surface finish. The principal definitions of the term are entirely different—the first based on material properties, the second based on tool life, and the third based on cutting speed. 1. Machinability is defined by the ease or difficulty with which the metal can be machined. In this light, specific energy, specific horsepower, and shear stress are used as measures, and, in general, the larger the shear stress or specific power values, the more difficult the material is to machine, requiring greater forces and lower speeds. In this definition, the material is the key. 2. Machinability is defined by the relative cutting speed for a given tool life while cutting some material, compared to a standard material cut with the same tool material. As shown in Figure 21-24, tool life curves are used to develop machinability ratings. For example, in steels, the material chosen for the standard material was B1112 steel, which has a tool life of 60 min at a cutting speed of 100 sfpm. Material X has a 70% rating, which implies that steel X has a cutting speed of 70% of B1112 for equal tool life. Note that this definition assumes that the tool fails when machining material X by whatever mechanism dominated the tool failure when machining the B1112. There is no guarantee that this will be the case. ISO standard 3685 has machinability index numbers based on 30 min of tool life with flank wear of 0.33 mm. 3. Cutting speed is measured by the maximum speed at which a tool can provide satisfactory performance for a specified time under specified conditions. (See ASTM standard E 618-81, ‘‘Evaluating Machining Performance of Ferrous Metals Using an Automatic Screw Bar Machine.’’) 4. Other definitions of machinability are based on the ease of removal of the chips (chip disposal), the quality of the surface finish of the part itself, the dimensional stability of the process, or the cost to remove a given volume of metal. Further definitions are being developed based on the probabilistic nature of the tool failure, in which machinability is defined by a tool reliability index. Using such indexes, various tool replacement strategies can be examined and optimum cutting rates obtained. These approaches account for the tool life variability by developing coefficients of variation for common combinations of cutting tools and work materials. The results to date are very promising. One thing is clear, however, from this sort of research: although many manufacturers of tools have worked at developing materials

Log VTn = C, V = 100 VTn = C, V = 70 Cutting speed V (fpm) 100 70

FIGURE 21-24 Machinability ratings defined by deterministic tool life curves.

B11

12,

Xt

he

unk

the

now

sta

nda

nm

rd m

ate

ate

rial

60 Tool life, T (min)

rial

Log

C21

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Page 599

SECTION 21.7

Cutting Fluids

599

that have greater tool life at higher speeds, few have worked to develop tools that have less variability in tool life at all speeds. The reduction in variability is fundamental to achieving smaller coefficients of variation, which typically are of the order of 0.3 to 0.4. This means that a tool with a 100-min average tool life has a standard deviation of 30 to 40 min, so there is a good probability that the tool will fail early. In automated equipment, where early, unpredicted tool failures are extremely costly, reduction of the tool life variability will pay great benefits in improved productivity and reduced costs.

RECONDITIONING CUTTING TOOLS In the reconditioning of tools by sharpening and recoating, care must be taken in grinding the tool’s surfaces. The following guidelines should be observed: 1. Resharpen to original tool geometry specifications. Restoring the original tool geometry will help the tool achieve consistent results on subsequent uses. Computer numerical control (CNC) grinding machines for tool resharpening have made it easier to restore a tool’s original geometry. 2. Grind cutting edges and surfaces to a fine finish. Rough finishes left by poor and abusive regrinding hinder the performance of resharpened tools. For coated tools, tops of ridges left by rough grinding will break away in early tool use, leaving uncoated and unprotected surfaces that will cause premature tool failure. 3. Remove all burrs on resharpened cutting edges. If a tool with a burr is coated, premature failure can occur because the burr will break away in the first cut, leaving an uncoated surface exposed to wear. 4. Avoid resharpening practices that overheat and burn or melt (called glazing over) the tool surfaces, because this will cause problems in coating adhesion. Polishing or wire brushing of tools causes similar problems. The cost of each recoating is about one-fifth the cost of purchasing a new tool. By recoating, the tooling cost per workpiece can be cut by between 20 and 30%, depending on the number of parts being machined.

& 21.7 CUTTING FLUIDS From the day that Frederick W. Taylor demonstrated that a heavy stream of water flowing directly on the cutting process allowed the cutting speeds to be doubled or tripled, cutting fluids have flourished in use and variety and have been employed in virtually every machining process. The cutting fluid acts primarily as a coolant and secondly as a lubricant, reducing the friction effects at the tool/chip interface and the work flank regions. The cutting fluids also carry away the chips and provide friction (and force) reductions in regions where the bodies of the tools rub against the workpiece. Thus, in processes such as drilling, sawing, tapping, and reaming, portions of the tool apart from the cutting edges come in contact with the work, and these (sliding friction) contacts greatly increase the power needed to perform the process, unless properly lubricated. The reduction in temperature greatly aids in retaining the hardness of the tool, thereby extending the tool life or permitting increased cutting speed with equal tool life. In addition, the removal of heat from the cutting zone reduces thermal distortion of the work and permits better dimensional control. Coolant effectiveness is closely related to the thermal capacity and conductivity of the fluid used. Water is very effective in this respect but presents a rust hazard to both the work and tools and also is ineffective as a lubricant. Oils offer less effective coolant capacity but do not cause rust and have some lubricant value. In practice, straight cutting oils or emulsion combinations of oil and water or wax and water are frequently used. Various chemicals can also be added to serve as wetting agents or detergents, rust inhibitors, or polarizing agents to promote formation of a protective oil film on the work. The extent to which the flow of a cutting fluid washes the very hot chips away from the cutting area is an important factor in heat removal. Thus, the application of a coolant should be copious and of some velocity.

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CHAPTER 21

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TABLE 21-6

Cutting Fluid Contaminants

Category

Contaminants

Effects

Solids

Metallic fines, chips Grease and sludge Debris and trash Hydraulic oils (coolant) Water (oils)

Scratch product’s surface Plug coolant lines Produce wear on tools and machines Decrease cooling efficiency Cause smoking Clog paper filters Grow bacteria faster

Bacteria Fungi Mold

Acidity coolant Break down emulsions Cause rancidity, dermatitis Require toxic biocides

Tramp fluids

Biologicals (coolants)

The possibility of a cutting fluid providing lubrication between the chip and the tool face is an attractive one. An effective lubricant can modify the process, perhaps producing a cooler chip, discouraging the formation of a built-up edge on the tool, and promoting improved surface finish. However, the extreme pressure at the tool/chip interface and the rapid movement of the chip away from the cutting edge make it virtually impossible to maintain a conventional hydrodynamic lubricating film at the tool/chip interface. Consequently, any lubrication action is associated primarily with the formation of solid chemical compounds of low shear strength on the freshly cut chip face, thereby reducing tool/chip shear forces or friction. For example, carbon tetrachloride is very effective in reducing friction in machining several different metals and yet would hardly be classified as a good lubricant in the usual sense. Chemically active compounds, such as chlorinated or sulfurized oils, can be added to cutting fluids to achieve such a lubrication effect. Extreme-pressure lubricants are especially valuable in severe operations, such as internal threading (tapping), where the extensive tool-work contact results in high friction with limited access for a fluid. In addition to functional effectiveness as coolant and lubricant, cutting fluids should be stable in use and storage, noncorrosive to work and machines, and nontoxic to operating personnel. The cutting fluid should also be restorable by using a closed recycling system that will purify the used coolant and cutting oils. Cutting fluids become contaminated in three ways (Table 21-6). All these contaminants can be eliminated by filtering, hydrocycloning, pasteurizing, and centrifuging. Coolant restoration eliminates 99% of the cost of disposal and 80% or more of new fluid purchases. See Figure 21-25 for a schematic of a coolant recycling system.

& 21.8 ECONOMICS OF MACHINING The cutting speed has such a great influence on the tool life compared to the feed or the depth of cut that it greatly influences the overall economics of the machining process. For a given combination of work material and tool material, a 50% increase in speed results in a 90% decrease in tool life, while a 50% increase in feed results in a 60% decrease in tool life. A 50% increase in depth of cut produces only a 15% decrease in tool life. Therefore, in limited-horsepower situations, depth of cut and then feed should be maximized, while speed is held constant and horsepower consumed is maintained within limits. As cutting speed is increased, the machining time decreases, but the tools wear out faster and must be changed more often. In terms of costs, the situation is as shown in Figure 21-26, which shows the effect of cutting speed on the cost per piece. The total cost per operation is comprised of four individual costs: machining costs, tool costs, tool-changing costs, and handling costs. The machining cost is observed to decrease with increasing cutting speed because the cutting time decreases. Cutting time is proportional to the machining costs. Both the tool costs and the tool-changing costs increase with increases in cutting speeds. The handling costs are independent of cutting speed. Adding up each of the individual costs results in a total unit cost curve that is

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Page 601

SECTION 21.8

DIRTY FLUIDS oils or coolants contaminated from usage

601

AUTOMATIC DISCHARGE HIGH-SPEED CENTRIFUGE removes solids to 1 micron and tramp fluids to 1/4 of 1%

PASTEURIZER reduces biologicals to levels of fresh coolants

MESH FILTER removes all coarse solids

FIGURE 21-25 reuse.

Economics of Machining

HYDROCYCLONE (coolant) or FILTER (oil) removes solids to 15–30 μm

CLEAN FLUIDS available for immediate reuse

A well-designed recycling system for coolants will return more than 99% of the fluid for

observed to go through a minimum point. For a turning operation, the total cost per piece C equals C ¼ C1 þ C2 þ C3 þ C4 ¼ Machining cost þ Tooling cost þ Tool-changing cost þ Handling cost per piece ð21-7Þ

Increasing

Note: This ‘‘C’’ is not the same ‘‘C’’ used in the Taylor Tool life equation. In this analysis, that ‘‘C’’ will be called ‘‘K.’’

C = total cost per piece

Cost per piece or unit cost ($/pc)

C21

FIGURE 21-26 Cost per unit for a machining process versus cutting speed. Note that the ‘‘C’’ in this figure and related equations is not the same ‘‘C’’ used in the Taylor tool life (equation 21-3).

Unit cost C1, Machining cost per piece

Nonproductive cost per piece C4

Tool cost per piece

C2

Tool-changing cost per piece C3

0 0

VM Cutting speed (sfpm) for minimum cost/piece

V Increasing

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Cutting Tools for Machining

Expressing each of these cost terms as a function of cutting velocity will permit the summation of all the costs. C1 ¼ T m Co Tm Ct C2 ¼ T Tm C3 ¼ tc Co T

where Co ¼ operating cost ($/min) Tm ¼ cutting time (min/piece) where T ¼ tool life (min/tool) Ct ¼ initial cost of tool ($) where tc ¼ time to change tool (min) Tm ¼ number of tool changes per piece T

C4

labor, overhead, and machine tool costs consumed while part is being loaded or unloaded, tools are being advanced, machine has broken down, and so on. Because Tm ¼ L/Nfr for turning ¼ pDL=12Vf r and T ¼ ðK=V Þ1=n , by rewriting equation 21-3, and using ‘‘K’’ for the constant ‘‘C’’, the cost per unit, C, can be expressed in terms of V: C¼

LpDCo Ct V 1=n tc Co V 1=n þ þ þ C4 12Vf r K1=n K1=n

ð21-8Þ

To find the minimum, take dc/dV ¼ 0 and solve for V:

n Co Vm ¼ K 1 n Co tt þ Ct

n ð21-9Þ

Thus, Vm represents a cutting speed that will minimize the cost per unit, as depicted in Figure 21-26. However, a word of caution here is appropriate. Note that this derivation was totally dependent upon the Taylor tool life equation. Such data may not be available because they are expensive and time consuming to obtain. Even when the tool life data are available, this procedure assumes that the tool fails only by whichever wear mechanism (flank or crater) was described by this equation and by no other failure mechanism. Recall that tool life has a very large coefficient of variation and is probabilistic in nature. This derivation assumes that for a given V, there is one T—and this simply is not the case, as was shown in Figure 21-18. The model also assumes that the workpiece material is homogeneous, the tool geometry is preselected, the depth of cut and feed rate are known and remain unchanged during the entire process, sufficient horsepower is available for the cut at the economic cutting conditions, and the cost of operating time is the same whether the machine is cutting or not cutting.

COST COMPARISONS Cost comparisons are made between different tools to decide which tool material to use for a given job. Suppose there are four different tools that can be used for turning hotrolled 8620 steel with triangular inserts. The four tool materials are shown in Table 21-7. Operating costs for the machine tool are $60/hr. The low-force groove insert has only three cutting edges available instead of six. It takes 3 min to change inserts and 0.5 min to unload a finished part and load in a new 6-in.-diameter bar stock. The length of cut is about 24 in. The student should study and analyze this table carefully so that each line is understood. Note that the cutting tool cost per piece is three times higher for the lowforce groove tool over the carbide but is really of no consequence, because the major cost per piece comes from two sources: the machining cost per piece and the nonproductive cost per piece.

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Review Questions

TABLE 21-7

Cost Comparison of Four Tool Materials, Based on Equal Tool Life of 40 Pieces per Cutting Edge Uncoated

Cutting speed (surface ft/min) Feed (in./rev) Cutting edges available per insert Cost of an insert ($/insert) Tool life (pieces/cutting edge)

TiC-Coated

A12O3-Coated

A12O3 LFG

400

640

1100

1320

0.020

0.02

0.024

0.028

6

6

6

3

4.80

5.52

6.72

6.72

192

108

60

40

Tool-change time per piece (min)

0.075

0.075

0.075

0.075

Nonproductive cost per piece ($/pc) Machining time per piece (min/pc)

0.50 4.8

0.50 2.7

0.50 1.50

0.50 1.00

Machining cost per piece ($/unit)

4.8

2.7

1.5

1.00

Tool-change cost per piece ($/pc)

0.08

0.08

0.08

0.08

Cutting tool cost per piece ($/pc)

0.02

0.02

0.03

0.06

Total cost per piece ($/pc)

5.40

3.30

2.11

1.64

Production rate (pieces/hr)

11

18

29

38

Improvement in productivity based on pieces/hr (%)

64

164

245

Source: Data from T. E. Hale et al., ‘‘High Productivity Approaches to Metal Removal,’’ Materials Technology, Spring 1980, p. 25.

& KEY WORDS aluminum oxide back rake angle BUE (built-up edge) carbides cast cobalt alloy ceramics cermets chemical vapor deposition (CVD) chip groove

coated tools crater wear cubic boron nitride (CBN) cutting fluids cutting tool materials depth-of-cut line (DCL) diamonds flank wear hardness high-speed steel (HSS)

hot hardness low-force groove (LFG) machinability metal cutting microchipping physical vapor deposition (PVD) polycrystalline cubic boron nitride (PCBN)

polycrystalline diamond (PCD) powder metallurgy (P/M) sintered carbides stellite tools tool life tool steels titanium carbide (TiC) titanium nitride (TiN)

& REVIEW QUESTIONS 1. For metal-cutting tools, what is the most important material property (i.e., the most critical characteristic)? Why? 2. What is hot hardness compared to hardness? 3. What is impact strength, and how is it measured? 4. Why is impact strength an important property in cutting tools? 5. Is a cemented carbide tool made by a powder metallurgy method? 6. What are the primary considerations in tool selection? 7. What is the general strategy behind coated tools? 8. What is a cermet? 9. How is a CBN tool manufactured? 10. F. W. Taylor was one of the discoverers of high-speed steel. What else is he well known for? 11. What casting process do you think was used to fabricate cast cobalt alloys? 12. Discuss the constraints in the selection of a cutting tool. 13. What does cemented mean in the manufacture of carbides? 14. What advantage do ground carbide inserts have over pressed carbide inserts? 15. What is a chip groove? 16. What is the DCL?

17. Suppose you made four beams out of carbide, HSS, ceramic, and cobalt. The beams are identical in size and shape, differing only in material. Which beam would do each of the following? a. Deflect the most, assuming the same load. b. Resist penetration the most. c. Bend the farthest without breaking. d. Support the greatest compressive load. 18. Multiple coats or layers are put on the carbide base for what different purposes? 19. What tool material would you recommend for machining a titanium aircraft part? 20. What makes the process that makes TiC coatings for tools a problem? See equation 21-1. 21. Why does a TiN-coated tool consume less power than an uncoated HSS under exactly the same cutting conditions? 22. For what work material are CBN tools more commonly used, and why? 23. Why is CBN better for machining steel than diamond? 24. What is the typical coefficient of variation for tool life data, and why is this a problem? 25. What is meant by the statement ‘‘Tool life is a random variable’’?

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26. The typical value of a coefficient of variation in metal-cutting tool life distributions is 0.3. How could it be reduced? 27. Machinability is defined in many ways. Explain how a rating is obtained. 28. What are the chief functions of cutting fluids? 29. How are CVD tools manufactured? 30. Why is the PVD process used to coat HSS tools? 31. Why is there no universal cutting tool material? 32. What is an 18-4-1 HSS composed of?

33. Over the years, tool materials have been developed that have allowed significant increases in MRR. Nevertheless, HSS is still widely used. Under what conditions might HSS be the material of choice? 34. Why is the rigidity of the machine tool an important consideration in the selection of the cutting tool material? 35. Explain how it can be that the tool wears when it may be four times as hard as the work material. 36. What is a honed edge on a cutting tool and why is it done?

& PROBLEMS 1. Figure 21-A gives data for cutting speed and tool life. Determine the constants for the Taylor tool life equation for these data. What do you think the tool material might have been?

55 50 V3 = 40.6 fpm 45 Cutting speed, V (fpm)

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p

40

n = tan υ =

y = log Vp – log Vr

35

y x q

V10 = 34.2 fpm 30

r

υ

x = log Tr – log Tp

s V60 = 26.8 fpm

25

T25 = 92 min 20 1

2

3

4

5 6 8 10 20 Tool life ,T (min)

2. Suppose you have a turning operation using a tool with a zero back rake and 5-degree end relief. The insert flank has a wear land on it of 0.020 in. How much has the diameter of the workpiece grown (increased) due to this flank wear, assuming the tool has not been reset to compensate for the flank wear?

End view

A

30

5060

80 100

FIGURE 21-A

3. In Figure 21-B, a single-point tool is shown. Identify points A through G using tool nomenclature.

Single-point tool

A = B = C =

B

D = E

E = G

F = G =

C Flank Top view Side view

D

F

FIGURE 21-B

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4. The following data have been obtained for machining an Si-Al alloy: Workpiece

Tool

Material

Material

Cutting Speed (m/min) for Tool Life (m/min) of

Sand casting Permanent-mold casting PMC with flood cooling Sand casting

Diamond polycrystal Diamond polycrystal Diamond polycrystal WC-K-20

20 min

30 min

60 min

731 591 608 175

642 517 554 161

514 411 472 139

Compute the C and n values for the Taylor tool life equation. How do these n values compare to the typical values? 5. In Figure 21-C, the insert at the top is set with a 0-degree side cutting-edge angle. The insert at the bottom is set so that the edge contact length is increased from a 0.250-in. depth of cut to 0.289 in. The feed was 0.010 ipr. a. Determine the side cutting-edge angle for the offset tool. b. What is the uncut chip thickness in the offset position? c. What effect will this have on the forces and the process?

Tube turning with insert tool α = 0° SCEA = 0° 0.250

Feed = 0.010 ipr

Tube turning with insert tool α = 0° SCEA = 0° 0.250 DOC

Feed = 0.010 ipr

0.289

FIGURE 21-C

6. Tool cost is often used as the major criterion for justifying tool selection. Either silicon nitride or PCBN insert tips can be used to machine (bore) a cylinder block on a transfer line at a rate of 312,000 part/yr (material: gray cast iron). The operation requires 12 inserts (2 per tool), as six bores are machined simultaneously. The machine was run at 2600 sfpm with a feed of 0.014 in. at 0.005-in. DOC for finishing. Here are some additional data:

Tips in use per part Tool life (parts per tool) Cost per tip

SiN

PCBN

12 200 $1.25

12 4700 $28.50

7.

8. 9. 10.

a. Which tool material would you recommend? b. On what basis? A 2-in.-diameter bar of steel was turned at 284 rpm, and tool failure occurred in 10 min. The speed was changed to 132 rpm, and the tool failed in 30 min of cutting. Assume that a straight-line relationship exists. What cutting speed should be used to obtain a 60-min tool life of V60? Table 21-7 shows a cost comparison for four tool materials. Show how the data in this table were generated. Problem 6 provided data stating the cutting speed for this job was 2600 sfpm. Use equation 21-9 to verify that speed. The outside diameter of a roll for a steel (AISI 1015) rolling mill is to be turned. In the final pass, Starting diameter ¼ 26.25 in.

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and Length ¼ 48.0 in. The cutting conditions will be Feed ¼ 0.0100 in./rev and Depth of cut ¼ 0.125 in. A cemented carbide cutting tool is to be used, and the parameters of the Taylor tool life equation for this setup are n ¼ 0.25 and C ¼ 1300. It is desirable to operate at a cutting speed such that the tool will not need to be changed during the cut. Determine the cutting speed that will make the tool life equal to the time required to complete this turning operation. (Problem suggested by Groover, Fundamentals of Modern Manufacturing:

Materials, Processes, and Systems, 2nd ed., John Wiley & Sons, 2002.) 11. Using data from Problems 8 and 10, estimate the necessary horsepower for the machine tool to make this cut. 12. Figure 21-B shows a sketch of a single-point tool and its associated tool signature. Put the signature from the tool in Figure 21-B in the same order as shown in Figure 21-D. Which tool would produce the larger Fc, given that both are cutting at the same V, fr, and DOC in the same material?

20° Normal side rake angle Normal side relief angle

20° End cutting-edge angle

Nose radius

0.125 15° Side cutting-edge angle

Normal back rake angle 10°

Normal end relief angle 8°

Tool Signature

10,

20,

8,

8,

20,

15,

125

Normal back rake angle Normal side rake angle Normal end relief angle Normal side relief angle End cutting-edge angle Side cutting-edge angle Nose radius

FIGURE 21-D

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Problems 13. What are the various ways a cutting tool can fail and what can be done to remedy this? See Figures 21-17 and 21-E.

Failure Excessive flank wear

Basic Remedy Tool material Cutting conditions

• Use a more wear-resistant grade coated carbide cermet • Decrease speed

Excessive crater wear

Tool material Tool design Cutting conditions

• Use a more wear-resistant grade coated carbide cermet • Enlarge the rake angle • Select the correct chip breaker • Decrease speed • Reduce the depth of cut and feed

Tool material Tool design Cutting conditions

• Use tougher grades

Tool material Tool design Cutting conditions

• Use tougher grades

Built-up edge

Tool material Cutting conditions

• Change to a grade that is adhesion resistant • Increase the cutting speed and feed • Use cutting fluids

Plastic deformation

Tool material Cutting conditions

• Change to highly thermal-resistant grades • Reduce the cutting speed and feed

Cutting-edge chipping Edge Failure

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Partial fracture of cutting edges

If carbide, (AC2000

AC3000)

• If built-up edge occurs, change to a less susceptible grade (cermet) • Reinforce the cutting edge (honing) • Reduce the rake angle • Increase speed (if caused by edge build-up)

if carbide, (AC2000

AC3000)

• Use holder with a large approach angle • Use larger shank-size holder • Reduce the depth of cut and feed

FIGURE 21-E

607

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Chapter 21

CASE STUDY

Comparing Tool Materials Based on Tool Life

T

per piece was $4.80 ($1.00/min 4.8 min/pc). The manufacturing engineer on the job, Brian Graney, has found three new tool materials being used in face mills. They are listed in Table CS-24 along with the data for the uncoated tool. The new materials are TiC-coated carbide, Al2O3 coated carbide, and a ceramic-coated insert with a single-side, low-force groove geometry. The expected cutting conditions, speeds, and feeds are given in the table along with Brian’s estimates of the production rates in pieces per hour for each of these new tool materials. The low-force groove geometry tools can only be used three times—they cannot be flipped over—so only three cutting edges are available per insert before it has to be replaced. Brian has argued that even though the ceramic-coated inserts cost more, they result in a lower cost per piece, considering all the costs. Determine the machining cost per piece, the tool changing cost per piece, and the tool cost per piece that make up the total cost per piece, and verify Brian’s belief that these coated tools will provide some cost savings.

he Zachary Milling Company is trying to decide on what kind of inserts to use in their milling cutters. These milling cutters often provide a marked productivity improvement compared to conventional HDD drills, particularly when they are combined with coated insert tools. The company is trying to determine which type of insert to use in the drill for machining some hot-rolled 8620 steel shafts using triangular inserts. The operating cost of the machine tool is $60/hr. It takes about 3 min to change the inserts and about 30 s to unload a finished part and load a new part in the machine. The company is currently using uncoated inserts at the following operating conditions: 400 sfpm and 0.020 ipr. These speeds and feeds resulted in each cutting edge producing about 40 pieces before the tool’s cutting edge became dull. The tool was then indexed. Because it was a triangular tool, each tool had six cutting edges available before it had to be replaced. At these speeds and feeds, the milling time was 4.8 min and the production rate was 11 part/hr, while the machining cost

TABLE CS-21

Cost Comparison of Four Tool Materials, Based on Equal Tool Life Uncoated

Cutting speed (surface ft/min)

TiC-Coated

Al2O3-coated

Al2O3 LFG

400

640

1100

1320

Feed (in/rev)

0.020

0.02

0.024

0.028

Cutting edges available per insert Cost of an insert ($/insert)

6 4.80

6 5.52

6 6.72

3 6.72

Tool life (pieces/cutting edge)

192

108

60

40

Tool change time per piece (min)

0.075

0.075

0.075

0.075

Nonproductive cost per piece ($/pc)

0.50

0.50

0.50

0.50

Machining time per piece (min/pc)

4.8

2.7

1.50

1.00

Machining cost per piece ($/pc)

4.80

Tool change cost per piece ($/pc)

0.08

Cutting tool cost per piece ($/pc) Total cost per piece ($)

0.02 5.40

Production rate (pieces/hr)

11

18

29

38

Improvement in productivity based on pieces/hr (%)

64

164

245

Source: Data from T. E. Hale et al., ‘‘High Productivity Approaches to Metal Removal,’’ Materials Technology, Spring 1980, p. 25.

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CHAPTER 22 TURNING AND BORING PROCESSES 22.1 INTRODUCTION 22.2 FUNDAMENTALS OF TURNING, BORING, AND FACING TURNING Example of Turning Boring Facing Parting Deflection Precision Boring Drilling Reaming

Knurling Special Attachments 22.3 LATHE DESIGN AND TERMINOLOGY Lathe Design Size Designation of Lathes Types of Lathes 22.4 CUTTING TOOLS FOR LATHES Lathe Cutting Tools Form Tools Turret-Lathe Tools

22.5 WORKHOLDING IN LATHES Workholding Devices for Lathes Lathe Centers Mandrels Lathe Chucks Collets Faceplates Mounting Work on the Carriage Steady and Follow Rests Case Study: Estimating the Machining Time for Turning

& 22.1 INTRODUCTION Turning is the process of machining external cylindrical and conical surfaces. It is usually performed on a machine tool called a lathe. An engine lathe is shown in Figure 22-1, using a cutting tool. The workpiece is held in a workholder as indicated in Figure 22-2, relatively simple work and tool movements are involved in turning a cylindrical surface. The workpiece is rotated and the single-point cutting tool is fed longitudinally into the workpiece and then travels parallel to the axis of workpiece rotation, reducing the diameter by the depth of cut (DOC). The tool feeds at a rate, fr, cutting at a speed, V, which is determined by the revolutions per minute (rpm) and the diameter of the workpiece, according to pD1 N s ð22-1Þ V¼ 12 If the tool is fed at an angle to the axis of rotation, an external conical surface results. This is called taper turning (see Figure 22-3). If the tool is fed to the axis of

Workholder (3-jaw chuck)

Cutting tool

Workpiece

Engine lathe Machine tool

FIGURE 22-1 Schematic of a standard engine lathe performing a turning operation, with the cutting tool shown in inset. (Courtesy J T. Black)

609

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Turning and Boring Processes L Machined surface

Ns rpm Work D1

FIGURE 22-2 Basics of the turning process normally done on a lathe. The dashed arrows indicate the feed motion of the tool relative to the work.

Straight turning

V

Chip

D2 Ns

D2

D1

DOC = d Tool fr

Too l

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Feed

fr

Turning single-point tool process

in./rev Tool

Taper turning

Facing

Facing, grooving

Contour turning

Form turning

End facing

Parting or cutoff

Drilling

Boring

Taper boring

Internal threading

External threading

Necking

Internal forming

Knurling

FIGURE 22-3 Basic turning machines can rotate the work and feed the tool longitudinally for turning and can perform other operations by feeding transversely. Depending on what direction the tool is fed and on what portion of the rotating workpiece is being machined, the operations have different names. The dashed arrows indicate the tool feed motion relative to the workpiece. (Courtesy J T. Black)

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Fundamentals of Turning, Boring, and Facing Turning

611

Second Turret

Main Spindle

Second Spindle

Main Turret

FIGURE 22-4 NC twin-spindle horizontal turning centers use coaxial spindles so work can be transferred from the main spindle to the second spindle.

rotation, using a tool that is wider than the depth of the cut, the operation is called facing, and a flat surface is produced on the end of the cylinder. By using a tool having a specific form or shape and feeding it radially or inward against the work, external cylindrical, conical, and irregular surfaces of limited length can also be turned. The shape of the resulting surface is determined by the shape and size of the cutting tool. Such machining is called form turning (see Figure 22-3). If the tool is fed all the way to the axis of the workpiece, it will be cut in two. This is called parting or cutoff, and a simple, thin tool is used. A similar tool is used for necking or partial cutoff. Boring is a variation of turning. Essentially, boring is internal turning. Boring can use single-point cutting tools to produce internal cylindrical or conical surfaces. It does not create the hole but, rather, machines or opens the hole up to a specific size. Boring can be done on most machine tools that can do turning. However, boring also can be done using a rotating tool with the workpiece remaining stationary. Also, specialized machine tools have been developed that will do boring, drilling, and reaming but will not do turning. Other operations, like threading and knurling, can be done on machines used for turning. In addition, drilling, reaming, and tapping can be done on the rotation axis of the work. In recent years, turning centers have been developed that use turrets to hold multiple-edge rotary tools in powered heads. Some new machine tools feature two opposing spindles with automatic transfer from one to the other and two turrets of tools (see Figure 22-4).

& 22.2 FUNDAMENTALS OF TURNING, BORING, AND FACING TURNING Turning constitutes the majority of lathe work. The cutting forces, resulting from feeding the tool from right to left, should be directed toward the headstock to force the workpiece against the workholder and thus provide better work support. If good finish and accurate size are desired, one or more roughing cuts are usually followed by one or more finishing cuts. Roughing cuts may be as heavy as proper chip thickness, cutting dynamics, tool life, lathe horsepower, and the workpiece permit. Large depths of cut and smaller feeds are preferred to the reverse procedure, because fewer cuts are required and less time is lost in reversing the carriage and resetting the tool for the following cut. On workpieces that have a hard surface, such as castings or hot-rolled materials containing mill scale, the initial roughing cut should be deep enough to penetrate the

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hard materials. Otherwise, the entire cutting edge operates in hard, abrasive material throughout the cut, and the tool will dull rapidly. If the surface is unusually hard, the cutting speed on the first roughing cut should be reduced accordingly. Finishing cuts are light, usually being less than 0.015 in. in depth, with the feed as fine as necessary to give the desired finish. Sometimes a special finishing tool is used, but often the same tool is used for both roughing and finishing cuts. In most cases, one finishing cut is all that is required. However, where exceptional accuracy is required, two finishing cuts may be made. If the diameter is controlled manually, a short finishing cut (14 in. long) is made and the diameter checked before the cut is completed. Because the previous micrometer measurements were made on a rougher surface, it may be necessary to reset the tool in order to have the final measurement, made on a smoother surface, check exactly. In turning, the primary cutting motion is rotational, with the tool feeding parallel to the axis of rotation (Figure 22-2). To determine the inputs to the machines, the depth of cut, cutting speed, and feed rate must be selected. The desired cutting speed establishes the necessary rpm (Ns) of the rotating workpiece. The feed fr is given in inches per revolution (ipr). The depth of cut is d, where D1 D2 in: 2

d ¼ DOC ¼

ð22-2Þ

The length of cut is the distance traveled parallel to the axis L plus some allowance or overrun A to allow the tool to enter and/or exit the cut. The inputs to the turning process are determined as follows. Based on the material being machined and the cutting tool material being used, the engineer selects the cutting speed, V, in feet per minute, the feed (fr), and the depth of cut. The rpm value for the machine tool can be determined by Ns ¼

12V pD1

ð22-3Þ

(using the larger diameter), where the factor of 12 is used to convert feet to inches. The cutting time is Tm ¼

LþA f r Ns

ð22-4Þ

where A is overrun allowance and fr is the selected feed in inches per revolution.

EXAMPLE OF TURNING The 1.78-in.-diameter steel bar is to be turned down to a 1.10-in. diameter on a standard engine lathe. The overall length of the bar is 18.750 in., and the region to be turned is 16.50 in. The part is made from cold-drawn free-machining steel (this means the chips breakup nicely) with a BHN of 250. Because you want to take the bar from 1.78 to 1.10, you have a total depth of cut, d, of 0.34 in. (0.68/2). You decide you want to make two cuts, a roughing pass and a finishing pass. Rough at d ¼ 0.300 and finish with d ¼ 0.04 in. Looking at the table in Figure 20-4, for selecting speed and feed, you select V ¼ 100 fpm and feed ¼ 0.020 ipr because you have decided to use high-speed steel cutting tools. The bar is held in a chuck with a feed through the hole in the spindle and is supported on the right end with a live center. The ends of the bar have been center drilled. Allowance should be 0.50 in. for approach (no overtravel). Allow 1.0 min to reset the tool after the first cut. To determine the inputs to the lathe, we calculate the spindle rpm: N s ¼ 12V=pD1 ¼ 12 100=3:14 1:78 ¼ 214 But your lathe does not have this particular rpm, so you select the closest rpm, which is 200. You don’t need any further calculations for lathe inputs as you input the feed in ipr directly. The time to make the cut is Tm ¼

L þ ALL 16:50 þ 0:50 ¼ ¼ 4:25 min f r Ns 0:020 200

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613

Fundamentals of Turning, Boring, and Facing Turning

You could reduce this time by changing to a coated carbide tool that would allow you to increase the cutting speed to 925 sfpm (see the table in Figure 20-4). The time for the second cut will be different if you change the feed and/or the speed to improve the surface finish. Again from a table of speeds and feeds, you select a speed of 0.925 and a feed rate of 0.007, so N s ¼ 12 925=3:14 1:10 ¼ 3213 with 3200 rpm the closest value: Tm ¼

L þ ALL ¼ 0:75 min 0:007 3200

The metal removal rate (MRR) is

pD21 pD22 L Volume removed MRR ¼ ¼ Time 4L=f r N

(omitting the allowance term). By rearranging and substituting Ns, an exact expression for MRR is obtained: D21 D22 MRR ¼ 12Vf r ð22-5Þ 4D1 Rewriting the last term,

D21 D22 D1 D2 D1 D2 ¼ 4D1 2 2D1

Therefore, because d¼

ðD1 D2 Þ D1 D2 and ffi 1 for small d 2D1 2

then, MRR ffi 12 Vf r d in:3 =min

ð22-6Þ

Note that equation 22-6 is an approximate equation that assumes that the depth of cut d is small compared to the uncut diameter D1.

BORING Boring always involves the enlarging of an existing hole, which may have been made by drilling or may be the result of a core in a casting. An equally important and concurrent purpose of boring may be to make the hole concentric with the axis of rotation of the workpiece and thus correct any eccentricity that may have resulted from the drill starting or drifting off the centerline. Concentricity is an important attribute of bored holes. When boring is done in a lathe, the work usually is held in a chuck or on a faceplate. Holes may be bored straight, tapered, with threads, or to irregular contours. Figure 22-5a shows the relationship of the tool and the workpiece for boring. Think Machined surface Machined surface Ns Boring bar

Headstock D2

fr

D1

D1

Ns

d Work surface fr

Tube D1

Solid

fr

Tool

d Tool

Boring a drilled hole

Facing from the cross side

D2

fr

Cutoff or parting

FIGURE 22-5 Basic movement of boring, facing, and cutoff in a lathe, where cutting is performed by one-single-point cutting tool at a time and the tool can be fed in any direction.

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of boring as internal turning while feeding the tool parallel to the rotation axis of the workpiece, with two important differences. First, the relief and clearance angles on the tool should be larger and the tool overhang (length to diameter) must be considered with regard to stability and deflection problems. As before V and fr are selected. For a cut of length L, the cutting time is Tm ¼

LþA f r Ns

ð22-7Þ

where Ns ¼ 12V/pD1 for D1, the diameter of bore, and A, the overrun allowance. The metal removal rate is L pD21 pD22 =4 MRR ¼ L=f r N where D2 is the original hole diameter; MRR ffi 12Vf r d

ð22-8Þ

(omitting allowance term), where d is the depth of cut. In most respects, the same principles are used for boring as for turning. Again, the tool should be set exactly at the same height as the axis of rotation. Larger end clearance angles help to prevent the heel of the tool from rubbing on the inner surface of the hole. Secondly, because the tool overhang will be greater, smaller feeds and depths of cut may be selected to reduce the cutting forces that cause tool vibration and chatter. In some cases, the boring bar may be made of tungsten carbide because of this material’s greater stiffness.

FACING Facing is the producing of a flat surface as the result of the tool being fed across the end of the rotating workpiece, as shown in Figure 22-5b. Unless the work is held on a mandrel, if both ends of the work are to be faced, it must be turned end for end after the first end is completed and the facing operation repeated. The cutting speed should be determined from the largest diameter of the surface to be faced. Facing may be done either from the outside inward or from the center outward. In either case, the point of the tool must be set exactly at the height of the center of rotation. Because the cutting force tends to push the tool away from the work, it is usually desirable to clamp the carriage to the lathe bed during each facing cut to prevent it from moving slightly and thus producing a surface that is not flat. In the facing of castings or other materials that have a hard surface, the depth of the first cut should be sufficient to penetrate the hard material to avoid excessive tool wear. In facing, the tool feeds perpendicular to the axis of the rotating workpiece. Because the rpm is constant, the cutting speed is continually decreasing as the axis is approached. The length of cut L is D1/2 or (D1 – D2)/2 for a tube. T m ¼ Cutting time ¼

LþA min f rN

D1 þA ¼ 2 f rN MRR ¼

VOL pD21 df r N ¼ 6Vf ; d in:3 =min ¼ Tm 4L

where d is the depth of cut and L ¼ D1/2 is the length of cut.

ð22-9Þ

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Fundamentals of Turning, Boring, and Facing Turning

615

PARTING Parting is the operation by which one section of a workpiece is severed from the remainder by means of a cutoff tool, as shown in Figure 22-5c. Because parting tools are quite thin and must have considerable overhang, this process is more difficult to perform accurately. The tool should be set exactly at the height of the axis of rotation, be kept sharp, have proper clearance angles, and be fed into the workpiece at a proper and uniform feed rate. In parting or cutoff work, the tool is fed (plunged) perpendicular to the rotational axis, as it was in facing. The length of cut for solid bars is D1/2. For tubes, L¼

D1 D2 2

In cutoff operations, the width of the tool is d in inches, the width of the cutoff operation. The equations for Tm and MRR are then basically the same as for facing.

DEFLECTION In boring, facing, and cutoff operations, the speeds, feeds, and depth of cut selected are generally less than those recommended for straight turning because of the large overhang of the tool often needed to complete the cuts. Recall the basic equation for deflection of a cantilever beam, modifying for machining, d¼

Pl3 F c l3 ¼ 3EI 3EI

ð22-10Þ

In equation 22-10, l represents the overhang of the tool, which greatly affects the deflection, so it should be minimized whenever possible. In equation 22-10, E ¼ modulus of elasticity I ¼ moment of inertia of cross section of tool P ¼ Fc ¼ applied load or cutting force where I ¼ pD21 =64 solidround bar I ¼ p D41 D42 =64 bar with hole D1 ¼ diameter of tube or bar D2 ¼ inside diameter of the tube Deflection is proportional to the fourth power of the boring bar diameter and the third power of the bar overhang. Select the largest bar diameter and minimize the overhand. Use carbide shank boring bars (E ffi 80,000,000 psi), and select tool geometries that direct cutting forces into the feed direction to minimize chatter. The reduction of the feed or depth of cut reduces the forces operating on the tools. The cutting speed usually controls the occurrence of chatter and vibration. See the dynamics of machining discussion in Chapter 20. Any imbalance in the cutting forces will deflect the tool to the side, resulting in loss of accuracy in cutoff lengths. At the outset, the forces will be balanced if there is no side rake on the tool. As the cutoff tool reaches the axis of the rotating part, the tool will be deflected away from the spindle, resulting in a change in the length of the part.

PRECISION BORING Sometimes bored holes are slightly bell mouthed because the tool deflects out of the work as it progresses into the hole. This often occurs in castings and forgings where the holes have draft angles so that the depth of cut increases as the tool progresses down the bore. This problem can usually be corrected by repeating the cut with the same tool setting; however, the total cutting time for the part is increased. Alternately, a more robust setup can be used. Large holes may be precision bored using the setup shown in

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Turning and Boring Processes Jaw

Workpiece

Chuck Quill

Lathe spindle

Bored surface

Boring bar pilot

Taper shank

Pilot bushing

Cutting tool (HSS tool bit) Jaw

FIGURE 22-6 Pilot boring bar mounted in tailstock of lathe for precision boring large hole in casting. The size of the hole is controlled by the rotation diameter of the cutting tool.

Figure 22-6, where a pilot bushing is placed in the spindle to mate with the hardened ground pilot of the boring bar. This setup eliminates the cantilever problems common to boring. Because the rotational relationship between the work and the tool is a simple one and is employed on several types of machine tools—such as lathes, drilling machines, and milling machines—boring is very frequently done on such machines. However, several machine tools have been developed primarily for boring, especially in cases involving large workpieces or for large-volume boring of smaller parts. Such machines as these are also capable of performing other operations, such as milling and turning. Because boring frequently follows drilling, many boring machines also can do drilling, permitting both operations to be done with a single setup of the work.

DRILLING Drilling, discussed in detail in Chapter 23, can be done on lathes with the drill mounted in the tailstock quill of engine lathes or the turret on turret lathes and fed against a rotating workpiece. Straight-shank drills can be held in Jacobs chucks, or drills with taper shanks mounted directly in the quill hole can drill holes online (center of rotation). Drills can also be mounted in the turrets of modern turret centers and fed automatically on the rotational axis of the workpiece or off-axis with power heads. It also is possible to drill on a lathe with the drill bit mounted and rotated in the spindle while the work remains stationary, supported on the tailstock or the carriage of the lathe. The usual speeds used for drilling should be selected for lathe work. Because the feed may be manually controlled, care must be exercised, particularly in drilling small holes. Coolants should be used where required. In drilling deep holes, the drill should be withdrawn occasionally to clear chips from the hole and to aid in getting coolant to the cutting edges. This is called peck drilling. See Chapter 23 for further discussion on drilling.

REAMING Reaming on a lathe involves no special precautions. Reamers are held in the tailstock quill, taper-shank types being mounted directly and straight-shank types by means of a drill chuck. Rose-chucking reamers are usually used (see Chapter 23). Fluted-chucking reamers may also be used, but these should be held in some type of holder that will permit the reamer to float (i.e., have some compliance) in the hole and conform to the geometry created by the boring process.

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SECTION 22.3

FIGURE 22-7 (a) Knurling in a lathe, using a forming-type tool, and showing the resulting pattern on the workpiece; (b) knurling tool with forming rolls. (Courtesy Armstong Industrial Hand Tools)

Lathe Design and Terminology

617

Two forming rolls (a)

(b)

KNURLING Knurling produces a regularly shaped, roughened surface on a workpiece. Although knurling also can be done on other machine tools, even on flat surfaces, in most cases it is done on external cylindrical surfaces using lathes. Knurling is a chipless, cold-forming process. See Figures 22-7 and 22-10 for examples. The two hardened rolls are pressed against the rotating workpiece with sufficient force to cause a slight outward and lateral displacement of the metal to form the knurl in a raised, diamond pattern. Another type of knurling tool produces the knurled pattern by cutting chips. Because it involves less pressure and thus does not tend to bend the workpiece, this method is often preferred for workpieces of small diameter and for use on automatic or semiautomatic machines.

SPECIAL ATTACHMENTS For engine lathes, taper turning and milling can be done on a lathe but require special attachments. The milling attachment is a special vise that attaches to the cross slide to hold work. The milling cutter is mounted and rotated by the spindle. The work is fed by means of the cross-slide screw. Tool-post grinders are often used to permit grinding to be done on a lathe. Taper turning will be discussed later. Duplicating attachments are available that, guided by a template, will automatically control the tool movements for turning irregularly shaped parts. In some cases, the first piece, produced in the normal manner, may serve as the template for duplicate parts. To a large extent, duplicating lathes using templates have been replaced by numerically controlled lathes and milling is done with power tools in numerical control turret lathes.

& 22.3 LATHE DESIGN AND TERMINOLOGY Knowing the terminology of a machine tool is fundamental to understanding how it performs the basic processes, how the workholding devices are interchanged, and how the cutting tools are mounted and interfaced to the work. Lathes are machine tools designed primarily to do turning, facing, and boring. Very little turning is done on other types of machine tools, and none can do it with equal facility. Lathes also can do facing, drilling, and reaming. Modern turning centers permit milling and drilling operations using live (also called powered) spindles in multiple-tool turrets, so their versatility permits multiple operations to be done with a single setup of the workpiece. Consequently, the lathe is probably the most common machine tool, along with milling machines. Lathes in various forms have existed for more than 2000 years, but modern lathes date from about 1797, when Henry Maudsley developed one with a leadscrew, providing controlled, mechanical feed of the tool. This ingenious Englishman also developed a change-gear system that could connect the motions of the spindle and leadscrew and thus enable threads to be cut.

LATHE DESIGN The essential components of an engine lathe (Figure 22-8) are the bed, headstock assembly (which includes the spindle), tailstock assembly, carriage assembly,

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Turning and Boring Processes Spindle speed levers Headstock Spindle nose Compound rest Cross slide Carriage Thread–chasing dial

Gearbox

FIGURE 22-8 Schematic of an engine lathe, a versatile machine tool that is easy to set up and operate and is intended for shortrun jobs.

Tailstock

Leadscrew Feed & thread levers

Spindle start & stop

Spindle start & stop

Feed rod

Apron

Half-nut lever

Feed lever feed fwd & rev

quick-change gearbox, and the leadscrew and feed rod. The bed is the base and backbone of a lathe. The bed is usually made of well-normalized or aged gray or nodular cast iron and provides a heavy, rigid frame on which all the other basic components are mounted. Two sets of parallel, longitudinal ways, inner and outer, are contained on the bed. On modern lathes, the ways are surface hardened and precision machined, and care should be taken to ensure that the ways are not damaged. Any inaccuracy in them usually means that the accuracy of the entire lathe is destroyed. The headstock, mounted in a fixed position on the inner ways, provides powered means to rotate the work at various rpm values. Essentially, it consists of a hollow spindle, mounted in accurate bearings, and a set of transmission gears—similar to a truck transmission—through which the spindle can be rotated at a number of speeds. Most lathes provide from 8 to 18 choices of rpm. On modern lathes all the rpm rates can be obtained merely by moving from two to four levers or the lathe has a continuously variable spindle rpm using electrical or mechanical drives. The accuracy of a lathe is greatly dependent on the spindle. It carries the workholders and is mounted in heavy bearings, usually preloaded tapered roller or ball types. The spindle has a hole extending through its length through which long bar stock can be fed. The size of this hole is an important dimension of a lathe because it determines the maximum size of bar stock that can be machined when the materials must be fed through the spindle. The spindle protrudes from the gearbox and contains means for mounting various types of workholding devices (chucks, face and dog plates, collets). Power is supplied to the spindle from an electric motor through a V-belt or silent-chain drive. Most modern lathes have motors of from 5 to 25 hp to provide adequate power for carbide and ceramic tools cutting hard materials at high cutting speeds. For the classic engine lathe, the tailstock assembly consists, essentially, of three parts. A lower casting fits on the inner ways of the bed, can slide longitudinally, and can be clamped in any desired location. An upper casting fits on the lower one and can be moved transversely upon it, on some type of keyed ways, to permit aligning the tailstock and headstock spindles (for turning tapers). The third major component of the assembly is the tailstock quill. This is a hollow steel cylinder, usually about 2 to 3 in. diameter, that can be moved longitudinally in and out of the upper casting by means of a handwheel and screw. The open end of the quill hole has a Morse taper. Cutting tools or a lathe center are held in the quill. A graduated scale is usually engraved on the outside of the quill to aid in controlling its motion in and out of the upper casting. A locking device permits clamping the quill in any desired position. In recent years, dual-spindle numerical control (NC) turning centers have emerged, where a subspindle replaces the tailstock assembly. Parts can be automatically transferred from the spindle to the subspindle for turning the back end of the part. See Chapter 39 for more on NC machining and turning centers.

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SECTION 22.3

Tool post

Lathe Design and Terminology

619

Cross slide

Compound rest Threading dial

Carriage

FIGURE 22-9 The cutting tools for lathe work are held in the tool post on the compound rest, which can translate and swivel. (Courtesy J T. Black)

Cross slide

Handwheels

The carriage assembly, together with the apron, provides the means for mounting and moving cutting tools. The carriage, a relatively flat H-shaped casting, rides on the outer set of ways on the bed. The cross slide is mounted on the carriage and can be moved by means of a feed screw that is controlled by a small handwheel and a graduated dial. The cross slide thus provides a means for moving the lathe tool in the facing or cutoff direction. On most lathes, the tool post is mounted on a compound rest (see Figure 22-9). The compound rest can rotate and translate with respect to the cross slide, permitting further positioning of the tool with respect to the work. The apron, attached to the front of the carriage, has the controls for providing manual and powered motion for the carriage and powered motion for the cross slide. The carriage is moved parallel to the ways by turning a handwheel on the front of the apron, which is geared to a pinion on the back side. This pinion engages a rack that is attached beneath the upper front edge of the bed in an inverted position. Powered movement of the carriage and cross slide is provided by a rotating feed rod. The feed rod, which contains a keyway, passes through the two reversing bevel pinions and is keyed to them. Either pinion can be activated by means of the feed reverse lever, thus providing ‘‘forward’’ or ‘‘reverse’’ power to the carriage. Suitable clutches connect either the rack pinion or the cross-slide screw to provide longitudinal motion of the carriage or transverse motion of the cross slide. For cutting threads, a leadscrew is used. When a friction clutch is used to drive the carriage, motion through the leadscrew is by a direct, mechanical connection between the apron and the leadscrew. A split nut is closed around the leadscrew by means of a lever on the front of the apron directly driving the carriage without any slippage. Modern lathes have quick-change gearboxes, driven by the spindle, that connect the feed rod and leadscrew. Thus, when the spindle turns a revolution, the tool (mounted on the carriage) translates (longitudinally or transversely) a specific distance in inches— that is, inches per revolution (ipr). This revolutions per minute, rpm or Ns, times the feed, fr, gives the feed rate, f, in inches per minute (ipm) that the tool is moving. In this way, the calculations for turning rpm and feed in ipr are ‘‘mechanically related.’’ Typical lathes may provide as many as 48 feeds, ranging from 0.002 to 0.118 in. (0.05 to 3 mm) per revolution of the spindle, and, through the leadscrew, leads from to 92 threads per inch.

SIZE DESIGNATION OF LATHES The size of a lathe is designated by two dimensions. The first is known as the swing. This is the maximum diameter of work that can be rotated on a lathe. Swing is approximately twice the distance between the line connecting the lathe centers and the nearest point on the ways. The maximum diameter of a workpiece that can be mounted between centers is somewhat less than the swing diameter because the workpiece must clear the carriage assembly as well as the ways. The second size dimension is the maximum

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distance between centers. The swing thus indicates the maximum workpiece diameter that can be turned in the lathe, while the distance between centers indicates the maximum length of workpiece that can be mounted between centers.

TYPES OF LATHES Lathes used in manufacturing can be classified as speed, engine, toolroom, turret, automatics, tracer, and numerical control turning centers. Speed lathes usually have only a headstock, a tailstock, and a simple tool post mounted on a light bed. They ordinarily have only three or four speeds and are used primarily for wood turning, polishing, or metal spinning. Spindle speeds up to about 4000 rpm are common. Engine lathes are the type most frequently used in manufacturing. Figure 22-1 and Figure 22-8 are examples of this type. They are heavy-duty machine tools with all the components described previously and have power drive for all tool movements except on the compound rest. They commonly range in size from 12- to 24-in. swing and from 24- to 48-in. center distances, but swings up to 50 in. and center distances up to 12 ft are not uncommon. Very large engine lathes (36- to 60-ft-long beds) are therefore capable of performing roughing cuts in iron and steel at depths of cut of 12 to 2 in. and at cutting speeds at 50 to 200 sfpm with WC tools run at 0.010 to 0.100 in./rev. To perform such heavy cuts requires rigidity in the machine tool, the cutting tools, the workholder, and the workpiece (using steady rests and other supports) and large horsepower (50 to 100 hp). Most engine lathes are equipped with chip pans and a built-in coolant circulating system. Smaller engine lathes, with swings usually not greater than 13 in., also are available in bench type, designed for the bed to be mounted on a bench or table. Toolroom lathes have somewhat greater accuracy and, usually, a wider range of speeds and feeds than ordinary engine lathes. Designed to have greater versatility to meet the requirements of tool and die work, they often have a continuously variable spindle speed range and shorter beds than ordinary engine lathes of comparable swing because they are generally used for machining relatively small parts. They may be either bench or pedestal type. Several types of special-purpose lathes are made to accommodate specific types of work. On a gap-bed lathe, for example, a section of the bed, adjacent to the headstock, can be removed to permit work of unusually large diameter to be swung. Another example is the wheel lathe, which is designed to permit the turning of railroad-car wheel-and-axle assemblies. Figure 22-10 shows a large vertical turning lathe machining a large steel casting. Vertical lathes are an excellent alternative to large horizontal CNC lathes.

FIGURE 22-10 This large vertical turning center is used for turning large circular parts rotated under vertically mounted tools. (# Alvis Upitis/SuperStock)

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SECTION 22.3

Lathe Design and Terminology

621

Gravity-aided seating of large/heavy workpieces allows a high degree of process repeatability. A smaller footprint, lower initial cost, and increased productivity are all advantages when compared to traditional horizontal lathes. Although engine lathes are versatile and very useful, the time required for changing and setting tools and for making measurements on the workpiece is often a large percentage of the cycle time. Often, the actual chip-production time is less than 30% of the total cycle time. Methods to reduce setup and tool-change time are now well known, reducing setups to minutes and unload/load steps to seconds. The placement of singlecycle machine tools into interim or lean manufacturing cells will increase the productivity of the workers because they can run more than one machine. Turret lathes, screw machines, and other types of semiautomatic and automatic lathes have been highly developed and are widely used in manufacturing as another means to improve cutting productivity. Turret Lathes. The basic components of a turret lathe are depicted in Figure 22-11. Basically, a longitudinally feedable, hexagon turret replaces the tailstock. The turret, on which six tools can be mounted, can be rotated about a vertical axis to bring each tool into operating position, and the entire unit can be translated parallel to the ways, either manually or by power, to provide feed for the tools. When the turret assembly is backed away from the spindle by means of a capstan wheel, the turret indexes automatically at the end of its movement, thus bringing each of the six tools into operating position in sequence. The square turret on the cross slide can be rotated manually about a vertical axis to bring each of the four tools into operating position. On most machines, the turret Adjustment Feed Spindle speed

Headstock Spindle revolves

Square turret (indexes)

Cross slide Rear tool post

Square turret (indexes) Reach over carriage

Hexagonal turret (indexes) Saddle

Ram

Ram-turret lathe (light duty)

Top view

Side-hung carriage

Saddle-turret lathe (heavy duty)

Bed

Spindle

Rear tool (1 tool station) post Bed

Ram

Headstock

FIGURE 22-11 Block diagrams of ram- and saddle-turret lathe.

Cross-slide turret (4 tool stations) square turret

Carriage

Hexagonal ram turret (6 tool stations)

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can be moved transversely, either manually or by power, by means of the cross slide, and longitudinally through power or manual operation of the carriage. In most cases, a fixed tool holder also is added to the back end of the cross slide; this often carries a parting tool. Through these basic features of a turret lathe, a number of tools can be set up on the machine and then quickly be brought successively into working position so that a complete part can be machined without the necessity for further adjusting, changing tools, or making measurements. The two basic types of turret lathes are the ram-type turret lathe and the saddletype turret lathe. In the ram-type turret lathe, the ram and turret are moved up to the cutting position by means of the capstan wheel, and the power feed is then engaged. As the ram is moved toward the headstock, the turret is automatically locked into position so that rigid tool support is obtained. Rotary stopscrews control the forward travel of the ram, one stop being provided for each face on the turret. The proper stop is brought into operating position automatically when the turret is indexed. A similar set of stops is usually provided to limit movement of the cross slide. The saddle-type turret lathe provides a more rugged mounting for the hexagon turret than can be obtained by the ramtype mounting. In saddle-type lathes, the main turret is mounted directly on the saddle, and the entire saddle and turret assembly reciprocates. Larger turret lathes usually have this type of mounting. However, because the saddle-turret assembly is rather heavy, this type of mounting provides less rapid turret reciprocation. When such lathes are used with heavy tooling for making heavy or multiple cuts, a pilot arm attached to the headstock engages a pilot hole attached to one or more faces of the turret to give additional rigidity. Turret-lathe headstocks can shift rapidly between spindle speeds and brake rapidly to stop the spindle very quickly. They also have automatic stock-feeding for feeding bar stock through the spindle hole. If the work is to be held in a chuck, some type of air-operated chuck or a special clamping fixture is often employed to reduce the time required for part loading and unloading. Single-Spindle Automatic Screw Machines. There are two common types of singlespindle screw machines. One, an American development and commonly called the turret type (Brown & Sharpe), is shown in Figure 22-12. The other is of Swiss origin and is referred to as the Swiss type. The Brown & Sharpe screw machine is essentially a small automatic turret lathe, designed for bar stock, with the main turret mounted in a vertical plane on a ram. Front and rear tool holders can be mounted on the cross slide.

Bar feeder

Upper slides Spindle Cross slide

Tool turret Turret cam

Upper slides

Spindle

Upper slide cam

FIGURE 22-12

Cross slides cam

Dog carriers for spindle reverse and turret index

s os Cr de sli

On the turret-type single-spindle automatic, the tools must take turns to make cuts.

Tool turret

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Lathe Design and Terminology

623

FIGURE 22-13 Close-up view of a Swiss-type screw machine, showing the tooling and radial tool sides, actuated by rocker arms controlled by a disk cam, shown in lower left. (Courtesy J T. Black)

All motions of the turret, cross slide, spindle, chuck, and stock-feed mechanism are controlled by cams. The turret cam is essentially a program that defines the movement of the turret during a cycle. These machines are usually equipped with an automatic rod-feeding magazine that feeds a new length of bar stock into the collet (the workholding device) as soon as one rod is completely used. Often, screw machines of the Brown & Sharpe type are equipped with a transfer or ‘‘picking’’ attachment. This device picks up the workpiece from the spindle as it is cut off and carries it to a position where a secondary operation is performed by a small, auxiliary power head. In this manner, screwdriver slots are put in screw heads, small flats are milled parallel with the axis of the workpiece, or holes are drilled normal to the axis. On the Swiss-type automatic screw machine, the cutting tools are held and moved in radial slides (Figure 22-13). Disk cams move the tools into cutting position and provide feed into the work in a radial direction only; they provide any required longitudinal feed by reciprocating the headstock. Most machining on Swiss-type screw machines is done with single-point cutting tools. Because they are located close to the spindle collet, the workpiece is not subjected to much deflection. Consequently, these machines are particularly well suited for machining very small parts. Both types of single-spindle screw machines can produce work to close tolerances, the Swiss-type probably being somewhat superior for very small work. Tolerances of 0.0002 to 0.0005 in. are not uncommon. The time required for setting up the machine is usually an hour or two and can be much less. One person can tend many machines, once they are properly tooled. They have short cycle times, frequently less than 30 s/piece. Multiple-Spindle Automatic Screw Machines. Single-spindle screw machines utilize only one or two tooling positions at any given time. Thus, the total cycle time per workpiece is the sum of the individual machining and tool-positioning times. On multiplespindle screw machines, sufficient spindles—usually four, six, or eight—are provided so that many tools can cut simultaneously. Thus, the cycle time per piece is equal to the maximum cutting time of a single tool position plus the time required to index the spindles from one position to the next. The two distinctive features of multiple-spindle screw machines are shown in Figure 22-14. First, the six spindles are carried in a rotatable drum that indexes in order to bring each spindle into a different working position. Second, a nonrotating tool slide contains the same number of tool holders as there are spindles and thus provides and

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Turning and Boring Processes Cutoff slide Front cutoff slide

End tool working slide

6 end slides Headstock 6 cross slides

All spindles on multiple-spindle automatic have the same tool path Spindle carrier

Lower cross slides The six-spindle automatic

Upper cross slides

Cross slide

Spindle arrangement for sixspindle automatic. The barstock is usually fed to a stop at position 6. The cutoff position is the one preceding the bar feed position.

Tooling sheet for making a part on a six-spindle.

FIGURE 22-14 The multiple-spindle, automatic screw machine makes all cuts simultaneously and then performs the noncutting functions (tool withdrawal, index, bar feed) at high speed.

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Cutting Tools for Lathes

625

positions a cutting tool (or tools) for each spindle. Tools are fed by longitudinal reciprocating motion. Most machines have a cross slide at each spindle position so that an additional tool can be fed from the side for facing, grooving, knurling, beveling, and cutoff operations. All motions are controlled automatically. Starting with the sixth position, follow the sequence of processing steps on the tooling sheet for making a part shown in Figure 22-14. With a tool position available on the end tool slide for each spindle (except for a stock-feed stop at position 6), when the slide moves forward, these tools cut essentially simultaneously. At the same time, the tools in the cross slides move inward and make their cuts. When the forward cutting motion of the end tool slide is completed, it moves away from the work, accompanied by the outward movement of the radial slides. The spindles are indexed one position, by rotation of the spindle carrier, to position each part for the next operation to be performed. At spindle position 5, finished pieces are cut off. Bar stock 1 in. in diameter is fed to correct length for the beginning of the next operation. Thus, a piece is completed each time the tool slide moves forward and back. Multiple-spindle screw machines are made in a considerable range of sizes, determined by the diameter of the stock that can be accommodated in the spindles. There may be four, five, six, or eight spindles. The operating cycle of the end tool slide is determined by the operation that requires the longest time. Once a multiple-spindle screw machine is set up, it requires only that the bar stock feed rack be supplied and the finished products checked periodically to make sure that they are within desired tolerances. One operator usually services many machines. Most multiple-spindle screw machines use cams to control the motions. Setting up the cams and the tooling for a given job may require from 2 to 20 hr. However, once such a machine is set up, the processing time per part is very short. Often, a piece may be completed every 10 s. Typically, a minimum of 2000 to 5000 parts are required in a lot to justify setting up and tooling a multiple-spindle automatic screw machine. The precision of multiple-spindle screw machines is good, but seldom as good as that of single-spindle machines. However, tolerances from 0.0005 to 0.001 in. on the diameter are typical.

& 22.4 CUTTING TOOLS FOR LATHES LATHE CUTTING TOOLS Most lathe operations are done using single-point cutting tools, such as the classic tool designs shown in Figure 22-15. On right-hand turning (and left-hand turning) and facing tools, the cutting usually takes place on the side of the tool; therefore, the side rake angle is of primary importance, particularly when deep cuts are being made. On the round-nose turning tools, cutoff tools, finishing tools, and some threading tools, cutting takes place on or near the tip of the tool, and the back rake is therefore of importance. Such tools are used with relatively light depths of cut. Because tool materials are expensive, it is desirable to use as little as possible. At the same time, it is essential that the cutting tool be supported in a strong, rigid manner to minimize deflection and possible vibration. Consequently, lathe tools are supported in various types of heavy, forged steel tool holders. The high-speed steel (HSS) tool bit should be clamped in the tool holder with minimum overhang; otherwise, tool chatter and a poor surface finish may result. In the use of carbide, ceramic, or coated carbides for higher speed cutting, throwaway inserts are used that can be purchased in a great variety of shapes, geometrics (nose radius, tool angles, and groove geometry), and sizes (see Figure 22-16 for some examples). When lathes are incorporated into lean manufacturing cells, requiring that different operations be performed and the time required for changing and setting tools may constitute as much as 50% of the total cycle time. Quick-change tool holders (Figure 22-17) can reduce the manual tool-changing time. The individual tools, preset in their holders, can be interchanged in the special tool post in a few seconds. With some systems, a second tool may be set in the tool post while a cut is being made with the first tool and can then be brought into proper position by rotating the post.

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Right-hand turning tool

(a)

Facing Right-hand facing tool

(b)

(c) Grooving Cutoff

Boring

FIGURE 22-15 Common types of forged tool holders: (a) righthand turning, (b) facing, (c) grooving cutoff, (d) boring, (e) threading. (Courtesy of Armstrong Industrial Hand Tools)

(d) H.S.S. Bit

(e) Threading

In lathe work, the nose of the tool should be set exactly at the same height as the axis of rotation of the work. However, because any setting below the axis causes the work to tend to ‘‘climb’’ up on the tool, most machinists set their tools a few thousandths of an inch above the axis, except for cutoff, threading, and some facing operations.

FORM TOOLS In Figure 22-15, the use of form tools was shown in automatic lathe work. Form tools are made by grinding the inverse of the desired work contour into a block of HSS or tool steel. A threading tool is often a form tool. Although form tools are relatively expensive to manufacture, it is possible to machine a fairly complex surface with a single inward feeding of one tool. For mass-production work, adjustable form tools of either flat or rotary types, such as are shown in Figure 22-18, are used. These are expensive to make initially but can be resharpened by merely grinding a small amount off the face and then raising or rotating the cutting edge to the correct position.

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SECTION 22.4 Insert shape

Cutting Tools for Lathes

627

Typical insert holder

Available cutting edges

Round 4–10 on a side 8–20 total

15⬚

Square insert

80⬚/100⬚ diamond 4 on a side 8 total Square 4 on a side 8 total

0⬚

Triangular insert

Triangle 3 on a side 6 total 55⬚ diamond 2 on a side 4 total

35° diamond

35⬚ diamond 2 on a side 4 total

5⬚

FIGURE 22-16 Typical insert shapes, available cutting edges per insert, and insert holders for throwaway insert cutting tools. (Adapted from Turning Handbook of High Efficiency Metal Cutting, Courtesy Armstong Industrial Hand Tools)

QUICK-CHANGE TOOL POST

TURNING, FACING, AND BORING TOOL HOLDER V-Slot holds round boring bars as well as square tool bits

FIGURE 22-17 Quick-change tool post and accompanying tool holders. (Courtesy Armstong Industrial Hand Tools)

TURNING AND FACING TOOL HOLDER Takes turning and facing tool bits

Form turning

KNURLING TOOL HOLDER Revolving head, selfcentering. 3 pairs of knurls

FIGURE 22-18 Circular and block types of form tools.

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The use of form tools is limited by the difficulty of grinding adequate rake angles for all points along the cutting edge. A rigid setup is needed to resist the large cutting forces that develop with these tools. Light feeds with sharp, coated HSS tools are used on multiple-spindle automatics, turret lathes, and transfer line machines.

TURRET-LATHE TOOLS In turret lathes, the work is generally held in collets and the correct amount of bar stock is fed into the machine to make one part. The tools are arranged in sequence at the tool stations with depths of cut all preset. The following factors should be considered when setting up a turret lathe. 1. Setup time: time required to install and set the tooling and set the stops. Standard tool holders and tools should be used as much as possible to minimize setup time. Setup time can be greatly reduced by eliminating adjustment in the setup. 2. Workholding time: time to load and unload parts and/or stock. 3. Machine-controlling time: time required to manipulate the turrets. Can be reduced by combining operations where possible. Dependent on the sequence of operations established by the design of the setup. 4. Cutting time: time during which chips are being produced. Should be as short as is economically practical and represent the greatest percentage of the total cycle time possible. 5. Cost: cost of the tool, setup labor cost, lathe operator labor cost, and the number of pieces to be made. There are essentially 11 tooling stations, as shown in Figure 22-19, with six in the turret, four in the indexable tool post, and one in the rear tool post. The tooling is more rugged

Threaded shaft 9

Cutoff and chamfer B D F

A

1 Turn D

C Form A

E

8 5 3

HEX 3

6

Center drill

4 End face and chamfer

Turn F See INSET

INSET

Rolls

2

7

Neck C and E

Turn B

FIGURE 22-19 Turret-lathe tooling setup for producing part shown. Numbers in circles indicate the sequence of the operations from 1 to 9. The letters A through F refer to the surfaces being machined. Operation 3 is a combined operation. The roll turner is turning surface F, while tool 3 on the square post is turning surface B. The first operation stops the stock at the right length. The last operation cuts the finished bar off and puts a chamfer on the bar, which will next be advanced to the stock stop.

Thread F

Stock stop

Tool-relief lever

Toolblock pivot

Cutter Roller turner has rolls to support the work against the cutting forces

Roll turner

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Workholding in Lathes

629

in turret lathes because heavy, simultaneous cuts are often made. Tools mounted in the hex turret that are used for turning are often equipped with pressure rollers set on the opposite side of the rotating workpiece from the tool to counter the cutting forces. The setup times are usually 1 or 2 hr, so turret lathes are most economical in producing lots too large for engine lathes but too small for automatic screw machines or automatic lathes. In recent years, much of this work has been assumed by numerical control lathes, turning centers, or lean manufacturing cells. For example, the component (threaded shaft) shown in Figure 22-19 could have also been made on an NC turret lathe with some savings in cycle time or in a manufacturing cell with further savings in cycle time.

& 22.5 WORKHOLDING IN LATHES WORKHOLDING DEVICES FOR LATHES Five methods are commonly used for supporting workpieces in lathes: 1. Held between centers. 2. Held in a chuck. 3. Held in a collet. 4. Mounted on a faceplate. 5. Mounted on the carriage. In the first four of these methods, the workpiece is rotated during machining. In the fifth method, which is not used extensively, the tool rotates while the workpiece is fed into the tool. A general discussion of workholding devices is found in Chapter 27, and the student involved in designing working devices should study the reference materials under ‘‘Tool Design.’’ For lathes, workholding is a matter of selecting from standard tooling.

LATHE CENTERS Workpieces that are relatively long with respect to their diameters are usually held between centers (see Figure 22-20). Two lathe centers are used, one in the spindle hole and the other in the hole in the tailstock quill. Two types are used, called dead and live. Dead centers are solid, that is, made of hardened steel with a Morse taper on one end so that it will fit into the spindle hole. The other end is ground to a taper. Sometimes the tip of this taper is made of tungsten carbide to provide better wear resistance. Before a center is placed in position, the spindle hole should be carefully wiped clean. The Dog plate, rotates

Dog, clamped to workpiece

Taper end Solid or “dead” lathe center Dog plate

Dog

Center

Live lathe center can rotate with the part

FIGURE 22-20 Work being turned between centers in a lathe, showing the use of a dog and dog plate. (Courtesy of South Bend Lathe)

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presence of foreign material will prevent the center from seating properly, and it will not be aligned accurately. A mechanical connection must be provided between the spindle and the workpiece to provide rotation. This is accomplished by a lathe dog and dog plate. The dog is clamped to the work. The tail of the dog enters a slot in the dog plate, which is attached to the lathe spindle in the same manner as a lathe chuck. For work that has a finished surface, a piece of soft metal, such as copper or aluminum, can be placed between the work and the dog setscrew clamp to avoid marring. Live centers are designed so that the end that fits into the workpiece is mounted on ball or roller bearings. It is free to rotate. No lubrication is required. Live centers may not be as accurate as the solid type and therefore are not often used for precision work. Before a workpiece can be mounted between lathe centers, a center hole must be drilled in each end. This is typically done in a drill press or on the lathe with a tool held in the rear turret. A combination center drill and countersink ordinarily is used, with care taken that the center hole is deep enough so that it will not be machined away in any facing operation and yet is not drilled to the full depth of the tapered portion of the center drill (see Chapter 23). Because the work and the center of the headstock end rotate together, no lubricant is needed in the center hole at this end. The center in the tailstock quill does not rotate; adequate lubrication must be provided. A mixture of white lead and oil is often used. Failure to provide proper lubrication at all times will result in scoring of the workpiece center hole and the center, and inaccuracy and serious damage may occur. Live centers are often used in the tailstock to overcome these problems. The workpiece must rotate freely, yet no looseness should exist. Looseness will usually be manifested in chattering of the workpiece during cutting. The setting of the centers should be checked after cutting for a short time. Heating and thermal expansion of the workpiece will reduce the clearances in the setup.

MANDRELS Workpieces that must be machined on both ends or are disk-shaped are often mounted on mandrels for turning between centers. Three common types of mandrels are shown in Figure 22-21. Solid mandrels usually vary from 4 to 12 in. in length and are accurately ground with a 1:2000 taper (0.006 in./ft). After the workpiece is drilled and/or bored, it is pressed on the mandrel. The mandrel should be mounted between centers so that the cutting force tends to tighten the work on the mandrel taper. Solid mandrels permit the work to be machined on both ends as well as on the cylindrical surface. They are available in stock sizes but can be made to any desired size. Gang (or disk) mandrels are used for production work because the workpieces do not have to be pressed on and thus can be put in position and removed more rapidly. However, only the cylindrical surface of the workpiece can be machined when this type of mandrel is used. Cone mandrels have the advantage that they can be used to center workpieces having a range of hole sizes.

LATHE CHUCKS Lathe chucks are used to support a wider variety of workpiece shapes and to permit more operations to be performed than can be accomplished when the work is held between centers. Two basic types of chucks are used, three-jaw and four-jaw (Figure 22-22).

Flat for dog

FIGURE 22-21 Three types of mandrels, which are mounted between centers for lathe work.

Tapered

Flat

Work

Plain solid mandrel

Gang mandrel

Flat

Cone mandrel

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631

3-jaw self-centering Actuates opening and closing Removable jaws

4-jaw with each jaw independent

FIGURE 22-22 The jaws on chucks for lathes (four-jaw independent or three-jaw selfcentering) can be removed and reversed.

Three-jaw self-centering chucks are used for work that has a round or hexagonal cross section. The three jaws are moved inward or outward simultaneously by the rotation of a spiral cam, which is operated by means of a special wrench through a bevel gear. If they are not abused, these chucks will provide automatic centering to within about 0.001 in. However, they can be damaged through use and will then be considerably less accurate. Each jaw in a four-jaw independent chuck can be moved inward and outward independent of the others by means of a chuck wrench. Thus, they can be used to support a wide variety of work shapes. A series of concentric circles engraved on the chuck face aid in adjusting the jaws to fit a given workpiece. Four-jaw chucks are heavier and more rugged than the three-jaw type, and because undue pressure on one jaw does not destroy the accuracy of the chuck, they should be used for all heavy work. The jaws on both three- and four-jaw chucks can be reversed to facilitate gripping either the inside or the outside of workpieces. Combination four-jaw chucks are available in which each jaw can be moved independently or can be moved simultaneously by means of a spiral cam. Two-jaw chucks are also available. For mass-production work, special chucks are often used in which the jaws are actuated by air or hydraulic pressure, permitting very rapid clamping of the work. See Figure 22-23 for a schematic. The rapid exchange of tooling is a key manufacturing strategy in manufacturing cells. Chuck jaw sets are dedicated and customized for specific parts. The first time a chuck jaw set is used, each jaw is marked with the number of the jaw slot where it was installed and an index mark that corresponds with the alignment of the jaw serrations and the first tooth on the chuck master jaw. The jaws can now be reinstalled on the chuck exactly where they were bored. The adjustability of the chuck body lets the operator dial in part concentrically without resetting the jaw.

COLLETS Collets are used to hold smooth cold-rolled bar stock or machined workpieces more accurately than with regular chucks. As shown in Figure 22-24, collets are relatively thin tubular steel bushings that are split into three longitudinal segments over about twothirds of their length. At the split end, the smooth internal surface is shaped to fit the

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Spindle Jaw one of three

Actuator

Spindle adaptor

Draw tube Chuck

Draw tube connector External wedge Hardened and ground master jaw

FIGURE 22-23 Hydraulically actuated through-hole three-jaw power chuck shown in section view to left and in the spindle of the lathe above connected to the actuator.

Adapter plate Chuck body Top jaw

Housing

piece of stock that is to be held. The external surface of the collet is a taper that mates with an internal taper of a collet sleeve that fits into the lathe spindle. When the collet is pulled inward into the spindle (by means of the draw bar), the action of the two mating tapers squeezes the collet segments together, causing them to grip the workpiece. Collets are made to fit a variety of symmetrical shapes. If the stock surface is smooth and accurate, good collets will provide very accurate centering, with runout less than 0.0005 in. However, the work should be no more than 0.002 in. larger or 0.005 in.

Round collet

FIGURE 22-24 Several types of lathe collets. (Courtesy of South Bend Lathe)

Square collet

Hexagon collet

Cutaway view of collet

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633

Collet Workpiece

Sleeve

Draw bar

Spindle Workpiece

(a)

Force indicator pins Mechanical grip

Full spindle bore Standard collet pads

FIGURE 22-25 (a) Method of using a draw-in collet in lathe spindle. (b) Schematic of a collet chuck in which the clamping force can be adjusted. [(a) Courtesy of South Bend Lathe]

(b)

smaller than the nominal size of the collet. Consequently, collets are used only on drillrod, cold-rolled, extruded, or previously machined stock. Collets that can open automatically and feed bar stock forward to a stop mechanism are commonly used on automatic lathes and turret lathes. An example of a collet chuck is shown in Figure 22-25. Another type of collet similar to a Jacobs drill chuck has a greater size range than ordinary collets; therefore, fewer are required.

FACEPLATES Faceplates are used to support irregularly shaped work that cannot be gripped easily in chucks or collets. The work can be bolted or clamped directly on the faceplate or can be supported on an auxiliary fixture that is attached to the faceplate. The latter procedure is time saving when identical pieces are to be machined.

MOUNTING WORK ON THE CARRIAGE When no other means are available, boring is occasionally done on a lathe by mounting the work on the carriage, with the boring bar mounted between centers and driven by means of a dog.

STEADY AND FOLLOW RESTS If one attempts to turn a long, slender piece between centers, the radial force exerted by the cutting tool or the weight of the workpiece itself may cause it to be deflected out of line. Steady rests and follow rests (Figure 22-26) provide a means for supporting such

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CHAPTER 22

Turning and Boring Processes Faceplate

Dog

Steady rest

Follow rest (a)

(b)

(c)

FIGURE 22-26 Cutting a thread on a long, slender workpiece, using a follow rest (left) and a steady rest (right) on an engine lathe. Note the use of a dog and faceplate to drive the workpiece. ((b) Courtesy of South Bend Lathe)

work between the headstock and the tailstock. The steady rest is clamped to the lathe ways and has three movable fingers that are adjusted to contact the work and align it. A light cut should be taken before adjusting the fingers to provide a smooth contactsurface area. A steady rest can also be used in place of the tailstock as a means of supporting the end of long pieces, pieces having too large an internal hole to permit using a regular dead center, or work where the end must be open for boring. In such cases, the headstock end of the work must be held in a chuck to prevent longitudinal movement. Tool feed should be toward the headstock. The follow rest is bolted to the lathe carriage. It has two contact fingers that are adjusted to bear against the workpiece, opposite the cutting tool, in order to prevent the work from being deflected away from the cutting tool by the cutting forces.

& KEY WORDS apron bed boring carriage carriage assembly chucks collets compound rest cross slide cutoff cutting tools deflection depth of cut (DOC)

dog plate drilling engine lathe faceplates facing feed rod follow rest form turning headstock knurling lathes lathe centers lathe dog

leadscrews mandrels metal removal rate (MRR) milling multiple-spindle screw machine parting pilot arm quick-change gearbox quill reaming single-spindle screw machine

spindle split nut steady rest swing tailstock taper turning threading turning turret lathe ways workholding

& REVIEW QUESTIONS 1. How is the tool–work relationship in turning different from that in facing? 2. What different kinds of surfaces can be produced by turning versus facing? 3. How does form turning differ from ordinary turning? 4. What is the basic difference between facing and a cutoff operation? 5. Which machining operations shown in Figure 22-3 do not form a chip?

6. Why is it difficult to make heavy cuts if a form turning tool is complex in shape? 7. Show how equation 22-6 is an approximate equation. 8. Why is the spindle of the lathe hollow? 9. What function does a lathe carriage have? 10. Why is feed specified for a boring operation typically less than that specified for turning if the MRR equations are the same? 11. What function is provided by the leadscrew on a lathe that is not provided by the feed rod?

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Problems 12. How can work be held and supported in a lathe? 13. How is a workpiece that is mounted between centers on a lathe driven (rotated)? 14. What will happen to the workpiece when turned, if held between centers, and the centers are not exactly in line? 15. Why is it not advisable to hold hot-rolled steel stock in a collet? 16. How does a steady rest differ from a follow rest? 17. What are the advantages and disadvantages of a four-jaw independent chuck versus a three-jaw chuck? 18. Why should the distance the cutting tool overhands from the tool holder be minimized? 19. What is the difference between a ram- and a saddle-turret lathe? 20. How can a tapered part be turned on a lathe? 21. Why might it be desirable to use a heavy depth of cut and a light feed at a given speed in turning rather than the opposite? 22. If the rpm for a facing cut (assuming given work and tool materials) is being held constant, what is happening during the cut to the speed? To the feed? 23. Why is it usually necessary to take relatively light feeds and depths of cut when boring on a lathe? 24. How does the corner radius of the tool influence the surface roughness?

635

25. What effect does a BUE have on the diameter of the workpiece in turning? 26. How does the multiple-spindle screw machine differ from the single-spindle machine? 27. Why does boring ensure concentricity between the hole axis and the axis of rotation of the workpiece (for boring tool), whereas drilling does not? 28. Why are vertical spindle machines better suited for machining large workpieces than horizontal lathes? 29. In which figures in this chapter is a workpiece being held in a three-jaw chuck? 30. How is the workpiece in Figure 22-13 being held? 31. In which figures in this chapter is a dead center shown? 32. In which figures in this chapter is a live center shown? 33. In which figures in this chapter showing setups do you find the following being used as a workholding device? a. Three-jaw chuck b. Collet c. Faceplate d. Four-jaw chuck 34. How many form tools are being utilized in the process shown in Figure 22-14 to machine the part? 35. From the information given in Figure 22-14, start with a piece of round bar stock and show how it progresses, operation by operation, into a finished part—a threaded shaft.

& PROBLEMS 1. Select the speed, feed, and depth of cut for turning wrought, low-carbon steel (hardness of 200 BHN) on a lathe with AISI tool material of HSS M2 or M3. (Hint: Refer back to Chapter 20 for recommended parameters.) 2. Calculate the rpm (Ns) to run the spindle on a lathe to generate a cutting speed of 80 sfpm if the outside diameter of the workpiece is 8 in. 3. The lathe has rpm settings of 20, 30, and 35 that are near the calculated value. What rpm would you pick, and what will actual cutting speed be? 4. Calculate the cutting time if the length of cut is 24 in., the feed rate is 0.030 ipr, and the cutting speed is 80 fpm. The allowance is 0.5 in and the diameter is 8 in. 5. Calculate the metal removal rate for machining at a speed of 80 fpm, feed of 0.030 ipr, at a depth of 0.625 in. Use any data from Problem 4 that you need. 6. If the cutting force is 11,135 lb, calculate the horsepower that a process operating at a speed of 80 fpm is going to use. 7. Explain how you would estimate the cutting force for a turning operation if you do not have a dynamometer. 8. Determine the speed, feed, and depth of cut when boring wrought, low-carbon steel (hardness of 120 BHN) on a lathe with AISI tool material of HSS M2 or M3. 9. At a speed of 90 fpm, feed of 0.030 ipr, and depth at 0.625, calculate the input parameters for a lathe if the diameter to be machined (bored) is 6 in. Find the rpm (Ns) to run the spindle to generate the cutting speed. 10. Calculate the cutting time for a 4-in. length of cut, given that the feed rate is 0.030 ipr at a speed of 90 fpm. 11. For a boring operation at V ¼ 90 sfpm, fr ¼ 0.030 ipr, and d ¼ 0.625 in., calculate the MMR.

12. Calculate the horsepower that a process is going to use if the cutting force is 563 lb and the speed is 90 fpm. 13. Explain how you would estimate the cutting force for this boring operation if you do not have a dynamometer. 14. A cutting speed of 100 sfpm has been selected for a turning cut. The workpiece is 8 in. (203.2 mm) long and a feed of 0.020 in. (0.51 mm) per revolution is used. Using cutting speeds and feeds, calculate the machining time to cut off the bar. 15. The following data apply for machining a part on a turret lathe and on an engine lathe:

Times, in minutes, to machine part Cost of special tooling Time to set up the machine Labor rates Machine rates (overhead)

Engine Lathe

Turret Lathe

30 min 0 30 min $8/hr $10/hr

5 min $300 3 hr $8/hr $12/hr

One of the jobs for the engineer is to estimate the run time for a sequence of machining processes. For example: a. How many pieces would have to be made for the cost of the engine lathe to just equal the cost of the turret lathe? This is the BEQ. b. What is the cost per unit at the BEQ? 16. A finish cut for a length of 10 in. on a diameter of 3 in. is to be taken in 1020 steel with a speed of 100 fpm and a feed of 0.005 ipr. What is the machining time? 17. A workpiece 10 in. in diameter is to be faced down to a diameter of 2 in. on the right end. The CNC lathe (see Chapter 39)

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controls the spindle speed and maintains the cutting speed at 100 fpm throughout the cut by changing the rpm. What should be the time for the cut? Now suppose the spindle rpm for the workpiece is set to give a speed of 100 fpm for the 10-in. diameter and is not changed during the cut. What is the machining time for the cut now? The feed rate is 0.005 ipr. 18. A hole 89 mm in diameter is to be drilled and bored through a piece of 1340 steel that is 200 mm long, using a horizontal boring, drilling, and milling machine. High-speed tools will be used. The sequence of operations will be center drilling;

Chapter 22

drilling with an 18-mm drill followed by a 76-mm drill; then boring to size in one cut, using a feed of 0.50 mm/rev. Drilling feeds will be 0.25 mm/rev for the smaller drill and 0.64 mm/ rev for the larger drill. The center drilling operation requires 0.5 min. To set or change any given tool and set the proper machine speed and feed requires 1 min. Select the initial cutting speeds, and compute the total time required for doing the job. (Neglect setup time for the fixture.) This is often referred to as the run time or the cycle time. (Hint: Check Chapter 20 for recommended speeds for turning.)

CASE STUDY

Estimating the Machining Time for Turning

A

s the plant manufacturing engineer at BRC Inc., Jay Strom has been called into the production department to provide an expert opinion on a machining problem. Unfortunately, the only tool or instrument available at the time is a 1-in. micrometer. Katrin Zachary, the production manager would like to know the minimum time required to machine a large forging. The 8-ft-long forging is to be turned down from an original diameter of 10 in. to a final diameter of 6 in. The forging has a BHN of 300 to 400. The turning is to be performed on a heavy-duty lathe, which is equipped with a 50 hp motor and a continuously variable speed drive on the spindle. The work will be held between centers, and the overall efficiency of the lathe has been determined to be 75%. (See Chapter 21.) The forging (or log) is made from medium-carbon, 4345 alloy steel. The steel manufacturer, some basic experimentation, and established knowledge of the product and its manufacture have provided the following information: 1. A tool-life equation developed for the most suitable type of tool material at a feed of 0.020 ipr and a rake angle of a ¼ 10 degrees. The equation VTn ¼ C generally fits the data, with V ¼ cutting speed and

T ¼ the time in minutes to tool failure. Two test cuts were run, one at V ¼ 60 sfpm, where T ¼ 100 min, and another at V ¼ 85 sfpm, where T ¼ 10 min. 2. According to the vendor, the dynamic shear strength of the material is on the order of 125,000 psi. 3. Jay decides to make two test cuts at the standard feed of 0.020 ipr. He assumes that the chip thickness ratio varies almost linearly between the speeds of 20 and 80 fpm, the values being 0.4 at the speed of 20 fpm and 0.6 at 80 fpm. The chip thickness values were determined by micrometer measurement in order to determine the value of rc. 4. The machined forging (log) will be used as a roller in a newspaper press and must be precisely machined. If the log deflects during the cutting more than 0.005 in., the roll will end up barrel-shaped after final grinding and polishing. How should Jay proceed to estimate the minimum time required to machine this forging, assuming that one finishing pass will be needed when the log has been reduced to 6 in. in diameter? The deflection due to cutting forces must be kept below 0.005 in. at the mid-log location. You can assume that Fc 0.5 ¼ Ff and Ff 0.5 ¼ FR and that FR causes the deflection.

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CHAPTER 23 DRILLING AND RELATED HOLE-MAKING PROCESSES 23.1 INTRODUCTION 23.2 FUNDAMENTALS OF THE DRILLING PROCESS 23.3 TYPES OF DRILLS Depth-to-Diameter Ratio Microdrilling

23.4 23.5 23.6 23.7

23.8 COUNTERBORING, COUNTERSINKING, AND SPOT FACING 23.9 REAMING Reaming Practice Case Study: Bolt-down Leg on a Casting

TOOL HOLDERS FOR DRILLS WORKHOLDING FOR DRILLING MACHINE TOOLS FOR DRILLING CUTTING FLUIDS FOR DRILLING

& 23.1 INTRODUCTION In manufacturing it is probable that more holes are produced than any other shape, and a large proportion of these are made by drilling. Of all the machining processes performed, drilling makes up about 25%. Consequently, drilling is a very important process. Although drilling appears to be a relatively simple process, it is really a complex process. Most drilling is done with a cutting tool called a twist drill that has two cutting edges, or lips, as shown in Figure 23-1. The twist drill has the most the most common drill geometry. The cutting edges are at the end of a relatively flexible tool. Cutting action takes place inside the workpiece. The only exit for the chips is the hole that is mostly filled by the drill. Friction between the margin and the hole wall produces heat that is additional to that due to chip formation. The counterflow of the chips in the flutes

Body

Shank length

Flute length Flute

(a)

Heel

Helix angle

Land Cutting diameter

Drill axis (b) Body clearance Lead of helix

Tang Overall length

(c)

(d)

Margin

Body diameter clearance Heel

Web or core thickness

FIGURE 23-1 Nomenclature and geometry of conventional twist drill. Shank style depends on the method used to hold the drill. Tangs or notches prevent slippage. (a) Straight shank with tang; (b) tapered shank with tang; (c) straight shank with whistle notch; (d) straight shank with flat notch.

Point

Diameter

C23

Point angle

Lip length

Land Outer corner

Lip

Outer corner

Nominal relief angle

Chisel edge corner

637

C23

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makes lubrication and cooling difficult. There are four major actions taking place at the point of a drill: 1. A small hole is formed by the web—chips are not cut here in the normal sense. 2. Chips are formed by the rotating lips. 3. Chips are removed from the hole by the screw action of the helical flutes. 4. The drill is guided by lands or margins that rub against the walls of the hole. In recent years, new drill-point geometries and TiN coatings have resulted in improved hole accuracy, longer life, self-centering action, and increased feed-rate capabilities. However, the great majority of drills manufactured are twist drills. One estimate has U.S. manufacturing companies consuming 250 million twist drills per year. When high-speed steel (HSS) drills wear out, the drill can be reground to restore its original geometry. However, if regrinding is not done properly, the original drill geometry may be lost, and so will drill accuracy and precision. Drill performance also depends on the machine tool used for drilling, the workholding device, the drill holder, and the surface of the workpiece. Poor surface conditions (sand pockets and/or chilled hard spots on castings, or hard oxide scale on hot-rolled metal) can accelerate early tool failure and degrade the hole-drilling process.

& 23.2 FUNDAMENTALS OF THE DRILLING PROCESS The process of drilling creates two chips. A conventional two-flute drill, with drill of diameter D, has two principal cutting edges rotating at an rpm rate of N and feeding axially. Using a table like Table 23-1, the starting cutting speeds and feed rates are obtained, depending on the type of drill, the tool material and the material being machines. The rpm of the drill is established by the selected cutting speed Ns ¼

12V pD

ð23-1Þ

where V is in surface feet per minute and D is in inches (mm), using equation (23-1). This equation assumes that V is the cutting speed at the outer corner of the cutting lip TABLE 23-1

Recommended Speeds and Feed Rates for HSS Twist Drills

Speeds for hss twist drills Material

Feeds for hss twist drills

Speed, sfm

Material

Speed

Diameter, in.

Feed, ipr

0.001–0.002

Aluminum alloys

200–300

Brass, bronze

150–300

heat treated to 35–40 Rc

30–40

Under 18

High-tensile bronze Zinc-base diecastings

70–150 300–400

heat treated to 40–45 Rc heat treated to 45–50 Rc

25–35 15–25

heat treated to 50–55 Rc

7–15

1 8 1 4 1 2

7–20

Maraging steel, heat treated

7–20

1 and over

High-temp alloys, solutiontreated & aged Cast iron, soft

High-tensile steel,

75–125

annealed

medium hard

50–100

Stainless steel,

hard chilled

10–20

free machining

malleable

80–90

Cr-Ni, nonhardenable

Magnesium alloys Monel or high-Ni steel

250–400 30–50

Bakelite and similar

100–300

Steel, 0.2%–0.3% C

80–100

0.4%–0.5% C

70–80

tool, 1.2% C

50–60

forgings

40–50

alloy, 300–400 Bhn

20–30

Source: Cleveland Twist Drill

Straight-Cr, martensitic Titanium, commercially pure

40–55 30–100 20–60 10–30 50–60

6Al-4V, annealed

25–35

6Al-4V, solution-trt & aged

15–20

Wood

300–400

14 12 1

0.002–0.004 0.004–0.007 0.007–0.015 0.015–0.025

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Fundamentals of the Drilling Process

639

Web Chisel edge Land

Margin

Lip Chisel edge angle Chisel edge

α

Tool α t Work

Section A-A X Chip

D

Thrust force

Outer corner

N

α = Rake angle D Outer corner

FIGURE 23-2 Conventional drill geometry viewed from the point showing how the rake angle varies from the chisel edge to the outer corner along the lip. The thrust force increases as the web is approached.

A WorkL piece

fr

A D 2

(point X in Figure 23-2). The velocity is very small near the center of the chisel end of the drill. The feed, fr, is given in inches per revolution (ipr). The depth of cut in drilling is equal to half the feed rate, or t ¼ fr/2 (see section A–A in Figure 23-2). The feed rate in inches per minute (ipm), fm, is frNs. The length of cut in drilling equals the depth of the hole, L, plus an allowance for approach and for the tip of drill, usually A ¼ D/2. Table 23-1 gives some typical values for speed and feed rates and for carbide indexable insert drills, a type of drill shown later in the chapter. For drilling, cutting time is given in equation 23-2: Tm ¼

ðL þ AÞ L þ A ¼ fm f r Ns

ð23-2Þ

The metal removal rate is MRR ¼

Volume Tm

pD2 L=4 ¼ ðomitting allowancesÞ L=f r N s

ð23-3Þ

which reduces to MRR ¼ ðpD2 =4Þf r N s in:3

ð23-4Þ

Substituting for N with equation 23-1, we obtain an approximate form MRR ffi 3DVf r

ð23-5Þ

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EXAMPLE OF DRILLING A cast iron plate is 2 in. thick and needs 4-in.-diameter holes drilled in it. An indexableinsert drill has been selected. Looking at Table 23-1, we select a cutting speed of 200 fpm and a feed of 0.005 ipr. Spindle rpm ¼ 12V=pD ¼ 12 200=3:14 1 ¼ 764 rpm What if the machine does not have this specific rpm? Pick the closest value: say, 750 rpm: Penetration rate or feed rateðin:=minÞ ¼ FeedðiprÞ rpm ¼ 0:005 750 ¼ 3:75 in:=min Maximum chip load ¼ feedðiprÞ=2 ¼ 0:005=2 ¼ 0:0025 in:=rev What if the machine does not have the specific feed rate? Pick the next lowest value as a starting value, say, 3.5 in./min: Material removal rate ðin:3 =minÞ ¼ ðp=4Þ ðDÞ2 FeedðiprÞ rpm ¼ ðp=4Þ 12 Feed rate ¼ 3:14=4 12 3:50 ¼ 2:75 in:3 =min The MRR can be used with the unit power for cast iron (see Chapter 20) to estimate the horsepower needed to drill the hole. Let HPs ¼ 0.33 for this cast iron: HP ¼ HPs MRR ¼ 0:33 2:75 ¼ 0:90 This value would typically represent 80% of the total motor horsepower (HP) needed, so in this case, a horsepower motor greater than 1.5 or 2 would be sufficient. In estimating the cost of a job, it is often necessary to determine the time to drill a hole: drill time=hole ¼

length drilled þ allowance feed rateðin:=minÞ þrapid traverse length of withdrawal rapid traverse rate þprorated downtime to change drills per hole

The last term prorated downtime is drill change downtime holes drilled per drill ðtool lifeÞ And the cost/hole is Drilling time=holes ðLabor þ Machine rateÞ þ Prorated cost of drill=hole

& 23.3 TYPES OF DRILLS The most common types of drills are twist drills. These have three basic parts: the body, the point, and the shank, shown in Figures 23-1 and 23-2. The body contains two or more spiral or helical grooves, called flutes, separated by lands. To reduce the friction between the drill and the hole, each land is reduced in diameter except at the leading edge, leaving a narrow margin of full diameter to aid in supporting and guiding the drill and thus aiding in obtaining an accurate hole. The lands terminate in the point, with the leading edge of each land forming a cutting edge. The flutes serve as channels through which the chips are withdrawn from the hole and coolant gets to the cutting edges. Although most drills have two flutes, some, as shown in Figure 23-3, have three, and some have only one. The principal rake angles behind the cutting edges are formed by the relation of the flute helix angle to the work. This means that the rake angle of a drill varies along

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SECTION 23.3

Types of Drills

641

FIGURE 23-3 Types of twist drills and shanks. Bottom to top: straight-shank, three-flute core drill; straight-shank; taper-shank; bit-shank; straight-shank, highhelix angle; straight-shank, straight-flute; taper-shank, subland drill. (Courtesy J T. Black)

the cutting edges (or lips), being negative close to the point and equal to the helix angle out at the lip. Because the helix angle is built into the twist drill, the primary rake angle cannot be changed by normal grinding. The helix angle of most drills is 24 degrees, but drills with larger helix angles—often more than 30 degrees—are used for materials that can be drilled very rapidly, resulting in a large volume of chips. Helix angles ranging from 0 to 20 degrees are used for soft materials, such as plastics and copper. Straightflute drills (zero helix and rake angles) are also used for drilling thin sheets of soft materials. It is possible to change the rake angle adjacent to the cutting edge by a special grinding procedure called dubbing. The cone-shaped point on a drill contains the cutting edges and the various clearance angles. This cone angle affects the direction of flow of the chips across the tool face and into the flute. The 118-degree cone angle that is used most often has been found to provide good cutting conditions and reasonable tool life when drilling mild steel, thus making it suitable for much general-purpose drilling. Smaller cone angles—from 90 to 118 degrees—are sometimes used for drilling more brittle materials, such as gray cast iron and magnesium alloys. Cone angles from 118 to 135 degrees are often used for the more ductile materials, such as aluminum alloys. Cone angles less than 90 degrees are frequently used for drilling plastics. Many methods of grinding drills have been developed that produce point angles other than 118 degrees. The drill produces a thrust force, T, and a torque, M. Drill torque increases with feed (in./rev) and drill diameter, while the thrust force is influenced greatly by the web or chisel end design, as shown in Figure 23-4. The relatively thin web between the flutes forms a metal column or backbone. If a plain conical point is ground on the drill, the intersection of the web and the cone

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130° S-form chisel 3375

S-point l

na

t en

2250

nv

Co

He lic al

io

Thrust force (lb)

C23

t

ce

r-fa

u Fo

al

ved

ie Rel

c heli

1125

0 .008

.015

.020

.027

Helical (S-shape chisel point) Can eliminate center drilling on NC machining centers Excellent hole geometry Close relationship between drill size and hole size Increased tool life Lower thrust requirements Leaves burr on breakthrough

Feed (ipr)

Secondary angle 30° – 40° (true) Primary angle 4° – 8° (true)

4-facial point

Relieved helical Reduces thrust force Eliminates chisel end Equal, rake angle

FIGURE 23-4 As the drill advances, it produces a thrust force. Variations in the drill-point geometry are aimed at reducing the thrust force.

Bickford Combination of helical and Racon point features Self-centering and reduced burrs Excellent hole geometry Increased tool life

Four-facet Good self-centering ability Breaks up chips for deep-hole drilling Can be generated in a single grinding operation: reduces thrust. Eliminates center drilling in NC

Racon (radiused conventional point) Increased feed rates Increased tool life (8–10 times in C.I.) Reduced burrs at breakthrough Not self-centering

produces a straight-line chisel end, which can be seen in the end view of Figure 23-2.The chisel point, which also must act as a cutting edge, forms a 56-degree negative rake angle with the conical surface. Such a large negative rake angle does not cut efficiently, causing excessive deformation of the metal. This results in high thrust forces and excessive heat being developed at the point. In addition, the cutting speed at the drill center is low, approaching zero. As a consequence, drill failure on a standard drill occurs both at the center, where the cutting speed is lowest, and at the outer tips of the cutting edges, where the speed is highest. When the rotating, straight-line chisel point comes in contact with the workpiece, it has a tendency to slide or ‘‘walk’’ along the surface, thus moving the drill away from the desired location. The conventional point drill, when used on machining centers or high-speed automatics, will require additional supporting operations like center drilling, burr removal, and tool change—all of which increase total production time and reduce productivity. Many special methods of grinding drill points have been developed to eliminate or minimize the difficulties caused by the chisel point and to obtain better cutting action and tool life (see Figure 23-4 for some examples). The center core or slot-point drill shown in Figure 23-5 has twin carbide tips brazed on a steel shank and a hole (or slot) in the center. The work material in the slot is not machined but, rather, fractured away. The center core drill has a self-centering action and greatly relieves the thrust force produced by the chisel edge of conventional twist drills. This drill operates at about 30 to 50% less thrust than that of conventional drills. All rake angles of the cutting edge are positive, which further reduces the cutting force.

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SECTION 23.3

Types of Drills

643

Body

Carbide tip 20° Convex rake face Noncutting zone

Twisted flute Margin

Conventional drill with large thrust force at web. Thrust force

Center core drill or slot point drill with greatly reduced thrust Center core removed by ductile fracture (tension)

Slot

Thrust force

FIGURE 23-5 Center core drills can greatly reduce the thrust force.

The conventional point also has a tendency to produce a burr on the exit side of a hole. Some type of chip breaker is often incorporated into drills. One procedure is to grind a small groove in the rake face, parallel with and a short distance back from the cutting edge. Drills with a special chip-breaker rib as an integral part of the flute are available. The rib interrupts the flow of the chip, causing it to break into short lengths. The split-point drill is a form of web thinning to shorten the chisel edge. This design reduces thrust and allows for higher feed rates. Web thinning uses a narrow grinding wheel to remove a portion of the web near the point of the drill. Such methods have had varying degrees of success, and they require special drill-grinding equipment. Also shown in Figure 23-4 is a four-facet self-centering point that works well in tougher materials. The facets refer to the number of edges on the clearance surfaces exposed to the cutting action. The self-centering drill lasts longer and saves machining time on numerical control (NC) centers as they can eliminate the need for center drills. A common aspect in drill-point terminology is total indicator runout (TIR). This is a measure of the cutting lips’ relative side-to-side accuracy. The original drill point produced by the manufacturer lasts only until the first regrind; thereafter, performance and life depend on the quality of regrind. Proper regrinding (reconditioning) of a drill is a complex and important operation. If satisfactory cutting and hole size are to be achieved, it is essential that the point angle, lip clearance, lip length, and web thinning be correct. As illustrated in Figure 23-6, incorrect sharpening often results in unbalanced cutting forces at the tip, causing misalignment and oversized holes. Drills, even small drills, should always be machine ground, never hand ground. Drill grinders, often computer controlled, should be used to ensure exact reproduction of the geometry established by the manufacturer of the drill. This is extremely important when drills are used on mass-production or numerically controlled machines. Companies invest huge sums in NC machining centers but overlook the value of a top-quality drillgrinding machine. Drill shanks are made in several types. The two most common types are the straight and the taper. Straight-shank drills are usually used for sizes up to 38-in. diameter and must be held in some type of drill chuck. Taper shanks are available on larger drills and are common on drills above 1 in. Morse tapers are used on taper-shank drills, ranging from a number 1 taper to a number 6. Taper-shank drills are held in a female taper in the end of the machine tool spindle. If the taper on the drill is different from the spindle taper, adapter sleeves are available. The taper assures the drill’s being accurately centered in the spindle. The tang at

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CHAPTER 23

Drilling and Related Hole-Making Processes FIGURE 23-6 Typical causes of drilling problems.

Outer corners break down: Cutting speed too high; hard spots in material; no cutting compound at drill point; flutes clogged with chips Cutting lips chip: Too much feed; lip relief too great Checks or cracks in cutting lips: Overheated or too quickly cooled while sharpening or drilling Chipped margin: Oversize jig bushing Drill breaks: Point improperly ground; feed too heavy; spring or backlash in drill press, fixture, or work; drill is dull; flutes clogged with chips Tang breaks: Imperfect fit between taper shank and socket caused by dirt or chips or by burred or badly worn sockets Drill breaks when drilling brass or wood: Wrong type drill; flutes clogged with chips Drill spilts up center: Lip relief too small; too much feed Drill will not enter work: Drill is dull; web too heavy; lip relief too small Hole rough: Point improperly ground or dull; no cutting compounds at drill point; improper cutting compound; feed too great; fixture not rigid Hole oversize: Unequal angle of the cutting edges; unequal length of the cutting edges; see part (a) Chip shape changes while drilling: Dull drill or cutting lips chipped

(a) Angle unequal

(b) Length unequal

Large chip coming from one flute, small chip from the other: Point improperly ground, one lip doing all the cutting

the end of the taper shank fits loosely in a slot at the end of the tapered hole in the spindle. The drill may be loosened for removal by driving a metal wedge, called a drift, through a hole in the side of the spindle and against the end of the tang. It also acts as a safety device to prevent the drill from rotating in the spindle hole under heavy loads. However, if the tapers on the drill and in the spindle are proper, no slipping should occur. The driving force to the drill is carried by the friction between the two tapered members. Standard drills are available in four size series, the size indicating the diameter of the drill body: Millimeter series: 0.01- to 0.50-mm increments, according to size, in diameters from 0.015 mm. Numerical series: no. 80 to no. 1 (0.0135 to 0.228 in.). Lettered series: A to Z (0.234 to 0.413 in.). Factional series: to 4 in. (and over) by 64ths. TiN coating of conventional drills greatly improves drilling performance. The increase in tool life of TiN-coated drills over uncoated drills in machining steel is more than 200 to 1000%.

DEPTH-TO-DIAMETER RATIO The depth of the hole to be drilled divided by the diameter of the drill is the depth-todiameter ratio. Most machinists consider a ratio of 3:1 to be deep-hole drilling, after which hole accuracy (location) drilling speed and tool life will be reduced. The bores of rifle barrels were once drilled using conventional drills. Today, deep-hole drills, or gundrills, are used when deep holes are to be drilled. The oldest of these deep-hole techniques is gundrilling. The original gundrills were very likely half-round drills, drilled axially with a coolant hole to deliver cutting fluids to the cutting edge (see Figure 23-7). Modern gundrills typically consist of an alloy-steel-tubing shank with a solid carbide or carbide-edged tip brazed or mechanically fixed to it. Guide pads following the cutting edge by about 90 to 180 degrees are also standard. The gundrill is a single-lipped tool, and its major feature is the delivery of coolant through the tool at extremely high pressures—typically from 300 to 1800 psi, depending on diameter—to force chips back down the flute. Successful application of a gundrill

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SECTION 23.3 Gundrill Tip

Straight clearance Secondary relief

645

Driver

Shank

Chamfer Fluid hole

Types of Drills

Inner cutting angle

Flute

Dia.

Cutting face Outer cutting angle

0.015 – 0.031 Shoulder Cutting face

FIGURE 23-7 The gundrill geometry is very different from that of conventional drills.

Outer land (margin)

Primary relief

depends almost entirely on the formation of small chips that can be effectively evacuated by the flow of cutting fluids. Standard gundrills are made in diameters from 0.0078 in. (2 mm) to 2 in. or more. Depth-to-diameter ratios of 100:1 or more are possible. In gundrilling tolerances for diameters of drilled holes under 1 in. can be held to 0.0005-in. total tolerance, and, should not exceed 0.001 in. over all. According to one source, ‘‘roundness accuracies of 0.00008 inch can be attained.’’ Because of the burnishing effect of the guide pads, excellent surface finishes can be produced. Hole straightness is affected by a number of variables, such as diameter, depth, uniformity of workpiece material, condition of the machine, sharpness of the gundrill, feeds and speeds used, and the specific technique used (rotation of the tool, of the work, or both), but deviation should not exceed about 0.002 in. TIR in a 4-in. depth at any diameter, and it can be held to 0.002 in./ft. Basic setup for a gundrilling operation, which is generally horizontal, requires a drill bushing very close to the work entry surface and may involve rotating the work or the tool, or both. Best concentricity and straightness are achieved by the work and the tool rotating in opposite directions. Other deep-hole drills are called BTA (Boring Trepanning Association) drills and ejector drills. A deep hole is one in which the length (or depth) of the hole is three or more times the diameter. Coolants can be fed internally through these drills to the cutting edges. See Figure 23-8 for schematic of an ejector drill and the machine tool used for gundrilling. The coolants flush the chips out the flutes. The special design of these drills reduces the tendency of the drill to drift, thus producing a more accurately aligned hole. The typical BTA deep-hole drilling tools are designed for single-lip end cutting of a hole in a single pass. Solid deep-hole drills have alloy-steel shanks with a carbideedged tip that is fixed to it mechanically. The cutting edge cuts through the center on one side of the hole, leaving no area of material to be extruded. The cutting is done by

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CHAPTER 23

Drilling and Related Hole-Making Processes Supporting pad Trailing ramp

Leading ramp Radial cutting clearance

Top clearance

Circle land or margin

Measuring pad

Chip mouth Primary cutting edge II or middle edge

Chip-breaker drain or groove Primary cutting edge or outer edge

B A

Section A–A

Secondary cutting edge or inner edge Wear pad recess or lag Section B–B

Depth of cutting recess

line

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B A

Width of cutting recess Clearance angle Solid boring

(a)

Clearance angle Leading Rake angle chamfer negative

Trepanning (b)

Trailing chamfer

Counterboring cutting tool (c)

Chip passages

Chips and coolant

(d)

Coolant passages to cutting edges

Boring bar

Pressure head Steady rest Tool head

Chip traps (e)

Magnetic separator drum Magnetic filter

Gear pumps

FIGURE 23-8 BTA drills for (a) boring, (b) trepanning, (c) counterboring, (d) deep-hole drilling with ejector drill, (e) horizontal deep-hole-drilling machine. (S. Azad and S. Chandeashekar, Mechanical Engineering, September 1985, pp. 62, 63)

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SECTION 23.3

TABLE 23-2

Types of Drills

647

Drilling Processes Compared Twist Drill

Pivot (micro)

Spade (inserted blade)

IndexableInsert Drill 5 83 5 8

0.078–1

Diameter, in.

0.020–2

0.001–0.020

1–6

Typical range

0.0059

24

10

Gundrill

Trepanning

134

18

Depth/Diameter Ratio Min. practical

No min

No min

No min

40 (horiz)

2–3

100

100

50

100

Ultimate

>50

20

10 (vert)

200

>100

>50

>100

>100 (horz) a

Maximum depth/diameter ratios in this table are estimates of what can be achieved with special attention and under ideal conditions Equality of tolerances should not be assumed for the different process.

the outer and inner cutting angles, which meet at a point. Theoretically, the depth of the hole has no limit, but practically, it is restricted by the torsional rigidity of the shank. Gundrills have a single-lip cutting action. Bearing areas and lifting forces generated by the coolant pressure counteract the radial and tangential loads. The single-lip construction forces the edge to cut in a true circular pattern. The tip thus follows the direction of its own axis. The trepanning gundrill leaves a solid core. Hole straightness is affected by variables such as diameter, depth, uniformity of the workpiece material, condition of the machine, sharpness of cutting edges, feeds and speeds used, and whether the tool or the workpiece is rotated or counter-rotated. Two-flute drills are available that have holes extending throughout the length of each land to permit coolant to be supplied, under pressure, to the point adjacent to each cutting edge. These are helpful in providing cooling and also in promoting chip removal from the hole in drilling to moderate depths. They require special fittings through which the coolant can be supplied to the rotating drill, and they are used primarily on automatic and semiautomatic machines. See Table 23-2 for comparison of drilling processes. Larger holes in thin material may be made with a hole cutter (Figure 23-9), where the large hole is produced by the thin-walled, multiple-tooth cutter with saw teeth and the metal hole with a twist drill. Hole cutters are often called hole saws. When starting to drill a hole, a drill can deflect rather easily because of the ‘‘walking’’ action of the chisel point. Hole location accuracy is lost. Consequently, to ensure that a hole is started accurately, a center drill (Figure 23-10) is used prior to a regular Quick Change Arbor Alloy Steel Body

High Speed Steel Teeth High Speed Steel Pilot Drill 9 FIGURE 23-9 High Speed Edge Hole Saw can cut holes 16 in. to 6 in. in diameter in any machinable material up to 118 in. thick. They can be used in portable electric or drill presses.

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Step 1 Centering and countersinking with a combination center drill and countersink. (Courtesy of Chicago-Latrobe)

Step 2 Drilling with a standard twist drill.

Step 3 Truing hole by boring.

Step 4 Final sizing and finishing with a reamer.

FIGURE 23-10 To obtain a hole that is accurate as to size and aligned on center (located), this four-step sequence of operations is usual. (Courtesy J T. Black)

chisel-point twist drill. The center drill and countersink tool have a short, straight drill section extending beyond a 60-degree taper portion. The heavy, short body provides rigidity so that a hole can be started with little possibility of tool deflection. The hole should be drilled only partway up on the tapered section of countersink. The conical portion of the hole serves to guide the drill being used to make the main hole. Combination center drills are made in four sizes to provide a starting hole of proper size for any drill. If the drill is sufficiently large in diameter, or if it is sufficiently short, satisfactory accuracy may often be obtained without center drilling. Special drill holders are available that permit drills to be held with only a very short length protruding. Because of its flexibility and endpoint geometry, a drill may start or drift off centerline during drilling. The use of a center (start) drill will help to ensure that a drill will start drilling at the desired location. Nonhomogeneities in the workpiece and imperfect drill geometries may also cause the hole to be oversize or off-line. For accuracy, it is necessary to follow center drilling and drilling by boring and reaming. Boring corrects the hole alignment, and reaming brings the hole to accurate size and improves the surface finish. Special combination drills can drill two or more diameters, or drill and countersink and/or counterbore, in a single operation (Figure 23-11). Countersinking and counterboring usually follow drilling. These operations are described in more detail later in this chapter. A step drill has a single set of flutes and is ground to two or more diameters. Subland drills have a separate set of flutes on a single body for each diameter or operation; they provide better chip flow, and the cutting edges can be ground to give proper cutting conditions for each operation. Combination drills are expensive and may be difficult to regrind, but they can be economical for production-type operations if they reduce work handling, setups, or separate machines and operations. Spade drills (Figure 23-12) are widely used for making holes 1 in. or larger in diameter at low speeds or with high feeds (Table 23-3). The workpiece usually has an

Subland drill

FIGURE 23-11 Specialpurpose subland drill (above) and some of the operations possible with other combination drills (below).

Drill multiple Multiple drill Drill and Drill and diameters countersink countersink counterbore and counterbore

Drill and chamfer

Drill, countersink, and counterbore

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SECTION 23.3

649

Types of Drills

C land

7° A web

Regular spade drill 7°

B rad.

4 1/6 2+ –0

1/3

D

130°

FIGURE 23-12 (Top) Regular spade drill; (middle) spade drill with oil holes; (bottom) spade drill geometry, nomenclature.

0.004 back 0.005 taper

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Spade drill with oil holes 25° 3°

existing hole, but a spade drill can drill deep holes in solids or stacked materials. Spade drills are less expensive because the long supporting bar can be made of ordinary steel. The drill point can be ground with a minimum chisel point. The main body can be made more rigid because no flutes are required, and it can have a central hole through which a fluid can be circulated to aid in cooling and in chip removal. The cutting blade is easier to sharpen; only the blades need to be TiN-coated. TABLE 23-3

Recommended Surface Speeds and Feeds for High-Speed Steel Spade Drills for Various Materials

Material

Surface Speed (ft per min)

Mild machinery steel 0.2 and 0.3 carbon

65–100

Cast iron, medium hard

55–100

Steel, annealed 0.4 to 0.5 carbon Tool steel, 1.2 carbon

55–80 45–60

Cast iron, hard, chilled Malleable iron

25–40 79–90

Steel forging

35–50

Brass and bronze, ordinary

200–300

Alloy steel

45–70

Bronze, high tensile

70–150

Stainless steel, free machining

50–70

Monel metal

35–50

Stainless steel, hard

25–40

Aluminum and its alloys

200–300

Cast iron, soft

80–150

Magnesium and its alloys

250–400

Material

Speed

Feed Rates for Spade Drilling (inches per revolution)

Drill Size (inches)

Cast Iron Malleable Iron Brass Bronze

Medium Steel Stainless Steel Monel Metal Drop-Forged Alloys Tool Steel (annealed)

1 to 114 114 to 34

0.010–0.020 0.010–0.024

0.008–0.014 0.008–0.018

0.006–0.012 0.008–0.017

134 to 212

0.010–0.030

0.010–0.024

0.010–0.017

212 to 4

0.012–0.032

0.012–0.030

0.010–0.017

4 to 6

0.012–0.032

0.010–0.024

0.008–0.017

Source: Waukesha Cutting Tools, Inc.

Tough Steel Drop Forging Aluminum

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C L

Rear Entry Coolant Port

Flat (2)

Shank Side Entry Coolant Port Mounting Surface CL

Flange

Flute (2) Drive Flat (Flange) Center Coolant Hole

Body Diameter (Secondary) Regash

Body Diameter (Primary) Outboard Insert

FIGURE 23-13 Design of an indexable insert drill with 2 inserts, the most common style. (Indexable Drill Applications, T. Benjamin, Machining Technology, Vol. 8, No. 2, SME, 1997)

Lead Angle Inboard Insert (Center Cutting)

Web Cutting Diameter

Spade drills are often used to machine a shallow locating cone for a subsequent smaller drill and at the same time to provide a small bevel around the hole to facilitate later tapping or assembly operations. Such a bevel also frequently eliminates the need for deburring. This practice is particularly useful on mass-production and numerically controlled machines. Carbide-tipped drills and drills with indexable inserts are also available (see Figure 23-13) with one- and two-piece inserts for drilling shallow holes in solid workpieces. Indexable insert drills can produce a hole four times faster than a spade drill because they run at high speeds/low feeds and are really more of a boring operation than a drilling process. However, to use indexable drills, you must have an extremely rigid machine tool and setup, adequate horsepower, and lots of cutting fluid. Indexable drills are roughing tools generating hole tolerances of and surface finish of 250 rpm or greater. The tool is designed for the inboard insert to cut past the centerline of the tool so the inboard tool is positioned radially below the center. See Table 23-4 for an indexable drilling troubleshooting guide. A high-pressure, pulsating coolant system can generate pressures up to 300 psi and works well with indexable drilling. It can have disadvantages, however. High pressure with pulsating action can decrease chip control and cause drill deflection. A highpressure coolant stream can flatten chips at the point of forming and forces them into the cut, causing recutting, insert chipping, and poor hole finishes. The pressure can force chips between the drill body and hole diameter, wrapping them around the drill. Friction then will weld the chips to the tool body or hole. The diameter of the hole and the length/diameter ratio usually determine what kind of drill to use. Figure 23-14 explores how drill selection depends on the depth of the hole and the diameter of the drill: Section A shows the drilling areas of relatively shallow holes and small diameters. About half of all the drilling process falls within the

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TABLE 23-4

Types of Drills

651

Indexable Drilling Troubleshooting Guidea

Problem

Source

Solution

Insert chipping or breakageb

Off-center drill, caused by misalignment

Maintain proper alignment. Concentricity not to exceed 0.005 TIR.

Improper seating of tool in tool holder, spindle, or turret

Check tool shank and socket for nicks and dirt. Check parting line between tool shank and socket with feeler gage. Check to see if tool is locked tightly.

Deflection because of too much overhand and Jack of rigidity

With indicator, check if tool can be moved by hand. Check if tool can be held shorter.

Improper seating of inserts in pockets

Clean pockets whenever indexing or changing inserts. Check pockets for nicks and burrs. Check if inserts rest completely on pocket bottoms.

Grooving on back stroke; drill body rubbing hole wall; over- or undersize holes Poor hole surface finish

Damaged insert screws

Check head and thread for nicks and burns. Do not overtorque screws.

Improper speeds and feeds

Check recommended guidelines for given materials.

Insufficient coolant supply

Check coolant flow.

Improper carbide grade in inboard station

Recommend straight grade for multiple-insert drills.

Off-center drill

Maintain proper alignment and concentricity. Check bottom of hole or disk for center stub.

Deflection

Check setup rigidity. Check speed and feed guidelines.

Vibrations

Check setup and part rigidity. Check seat in spindle or tool holder.

Insufficient coolant pressure and volume

Check speeds and feeds. Increase coolant pressure and flow. Is coolant flow constant?

Recutting chips, causing drill to jump

Increase coolant flow. Add coolant grooves.

Poor chip control; chips trapped in hole

Mostly speed or feed.

Make sure coolant reaches inserts at all times.

Chatter

Mostly feed rate.

Very short, thick, flat chips

Feed rate too high in relation to cutting speed

Lower feed or increase speed.

Long and stringy chips

Feed rate too low in relation to cutting speed

Increase feed rate or decrease speed.

Unable to loosen insert locking screws

Seized threads, caused by coolant or heat

Use dimple inserts Apply water and heat-resistant lubricant to threads.

a b

Source: ‘‘Fundamentals of Indexable Drilling,’’ K.L. Anderson, Machining Technology, vol. 2, no. 3, 1991. If constant chipping occurs, especially on an inner insert, and conditions are optimum, try an uncoated-carbide insert or a grade with higher transverse rapture strength.

Sector

(50) 2⬙

Hole diameter D (in. [mm])

C23

C

D

A

B

A

Twist drill (HSS) Center core drill

B

Twist drill Gundrill BTA Ejector drill

C

Twist drill Indexable insert drill Spade drill Center core drill

D

BTA Ejector drill

1⬙

5 (8)16 ⬙

FIGURE 23-14

1

2 3 4 Hole depth to diameter (L/O)

Typical drill types

5

Drill selection depends on hole diameter and hole depth.

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Microdrill Note back taper

Dia.

Point angle, 118°–135°

FIGURE 23-15 Pivot microdrill for drilling very-small-diameter holes.

Clearance angle 8°–15°

Shank

category of this section. It is the section for which the majority of the work is done by twist drills and a very few cemented carbide drills. Section B is the drilling of deep holes for which cemented carbide gundrills are used. Section C is that of shallow holes having large diameters, for which spade drills are used. Section D is that of deep holes having large diameters, for which BTA tools are used.

MICRODRILLING As the term suggests, microdrilling involves very-small-diameter cutting tools, including drills, end mills, routers, and other special tools. Drills from 0.002 in. (0.05 mm) and mills to 0.005 in. in diameter are used to produce geometries involving dimensions at which many workpiece materials no longer exhibit uniformity and homogeneity. Grain borders, inclusions, alloy or carbide segregates, and microscopic voids are problems in microdrilling, where holes of 0.02 to 0.0001 in. have been drilled using pivot drills, as shown in Figure 23-15. Pivot drills are two-lipped (two-fluted), end-cutting tools of relatively simple geometry. Web thickness tapers toward the point, and a generous back-taper is incorporated. For softer workpiece materials, point angles are typically 118 degrees and lip clearance is 15 degrees. For steels and general use in harder metals, 135-degree points and 8-degree clearance are recommended. The chisel edge is similar to that of a twist drill. Pivot drills are made of tungsten-alloy tool steel in standard sizes from 0.0001 to 0.125 in. and of sintered tungsten carbide from 0.001 to 0.125 in. Small drills easily deflect, and getting accurate and precise holes requires a machine with a high-quality spindle and very sensitive feeding pressure. In the medical components field, much of this machining work is performed on computer numerical control (CNC) Swiss-type turning machines. Speeds and feeds are greatly reduced with frequent pecking to clear the chips. Use a light, lard-based, sulfurized cutting oil.

& 23.4 TOOL HOLDERS FOR DRILLS Straight-shank drills must be held in some type of drill chuck (Figure 23-16). Chucks are adjustable over a considerable size range and have radial steel fingers. When the chuck is tightened by means of a chuck key, these fingers are forced inward against the drill. On smaller drill presses, the chuck often is permanently attached to the machine spindle, whereas on larger drilling machines the chucks have a tapered shank that fits into the female Morse taper of the machine spindle. Special types of chucks in semiautomatic or fully automatic machines permit quite a wide range of sizes of drills to be held in a single chuck. Chucks using chuck keys require that the machine spindle be stopped in order to change a drill. To reduce the downtime when drills must be changed frequently, quickchange chucks are used. Each drill is fastened in a simple round collet that can be inserted into the chuck hole while it is turning by merely raising and lowering a ring on the chuck body. With the use of this type of chuck, center drills, drills, counterbores,

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Synthetic rubber support for jaws

Collet chuck

3 jaw Jacobs chuck

653

Tool Holders for Drills

Chuck key

(a)

(b)

FIGURE 23-16 Two of the most commonly used types of drill chucks are the three-jaw Jacobs chuck (above) and the collet chuck with synthetic rubber support for jaws. (Image provided by Jacobs Chuck, Apex Tool Group, Sparks, MD)

reamers, and so on, can be manually changed in quick succession. For carbide drills, collet-type holders with thrust bearings are recommended (Figure 23-17). For drills using an internal coolant supply, a very rigid chuck with either an inducer or throughspindle coolant source is recommended. Conventional holders such as keyless chucks cannot be used because the gripping strength is limited. Collet holders should be cleaned periodically with oil to remove small chips. The entire flute length must protrude from the chuck. At maximum hole depth, the length of flute protruding from the hole must be at least one to one-and-a-half times the drill diameter. Radial runout at the drill tip must not exceed 0.001 in. Correct chucking with spring collet.

Chips cannot be removed if the flute is chucked.

Good

Bad

The drill chuck rigidity is important. Thicker shanks can offer higher rigidity.

Slip

Rotate by hand one rev.

The runout of the drill when held in the chuck should be less than 0.001 in. Total indicator runout (TIR)

Dimension A should be 1 to 1.5 times drill diameter (D).

The drill point should be within 0.004 in. maximum of the center of the workpiece when the work is rotating.

A Within 0.004 in. maximum

Dial indicator is within 0.001 in. D

FIGURE 23-17

Here are some suggestions for correct chucking of carbide drills.

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& 23.5 WORKHOLDING FOR DRILLING Work that is to be drilled is ordinarily held in a vise or in specially designed workholders called jigs. Workholding devices are the subject of Chapter 27, where the design of workholding devices is discussed. Many examples of drill jigs are shown. With regard to safety, the work should not be held on the table by hand unless adequate leverage is available, even in light drilling operations. This is a dangerous practice and can lead to serious accidents, because the drill has a tendency to catch on the workpiece and cause it to rotate, especially when the drill exits the workpiece. Work that is too large to be held in a jig can be clamped directly to the machine table using suitable bolts and clamps and the slots or holes in the table. Jigs and workholding devices on indexing machines must be free from play and firmly seated.

& 23.6 MACHINE TOOLS FOR DRILLING The basic work and tool motions required for drilling—relative rotation between the workpiece and the tool, with relative longitudinal feeding—occur in a wide variety of machine tools. Thus, drilling can be done on a variety of machine tools such as lathes, horizontal and vertical milling machines, boring machines, and machining centers. This section will focus on those machines that are designed, constructed, and used primarily for drilling. First, the machine tools must have sufficient power (torque) and thrust to perform the cut. It is the task of the engineer to select the correct machine or select the cutting parameters (speed and feed) based on the type and size of the drill, drill material, and the work material (hardness). Because of the complex geometry of the drill, empirical equations are widely used. Figure 23-18 shows the type of information provided by cutting tool manufacturers to calculate (estimate) thrust in drilling. Specific cutting force values Ks are given in Figure 23-18, while empirical constants X and Y are obtained from cutting tool manufacturers. Much of these data have been developed for highspeed-steel tools. When using solid carbide tools, rigid machines such as machining centers or NC turning machines are recommended, whereas a radial drilling machine is not recommended (not rigid enough).

Feed (ipr) Material

PlainCarbon Steel FreeMachining Steels Alloy Steels

Brinell Hardness

E C

0.004 0.35 3.3

0.005 0.39 3.5

0.006 0.47 3.8

0.008 0.60 4.4

0.010 0.70 4.4

0.012 0.80 4.4

0.016 0.90 4.0

0.020 1.08 3.6

140–220

444230 435510 431150 426790 418070 409350 391910 374460

220–300

493590 483900 479060 474210 464520 454830 435450 416070

120–180

296150 290340 287440 284530 278710 272900 261270 249640

180–260

345510 338730 335340 331950 325160 318380 304820 291250

260–340

493590 483900 479060 474210 464520 454830 435450 416070

150–200

370190 362930 359300 355660 348390 341120 326590 312050

Steels

200–300

444230 435510 431150 426790 418070 409350 391910 374460

Cast Iron

180–250

345510 338730 335340 331950 325160 318380 304820 291250

Stainless

Aluminum

148080 145170 143720 142260 139360 136450 130640 124820

Titanium

320830 314530 311390 308240 301940 295640 283040 270450

HighTemperature Alloys

542950 532290 526970 521630 510970 500310 478990 457680

Fv=Axial thrust =D1.15*Ks*fr0.8 where D=Drill diameter (inches) Ks=Specific cutting energy from table (in-lb/in2) fr=Feed (in./rev)

Values in in.-lb/in2.

FIGURE 23-18 Tools)

Estimating the thrust force, Fv, in drilling using, Ks values in in-lb/in2. (Waukesha Cutting

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Machine Tools for Drilling

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Rigidity is especially important in avoiding chatter. A lack of rigidity in the cutting tool, the workpiece workholding device or the machine tool permits the affected members to deflect due to the cutting forces. Conditions for chatter are discussed in Chapter 20. The cutting lips can have a hammering action against the work. Using the shortest tool possible can help. In addition, backlash in the feed mechanism should be kept at a minimum to reduce strain on the drill when it breaks through the bottom of the hole. The common name for the machine tool used for drilling is the drill press. Drill presses consist of a base, a column that supports a powerhead, a spindle, and a worktable. On small machines, the base rests on a workbench, whereas on larger machines it rests on the floor (Figure 23-19). The column may be either round or of box-type construction—the latter being used on larger, heavy-duty machines, except in radial types. The powerhead contains an electric motor and means for driving the spindle in rotation at several speeds. On small drilling machines, this may be accomplished by shifting a belt on a step-cone pulley, but on larger machines a geared transmission is used. The heart of any drilling machine is its spindle. In order to drill satisfactorily, the spindle must rotate accurately and also resist whatever side forces result from the drilling process. In virtually all machines, the spindle rotates in preloaded ball or taper-roller bearings. In addition to powered rotation, provision is made so that the spindle can be moved axially to feed the drill into the work. On small machines, the spindle is fed by hand, using the handles extending from the capstan wheel; on larger machines, power feed is provided. Except for some small bench types, the spindle contains a hole with a Morse taper in its lower end into which taper-shank drills or drill chucks can be inserted. The worktables on drilling machines may be moved up and down on the column to accommodate work of various sizes. On round-column machines, the table can usually be rotated out of the way so that workpieces can be mounted directly on the base. On some box-column machines, the table is mounted on a subbase so that it can be moved in two directions in a horizontal plane by means of feed screws. Figure 23-19 shows examples of common types of drilling machines used in production environments. Drilling machines are usually classified as bench, upright with single spindle, turret or NC turret, gang, multispindle, deep-hole, and transfer. With bench drill presses, holes up to 12 in. in diameter can be drilled. The same type of machine can be obtained with a long column so that it can stand on the floor. The size of bench and upright drilling machines is designated by twice the distance from the centerline of the spindle to the nearest point on the column, this being an indication of the maximum size of the work that can be drilled in the machines. For example, a 15-in. drill press will permit a hole to be drilled at the center of a workpiece 15 in. in diameter. Sensitive drilling machines are essentially smaller, plain, bench-type machines with more accurate spindles and bearings. They are capable of operating at higher speeds, up to 30,000 rpm. Very sensitive hand-operated feeding mechanisms are provided for use in drilling small holes. Such machines are used for tool and die work and for drilling very small holes, often less than a few thousandths of an inch in diameter, when high spindle speeds are necessary to obtain proper cutting speed and sensitive feel to provide delicate feeding to avoid breakage of the very small drills. Upright drilling machines usually have spindle speed ranges from 60 to 3500 rpm and power feed rates, from 4 to 12 steps, from about 0.004 to 0.025 in./rev. Most modern machines use a single-speed motor and a geared transmission to provide the range of speeds and feeds. The feed clutch disengages automatically when the spindle reaches a preset depth. Worktables on most upright drilling machines contain holes and slots for use in clamping work and nearly always have a channel around the edges to collect cutting fluid, when it is used. On box-column machines, the table is mounted on vertical ways on the front of the column and can be raised or lowered by means of a crank-operated elevating screw. In mass production, gang-drilling machines are often used when several related operations—such as drilling holes of different sizes, reaming, or counterboring—must be done on a single part. These consist essentially of several independent columns, heads, and spindles mounted on a common base and having a single table. The work can be slid

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Power head

Column RAM

Speed control 10-tool turret

Speed indicator Feed control

Z

Quill Spindle Crank to adjust table height Worktable

CNC controller Table

X Column (round) Y

Base

(a)

Base

(b) Arm-elevating screw Column Drive motor

Head moves in or out

Arm Arm moves up or down

Gang-drilling machine

(c)

FIGURE 23-19 Examples of drilling machines: (a) upright column drilling machine; (b) CNC turret drilling machine; (c) gang-drilling machine; (d) radial drill press; (e) multiple-spindle drilling machine. (Courtesy J T. Black)

Arm swings about column Spindle and drill rotate

Column clamps

Spindle feeds down Work clamps to base

(d)

Multiple-spindle drilling machine

(e)

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Cutting Fluids for Drilling

657

into position for the operation at each spindle. They are available with or without power feed. One or several operators may be used. This machine would be an example of a simple small cell, except the machines are usually not single-cycle automatics. Turret-type, upright drilling machines are used when a series of holes of different sizes, or a series of operations (such as center drilling, drilling, reaming, and spot facing), must be done repeatedly in succession. The selected tools are mounted in the turret. Each tool can quickly be brought into position merely by rotation of the turret. These machines automatically provide individual feed rates for each spindle and are often numerically controlled. Radial drilling machine tools are used on large workpieces that cannot be easily handled manually. As shown in Figure 23-19, these machines have a large, heavy, round, vertical column supported on a large base. The column supports a radial arm that can be raised and lowered by power and rotated over the base. The spindle head, with its speed- and feed-changing mechanism, is mounted on the radial arm. It can be moved horizontally to any desired position on the arm. Thus, the spindle can be quickly positioned properly for drilling holes at any point on a large workpiece mounted either on the base of the machine or even sitting on the floor. Plain radial drilling machines provide only a vertical spindle motion. On semiuniversal machines, the spindle head can be pivoted at an angle to a vertical plane. On universal machines, the radial arm is rotated about a horizontal axis to permit drilling at any angle. Radial drilling machines are designated by the radius of the largest disk in which a center hole can be drilled when the spindle head is at its outermost position. Sizes from 3 to 12 ft are available. Radial drilling machines have a wide range of speeds and feeds, can do boring, and include provisions for tapping internal threading. Multiple-spindle drilling machines (Figure 23-19) are mass-production machines with as many as 50 spindles driven by a single power head and fed simultaneously into the work. Figure 23-20 shows an adjustable multiple-spindle head that can be mounted on a regular single spindle-drill press. Figure 23-20 shows the methods of driving and positioning the spindles, which permit them to be adjusted so that holes can be drilled at any location within the overall capacity of the head. Special drill jigs are often designed and built for each job to provide accurate guidance for each drill. Although such machines and workholders are quite costly, they can be cost-justified when the quantity to be produced will justify the setup cost and the cost of the jig. Reducing setup on these machines is difficult. Numerically controlled drill presses other than turret drill presses are not common because drilling and all its related processes can be done on vertical or horizontal NC machining centers equipped with automatic tool changers (see Chapter 40). Special machines are used for drilling long (deep) holes, such as are found in rifle barrels, connecting rods, and long spindles. High cutting speeds, very light feeds, and a copious flow of cutting fluid ensure rapid chip removal. Adequate support for the long, slender drills is required. In most cases horizontal machines are used. The work is rotated in a chuck with steady rests providing support along its length, as required. The drill does not rotate and is fed into the work. Vertical machines are also available for shorter workpieces. Notice the similarity between this process and boring.

& 23.7 CUTTING FLUIDS FOR DRILLING For shallow holes, the general rules relating to cutting fluids, as given in Chapter 21, are applicable. When the depth of the hole exceeds one diameter, it is desirable to increase the lubricating quality of the fluid because of the rubbing between the drill margins and the wall of the hole. The effectiveness of a cutting fluid as a coolant is quite variable in drilling. While the rapid exit of the chips is a primary factor in heat removal, this action also tends to restrict entry of the cutting fluid. This is of particular importance in drilling materials that have poor heat conductivity. Recommendations for cutting fluids for drilling are given in Table 23-5. If the hole depth exceeds two or three diameters, it is usually advantageous to withdraw the drill each time it has drilled about one diameter of depth to clear chips from the hole. Some machines are equipped to provide this ‘‘pecking’’ action automatically.

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Adjustable drill head Spindle: 6 Production: 50 pieces

Geared drill head Spindle: 8 Production: 80,000 pieces

Gearless drill head Spindle: 16 Production: 30,000 pieces

An adjustable drill head should be considered for low-production jobs. However, many short-run jobs such as this would be required to justify a multiple-spindle head.

A geared drill head is most appropriate in this situation, where there is a large difference in sizes and a high daily production.

Only a gearless head can perform this operation in one pass, due to the close proximity of the spindle centers.

FIGURE 23-20 Three basic types of multiple-spindle drill heads: (left) adjustable; (middle) geared; (right) gearless. (Courtesy of Zagar Incorporated)

Where cooling is desired, the fluid should be applied copiously. For severe conditions, drills containing coolant holes have a considerable advantage. Not only is the fluid supplied near the cutting edges, but the coolant flow aids in flushing the chips from the hole. Where feasible, drilling horizontally has distinct advantages over drilling vertically downward. TABLE 23-5

Cutting Fluids for Drilling

Work Material

Cutting Fluid

Aluminum and its alloys

Soluble oil, kerosene, and lard-oil compounds; light, nonviscous neutral oil; kerosene and soluble oil mixtures

Brass

Dry or a soluble oil; kerosene and lard-oil compounds; light, nonviscous neutral oil

Copper Cast iron

Soluble oil, strained lard oil, oleic-acid compounds Dry or with a jet of compressed air for cooling

Malleable iron

Soluble oil, nonviscous neutral oil

Monel metal

Soluble oil, sulfurized mineral oil

Stainless steel

Soluble oil, sulfurized mineral oil

Steel, ordinary

Soluble oil, sulfurized oil, high extreme-pressure-value mineral oil

Steel, very hard

Soluble oil, sulfurized oil, turpentine

Wrought iron

Soluble oil, sulfurized oil, mineral-animal oil compound

Neat oil can be used effectively with the solid carbide drills for low-speed drilling (up to 130 sfpm). If the work surface becomes hard or blue in color, decrease the rpm and use neat oil. For heavy-duty cutting, emulsion-type oil containing some extreme pressure additive is recommended. A volume of 3.0 gal/min at a pressure of 37–62 lb/in.2 is recommended. A double stream supply of fluid is recommended.

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Reaming

659

(a) Counterbore

Countersink

Spot face

(b)

FIGURE 23-21 (a) Surfaces produced by counterboring, countersinking, and spot facing. (b) Counterboring tools: (bottom to top) interchangeable counterbore; solid, taper-shank counterbore with integral pilot; replaceable counterbore and pilot; replaceable counterbore, disassembled. (Courtesy of ExCell-O Corporation and Chicago Latrobe Twist Drill Works)

& 23.8 COUNTERBORING, COUNTERSINKING, AND SPOT FACING Drilling is often followed by counterboring, countersinking, or spot facing. As shown in Figure 23-21, each provides a bearing surface at one end of a drilled hole. They are usually done with a special tool having from three to six cutting edges. Counterboring provides an enlarged cylindrical hole with a flat bottom so that a bolt head, or a nut, will have a smooth bearing surface that is normal to the axis of the hole; the depth may be sufficient so that the entire bolt head or nut will be below the surface of the part. The pilot on the end of the tool fits into the drilled hole and helps to ensure concentricity with the original hole. Two or more diameters may be produced in a single counterboring operation. Counterboring also can be done with a single-point tool, although this method ordinarily is used only on large holes and essentially is a boring operation. Some counterboring tools are shown in Figure 23-21b. Countersinking makes a beveled section at the end of a drilled hole to provide a proper seat for a flat-head screw or rivet. The most common angles are 60, 82, and 90 degrees. Countersinking tools are similar to counterboring tools except that the cutting edges are elements of a cone, and they usually do not have a pilot because the bevel of the tool causes them to be self-centering. Spot facing is done to provide a smooth bearing area on an otherwise rough surface at the opening of a hole and normal to its axis. Machining is limited to the minimum depth that will provide a smooth, uniform surface. Spot faces thus are somewhat easier and more economical to produce than counterbores. They are usually made with a multiedged end-cutting tool that does not have a pilot, although counterboring tools are frequently used.

& 23.9 REAMING Reaming removes a small amount of material from the surface of holes. It is done for two purposes: to bring holes to a more exact size and to improve the finish of an existing

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Chucking reamer Helical flutes (r.h. helix shown)

Taper shank

Reamer diameter

Body

Chamfer Chamfer angle relief

Cutter sweep

Land width Margin

Shank length

Radial rake angle

Helix angle Straight shank Shank length

Chamfer relief angle

Flute length

Overall length

Chamfer length

Hand reamer, pilot and guide Cutter sweep Guide

Reamer diameter Straight flutes

Starting taper

Neck

Pilot Axis

Squared shank

Neck Shank length

Flute length

Cutter sweep

Overall length

FIGURE 23-22

Standard nomenclature for hand and chucking reamers. (Courtesy J T. Black)

hole. Multiedge cutting tools are used, as shown in Figure 23-22. No special machines are built for reaming. The same machine that was employed for drilling the hole can be used for reaming by changing the cutting tool. To obtain proper results, only a minimum amount of materials should be left for removal by reaming. As little as 0.005 in. is desirable, and in no case should the amount exceed 0.015 in. A properly reamed hole will be within 0.001 in. of correct size and have a fine finish. The principal types of reamers are shown in Figures 23-22 and 23-23. Hand reamers are intended to be turned and fed by hand and to remove only a few thousandths of

FIGURE 23-23 Types of reamers (top to bottom): straight-fluted rose reamer, straight-fluted chucking reamer, straight-fluted taper reamer, straight-fluted hand reamer, expansion reamer, shell reamer, adjustable insert-blade reamer. (Courtesy J T. Black)

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Reaming

661

an inch of metal. They have a straight shank with a square tang for a wrench. They can have straight or spiral flutes and can be solid or expandable. The teeth have relief along their edges and thus may cut along their entire length. However, the reamer is tapered from 0.005 to 0.010 in. in the first third of its length to assist in starting it in the hole, and most of the cutting therefore takes place in this portion. Machine or chucking reamers are for use with various machine tools at slow speeds. The best feed is usually two to three times the drilling feed. Machine reamers have chamfers on the front end of the cutting edges. The chamfer causes the reamer to seat firmly and concentrically in the drilled hole, allowing the reamer to cut at full diameter. The longitudinal cutting edges do little or no cutting. Chamfer angles are usually 45 degrees. Reamers have straight or tapered shanks and straight or spiral flutes. Rosechucking reamers are ground cylindrical and have no relief behind the outer edges of the teeth. All cutting is done on the beveled ends of the teeth. Fluted-chucking reamers, on the other hand, have relief behind the edges of the teeth as well as beveled ends. They can, therefore, cut on all portions of the teeth. Their flutes are relatively short, and they are intended for light finishing cuts. For best results they should not be held rigidly but permitted to float and be aligned by the hole. Shell reamers often are used for larger sizes in order to save cutting tool material. The shell, made of tool steel for smaller sizes and with carbide edges for larger sizes or for mass-production work, is held on an arbor that is made of ordinary steel. One arbor may be used with any number of shells. Only the shell is subject to wear and needs to be replaced when worn. They may be ground as rose or fluted reamers. Expansion reamers can be adjusted over a few thousandths of an inch to compensate for wear or to permit some variation in hole size to be obtained. They are available in both hand and machine types. Adjustable reamers have cutting edges in the form of blades that are locked in a body. The blades can be adjusted over a greater range than expansion reamers. This permits adjustment for size and to compensate for regrinding. When the blades become too small from regrinding, they can be replaced. Both tool steel and carbide blades are used. Taper reamers are used for finishing holes to an exact taper. They may have up to eight straight or spiral flutes. Standard tapers, such as Morse, Jarno, or Brown & Sharpe, come in sets of two. The roughing reamer has nicks along the cutting edge to break up the heavy chips that result as a cylindrical hole is cut to a taper. The finishing reamer has smooth cutting edges.

REAMING PRACTICE If the material to be removed is free cutting, reamers of fairly light construction will give satisfactory results. However, if the material is hard, then tough, solid-type reamers are recommended, even for fairly large holes. To meet quality requirements, including both finish and accuracy (tolerances on diameter, roundness, straightness, and absence of bell-mouth at ends of holes), reamers must have adequate support for the cutting edges, and reamer deflection must be minimal. Reaming speed is usually two-thirds the speed for drilling the same materials. However, for close tolerances and fine finish, speeds should be slower. Feeds are usually much higher than those for drilling and depend on material. A feed of between 0.0015 and 0.004 in. per flute is recommended as a starting point. Use the highest feed that will still produce the required finish and accuracy. Recommended cutting fluids are the same as those for drilling. Reamers, like drills, should not be allowed to become dull. The chamfer must be reground long before it exhibits excessive wear. Sharpening is usually restricted to the starting taper or chamfer. Each flute must be ground exactly even, or the tool will cut oversize. Reamers tend to chatter when not held securely, when the work or workholder is loose, or when the reamer is not properly ground. Irregularly spaced teeth may help reduce chatter. Other cures for chatter in reaming are to reduce the speed, vary the feed rate, chamfer the hole opening, use a piloted reamer, reduce the relief angle on the chamfer, or change cutting fluid. Any misalignment between the workpiece and the reamer will cause chatter and improper reaming.

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Drilling and Related Hole-Making Processes

& KEY WORDS adjustable reamer BTA drills center core drill chisel end chuck chuck reamer combination drill counterboring countersinking deep-hole drills drift drill press drilling dubbing

ejector drills expansion reamer finishing reamer flute fluted-chucking reamer gang-drilling machine gundrill gundrill trepanning hand reamer helix angle hole cutter hole saw indexable insert drill jig

lands lip machine reamer microdrilling multiple-spindle drilling machine quick-change chuck radial drilling machine reaming rose-chucking reamer roughing reamer shell reamer spade drill spindle

spot facing step drill straight shank subland drill tang tape reamer taper shank thrust force trepanning torque turret-type, upright drilling machine twist drill web web thinning

& REVIEW QUESTIONS 1. What functions are performed by the flutes on a standard twist drill? 2. What determines the rake angle of a drill? See Figure 23-2. 3. Basically, what determines what helix angle a drill should have? 4. When a large-diameter hole is to be drilled, why is a smallerdiameter hole often drilled first? 5. Equation 23-4 for the MRR for drilling can be thought of as ___ ___, where frNs is the feed rate of the drill bit. 6. Are the recommended surface speeds for spade drills given in Table 23-3 typically higher or lower than those recommended for twist drills? How about the feeds? Why? 7. What can happen when an improperly ground drill is used to drill a hole? 8. Why are most drilled holes oversize with respect to the nominally specified diameter? 9. What are the two primary functions of a combination center drill? 10. What is the function of the margins on a twist drill? 11. What factors tend to cause a drill to ‘‘drift’’ off the centerline of a hole? 12. The drills shown in Figure 23-13 have coolant passages in the flutes. What is the purpose of these holes? 13. In drilling, the deeper the hole, the greater the torque. Why? 14. Why do cutting fluids for drilling usually have more lubricating qualities than those for most other machining operations? 15. How does a gang-drilling machine differ from a multiplespindle drilling machine?

16. How does a multiple-spindle drilling machine differ from a numerical control (NC) drilling machine with a tool changer that would hold all the drills found in the multiple-spindle machine? See Chapter 40 for discussion on NC. 17. How does the thrust force vary with feed? Why? 18. Holding the workpiece by hand when drilling is not a good idea. Why? 19. What is the rationale behind the operation sequence shown in Figure 23-10? 20. In terms of thrust, what is unusual about the slot-point drill compared to other drills? 21. What is the purpose of spot facing? 22. How does the purpose of counterboring differ from that of spot facing? 23. What are the primary purposes of reaming? 24. What are the advantages of shell reamers? 25. A drill that operated satisfactorily for drilling cast iron gave very short life when used for drilling a plastic. What might be the reason for this? 26. What precautionary procedures should be used when drilling a deep, vertical hole in mild steel when using an ordinary twist drill? 27. What is the advantage of a spade drill? Is it really a drill? 28. What is a ‘‘pecking’’ action in drilling? 29. Why does drill feed increase with drill size? 30. Suppose you specified a drilling feed rate that was too large. What kinds of problems do you think this might cause? See Figure 23-6 and Table 23-4 for help.

& PROBLEMS 1. Suppose you wanted to drill a 1.5-in.-diameter hole through a piece of 1020 cold-rolled steel that is 2 in. thick, using an indexable insert drill. What values of feed and cutting speed will you specify, along with an appropriate allowance. Is this the correct tool? What other drill types could be used? 2. How much time will be required to drill the hole in Problem 1 using the insert drill?

3. What is the metal removal rate when a 1.5-in.-diameter hole, 2 in. deep, is drilled in 1020 steel at a cutting speed of 200 fpm with a feed of 0.010 ipr? What is the cutting time? 4. If the specific horsepower for the steel in Problem 3 is 0.9, what horsepower would be required, assuming 80% efficiency in the machine tool?

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Problems 5. If the specific power of an AISI 1020 steel of 0.9, and 80% of the output of the 1.0-kW motor of a drilling machine is available at the tool, what is the maximum feed that can be used in drilling a 1-in.-diameter hole with a carbide drill? (Use the cutting speed suggested in Problem 3.) 6. Show how the approximate equation 23-5 for MRR in drilling was obtained. What assumption was needed? 7. A workpiece must have 10 holes finished in it. Manual layout time is 12 hr/piece. To drill and ream all the holes requires 1 hr on the machine for each piece, not counting layout or setup. The labor rate is $10/hr and the machine rate is $20/hr. If a jig is used, the labor cost to lay out each piece can be saved. Both methods give the same-quality product, but this jig saves 40 min in processing time on the machine. How large a lot justifies the use of a jig that costs $150 to make (labor and materials)? 8. A part has two holes located for drilling by manual layout. If a drill jig is used, 0.5 min in processing time is saved for each piece. The labor rate is $9/hr. The overhead rate on the labor saved is 100%. Setup time is no more with than without the jig. The combined rate for interest, insurance, taxes, and maintenance is 35%. The cost of the jig is $500. a. How many pieces must be made in one lot to make the jig worthwhile? b. How many pieces must be made on the jig in one lot each month to earn the cost of the jig in 2 yr? 9. Manufacturer’s charts will help determine the best feed and speed to run the drills. For example, a 1.5-in hole is to be drilled in 4140 steel annealed to BHN 275. For the spade drill, speed is 80 sfpm; feed, 0.009 ipr; and spindle rotation, 204 rpm. For the indexable insert drill, speed is 358 sfpm; feed, 0.007 ipr; and spindle rotation, 891 rpm. Typically, an indexable insert drill can produce a hole four times faster than a spade drill but may cost (with inserts) 50 to 75% more than the equivalent spade blade and holder. For making only a couple of holes, the extra cost is not usually justified. Determine the number of holes needed to justify the extra cost of the indexable insert drill. Some additional cost data are given in the accompanying table. Ignore tool life and assume that the blades and the indexable drills make about the same number of holes. (Why is this

663

a reasonable assumption?) The holes are 3 in. deep, with no allowance needed. Cost of drills: Spade Drill

Indexable-Insert Drill

$139.00 holder þ21.90 per blade $160.90

$273.00 drill þ12.80 per two inserts $285.80

Assume for this example that a machine rate of $45/hr includes the cost of labor and machine burden. 10. Assume that you are drilling eight holes, equally spaced in a bolt-hole circle. That is, there would be holes at 12, 3, 6, and 9 o’clock and four more holes equally spaced between them. The diameter of the bolt hole circle is 6 in. The designer says that the holes must be 45 degrees 1 degree from each other around the circle. a. Compute the tolerance between hole centers. b. Do you think a typical multiple-spindle drill setup could be used to make this bolt circle—using eight drills all at once? Why or why not? c. Do you think that the use of a jig may help improve the situation? d. Do you think a CNC drilling process could do the holes best? 11. A part with seven holes can be machined on a numerically controlled turret drill press in 3 min (estimated time based on similar parts). The rate on the CNC machine for labor is $34/hr. Currently, the part is being machined on a gang drill press with a special jig in 10 min/piece. The jig for the gang drill costs $300; the combined rate for depreciation, interest, insurance, and taxes is 135%; and the hourly rate for the gang drill and operator is $16. Setup time is about the same for both machines. For how many pieces is it economical to switch to the CNC? 12. It is estimated that a jig for machining a part with three holes costs $400 and with it the operation takes 15 min/part. The operation can be done without a jig on a numerically controlled drill press in 5 min. Assume that any other conditions are the same as in Problem 11. How many pieces are needed to cost-justify the use of a jig?

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Drilling and Related Hole-Making Processes

Chapter 23

CASE STUDY

Bolt-down Leg on a Casting Larry Cornelius and Marcelo Veras are consulting engineers and have just received the drawing shown in Figure CS-23. This is one of four legs on a casting made by the the Lorac Warpug Company. These legs are used to attach the device to the floor. The section drawing to the right shows the typical loading to which the leg is subjected. The company is currently drilling the bolt hole and then counterboring the land, but manufacturing has experienced some difficulty in machining the four holes. They report a lot of drill breakage. Quality control reports that distances between the four holes are frequently too large. Sales has recently reported that a substantial number of in-service failures have occurred with these

legs. Larry and Marcelo have obtained a sketch from sales showing where the legs typically fail. This casting is manufactured from gray cast iron using the sand casting process. 1. What machining difficulties should Larry and Marcelo suspect this leg to have? 2. Why were the distances between the holes too large? 3. What should Larry and Marcelo recommend for solving these problems in the future in terms of materials, design, and manufacturing methods? 4. What should Larry and Marcelo recommend be done with the units in the field to stop the failures?

Loading M Counterbored land A

Counterbored land

A⬘

Section A – A⬘ Drilled hole

Sketch received from sales

Break

FIGURE CS-23 Shows the design of one of four legs on a casting made by the Cruftsmobile Company.

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CHAPTER 24 MILLING 24.1 INTRODUCTION 24.2 FUNDAMENTALS OF MILLING PROCESSES Face Milling End Milling Example Up Versus Down Milling Milling Surface Finish

Profilers and Duplicators Milling Machine Selection Accessories for Milling Machines Case Study: HSS versus Tungsten Carbide Milling

24.3 MILLING TOOLS AND CUTTERS 24.4 MACHINES FOR MILLING Basic Milling Machine Construction Bed-Type Milling Machines Planer-Type Milling Machines Rotary-Table Milling Machines

& 24.1 INTRODUCTION Milling is a basic machining process by which a surface is generated by progressive chip removal. The workpiece is fed into a rotating cutting tool. Sometimes the workpiece remains stationary, and the cutter is fed to the work. In nearly all cases, a multiple-tooth cutter is used so that the material removal rate is high. Often the desired surface is obtained in a single pass of the cutter or work and, because very good surface finish can be obtained, milling is particularly well suited and widely used for mass-production work. Many types of milling machines are used, ranging from relatively simple and versatile machines that are used for general-purpose machining in job shops and tool and die work (these are NC or CNC machines) to highly specialized machines for mass production. Unquestionably, more flat surfaces are produced by milling than by any other machining process. The cutting tool used in milling is known as a milling cutter. Equally spaced peripheral teeth will intermittently engage and machine the workpiece. This is called interrupted cutting. The workpieces are typically held in fixtures, as described in Chapter 27.

& 24.2 FUNDAMENTALS OF MILLING PROCESSES Milling operations can be classified into broad categories called peripheral milling, end milling, and face milling. Each has many variations. In peripheral milling, the surface is generated by teeth located on the periphery of the cutter body (Figure 24-1). The surface is parallel with the axis of rotation of the cutter. Both flat and formed surfaces can be produced by this method, the cross section of the resulting surface corresponding to the axial contour of the cutter. This process, often called slab milling, is usually performed on horizontal spindle milling machines. In slab milling, the tool rotates (mills) at some rpm (Ns) while the work feeds past the tool at a table feed rate, fm, in inches per minute (ipm), which depends on the feed per tooth, ft. As in the other processes, the cutting speed, V, and feed per tooth are selected by the engineer or the machine tool operator. As before, these variables depend on the work material, the tool material, and the specific process. The cutting velocity is that which occurs at the cutting edges of the teeth in the milling center. The rpm of the spindle is determined from the surface cutting speed, where D is the cutter diameter in inches according to Ns ¼

12V pD

ð24-1Þ

The depth of cut, called DOC or d in Figure 24-1, is simply the distance between the old and new machined surface.

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CHAPTER 24

Milling

Cutting edge

Horizontal Spindle

D

RPM cutter

Face Flank

Arbor

n teeth in cutter

Table Ns

V d=DOC

fm

Workpiece (a) Horizontal-spindle milling machine

D

d=DOC

(b) Slab milling—multiple tooth

Cutter

ft D 2

d

fm L

Tooth

C24

Work

LA

Material removed by one tooth

fm

(d) Feed per tooth

(c) Allowances for cutter approach

FIGURE 24-1 Peripheral milling can be performed on a horizontal-spindle milling machine. The cutter rotates at rpm Ns, removing metal at cutting speed, V. The allowance for starting and finishing the cut depends on the cutter diameter and depth of cut, d. The feed per tooth, ft, and cutting speed are selected by the operator or process planner. (Courtesy J T. Black)

The width of cut is the width of the cutter or the work, in inches, and is given the symbol W. The length of the cut, L, is the length of the work plus some allowance, LA, for approach and overtravel. The feed of the table, fm, in inches per minute, is related to the amount of metal each tooth removes during a revolution, the feed per tooth, ft, according to f m ¼ f t Nsn ð24-2Þ where n is the number of teeth in the cutter (teeth rev.). The cutting time is Tm ¼

L þ LA fm

ð24-3Þ

The length of approach is

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D2 D DOC ¼ dðD dÞ LA ¼ 2 4

ð24-4Þ

The metal removal rate (MRR) is MRR ¼

Volume LWd ¼ ¼ Wf m d in:3 =min Tm Tm

ð24-5Þ

ignoring LA. Values for ft are given in Table 24-1, along with recommended cutting speeds in feet per minute.

Speed, fpm

Feed per tooth Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth

Speed, fpm

Feed per tooth Speed, fpm

Feed per tooth

Speed, fpm

Soft/hard

100–150 BHN steel

150–250

BHN steel

250–350

BHN steel

350–450

BHN steel

Cast iron, hard BHN 180–225

Cast iron, medium

BHN 180–225

Cast iron, soft

BHN 150–180

Bronze

Soft/hard

Brass Soft/hard

Aluminum alloy

Soft/hard

2000 UP

.010–.040

.010–.020 500–1500

300–100

.010–.020

275–400

.015–.025

200–275

.008–.015

.005–.010 125–200

125–180

.008–.015

180–300

.008–.015

300–450

.010–.015

.010–.015 450–800

150–350

.008–.015

200–300

.005–.015

Face Mills

2000 UP

.010–.030

.010–.020 500–1500

300–800

.010–.020

250–350

.010–.020

175–250

.008–.015

.005–.010 100–175

100–150

.007–.012

150–300

.007–.012

300–450

.008–.015

.008–.015 450–600

150–350

.005–.015

200–300

.005–.015

Slab Mills

2000 UP

.003–.015

.005–.010 500–1500

300–1000

.005–.010

275–400

.005–.012

200–275

.005–.010

.003–.008 125–200

100–150

.004–.008

150–300

.005–.010

300–450

.005–.010

.005–.010 450–600

150–350

.003–.010

200–350

.005–.010

End Mills

2000 UP

.008–.025

.008–.012 500–1500

300–1000

.008–.012

275–400

.008–.015

200–275

.005–.012

.003–.010 125–200

125–180

.005–.012

160–300

.005–.012

300–450

.007–.012

.008–.012 450–800

150–350

.005–.010

200–300

.005–.010

Full and Half-Side Mills

Generally lower end of range used for inserted blade cutters, higher end of range for indexable insert cutters.

Feed per tooth

Cast steel

a

Feed per tooth

Speed, fpm

Soft/hard

Material

Malleable iron

Feed (in./tooth) Speed (fpm)

Carbide Cutters

2000 UP

.003–.006

.003–.004 500–1500

300–1000

.003–.004

250–350

.003–.004

200–250

.003–.004

.002–.003 125–200

100–150

.001–.004

150–300

.002–.005

300–450

.003–.006

.003–.006 350–600

150–300

.002–.004

200–350

.003–.004

Saws

2000 UP

.008–.015

.008–.015 500–1500

200–800

.008–.015

250–350

.008–.015

175–250

.006–.012

.005–.010 100–175

100–150

.003–.008

150–300

.003–.008

300–450

.004–.010

.004–.010 350–600

150–300

.005–.010

175–275

.005–.010

Form Mills

300–1200

.010–.040

.010–.025 150–300

50–225

.010–.025

80–120

.015–.030

60–80

.010–.020

.005–.012 40–60

20–35

.003–.008

35–60

.005–.010

50–70

.010–.020

.015–.030 80–130

40–60

.010–.015

60–100

.005–.015

Face Mills

300–1200

.015–.040

.008–.020 100–300

50–200

.008–.020

70–110

.010–.025

50–70

.008–.015

.005–.010 35–50

20–35

.005–.008

35–50

.005–.010

50–70

.008–.015

.008–.015 80–130

40–60

.010–.015

60–90

.005–.015

Slab Mills

300–1200

.015–.040

.005–.015 150–350

50–250

.003–.010

80–120

.004–.010

60–90

.003–.010

.003–.008 40–60

20–40

.003–.101

40–60

.003–.010

60–80

.003–.010

.003–.010 80–140

40–60

.005–.010

60–100

.003–.015

End Mills

300–1200

.010–.030

.008–.015 150–350

50–225

.008–.015

80–120

.010–.020

60–80

.008–.015

.005–.010 40–60

20–35

.003–.008

35–50

.005–.010

50–70

.010–.015

.010–.020 80–130

40–60

.005–.010

60–100

.006–.012

Full and Half-Side Mills

High-speed-Steel Cutters

300–1000

.004–.008

.003–.005 150–300

50–250

.003–.005

70–110

.002–.005

60–70

.003–.005

.002–.004 35–60

20–35

.001–.004

35–50

.002–.005

50–70

.003–.006

.003–.006 70–100

40–60

.002–.005

60–100

.003–.006

Saws

300–1200

.010–.020

.008–.015 100–300

50–200

.008–.015

60–80

.010–.015

50–60

.008–.012

.005–.010 35–50

20–35

.003–.008

35–50

.005–.010

50–70

.006–.010

.008–.010 70–100

40–60

.008–.012

60–80

.005–.010

Form Mills

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Suggested Starting Feeds and Speeds Using High-Speed Steel and Carbide Cuttersa

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TABLE 24-1

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CHAPTER 24

Milling

FACE MILLING In face milling and end milling, the generated surface is at right angles to the cutter axis (Figure 24-2). Most of the cutting is done by the peripheral portions of the teeth, with the face portions providing some finishing action. Face milling is done on both horizontal- and vertical-spindle machines. The tool rotates (face mills) at some rpm (Ns) while the work feeds past the tool. The rpm is related to the surface cutting speed, V, and the cutting tool diameter, D, according to equation 24-1. The depth of cut is d, in inches, as shown in Figure 24-2b. The width of cut is W, in inches, and may be width of the workpiece or width of the cutter, depending on the setup. The length of cut is the length of the workpiece, L, plus an allowance, LA, for approach and overtravel, LO, in inches. The feed rate of the table, fm, in inches per minute, is related to the amount of metal each tooth removes during a pass over the work, called the feed per tooth, ft,. As before fm ¼ ftNsn, where the number of teeth in the cutter is n. The cutting time is Tm ¼

L þ LA þ Lo min fm

ð24-6Þ

Column V Ns

Table

Six-tooth face mill

e

c Workpie

W

Ns

d

(a) Vertical-spindle milling machine

fm

Feed

Top views LO

L

(b) Face milling over part of surface

LA

W

LO

L

LA

D Machined surface

D D 2

(c) Allowance for partial coverage

W

(d) Allowance for full coverage

FIGURE 24-2 Face milling is often performed on a vertical spindle milling machine using a multiple-tooth cutter (n ¼ 6 teeth) rotating Ns at rpm to produce cutting speed, V. The workpiece feeds at rate fm, in inches per minute past the tool. The allowance depends on the tool diameter and the width of cut. (Courtesy J T. Black)

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SECTION 24.2 Top view

Fundamentals of Milling Processes

669

Cutter Spindle Machined surface

Workpiece

FIGURE 24-3 Face milling viewed from above with vertical spindle-machine.

fm =feed rate =19 in./min

The metal removal rate is MRR

Volume LWd ¼ ¼ f m Wd in:3 =min Tm Tm

When calculating the MRR, ignore LO and LA. The length of approach is usually equal to the length of overtravel, which usually equals D/2 in. For a setup where the tool does not completely pass over the workpiece, Lo ¼ LA ¼

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D W ðD W Þ for W < 2

ð24-7Þ

D D for W 2 2

ð24-8Þ

Lo ¼ LA ¼

Here is an example of a face milling calculation. A 4-in.-diameter, six-tooth face mill is selected, using carbide inserts (Figure 24-3). The material being machined is low-alloy steel, annealed. Using cutting data recommendations, the cutting speed chosen is 400 sfpm with a feed of 0.008 in./tooth at a d of 0.12 in. Determining rpm at the spindle, Ns ¼

12V 12 400 ¼ ¼ 392 rpm pD 3:14 4

Determining the feed rate of the table, fm ¼ nNsft, f m ¼ 0:008 6 392 ¼ 19 in:=min If slab or side milling were being performed, with the same parameters selected as given earlier, the setup would be different but the spindle rpm and table feed rate the same. The cutting time would be different because the allowances for face milling are greater than for slab milling. In milling, power consumption is usually the limiting factor. A thick chip is more power efficient than a thin chip.

END MILLING EXAMPLE End milling is a very common operation performed on both vertical- and horizontalspindle milling machines or machining centers. Figure 24-4 shows a vertical spindle end milling process, cutting a step in the workpiece. This cutter can cut on both the sides and ends of the tool. If you were performing this operation on a block of metal (for example, 430F stainless steel), you (the manufacturing engineer) would select a specific machine tool. You would have to determine how many passes (rough and finish cuts) were needed to produce the geometry specified in the design. Why? The number of passes determines the total cutting time for the job. Using a vertical-spindle milling machine, an end mill can produce a step in the workpiece. In Figure 24-4, an end mill with six teeth on a 2-in. diameter is used to cut a step in 430F stainless. The d (depth of cut) is 0.375 in., and the depth of immersion

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CHAPTER 24

Milling

(not to scale)

End View

Side View

Rotating Cutting Tool – End Mill = 2 in. diam., 6 teeth

1.25 Inches DOI

0.375

Fixture or vise

Work feed rate (in/min)

d Workpiece 430F stainless

Vise

d = Depth of cut DOI = Depth of immersion

Vise

FIGURE 24-4 End milling a step feature in a block using a flat-bottomed, end mill cutter in a vertical spindle-milling machine. On left, photo. In middle, end view, table moving the block into the cutter. On right, side view, workpiece feeding right to left into tool. (Courtesy J T. Black)

(DOI) is 1.25 in. The tool deflects due to the cutting forces, so the cut needs to be made at full immersion. The engineer should check to see if there is enough power for a full DOC. Can the step be cut in one pass, or will multiple cuts be necessary? The vertical milling machine tool available has a 5-hp motor with 80% efficiency. The specific horsepower for 430F stainless is 1.3 hp/in.3/min. The maximum amount of material that can be removed per pass is usually limited by the available power. Using the horsepower equation from Chapter 20, hp ¼ HPs MRR ¼ HPs f m WD ¼ HPs f m DOI d

ð24-9Þ

Select ft ¼ 0.005 ipt and V ¼ 250 fpm from Table 24-1. Calculate the spindle rpm: Ns ¼

12 250 ¼ 477 rpm of cutter 3:14 2

Next, assuming the machine tool has this rpm available, calculate the table feed rate: f m ¼ f t n N s ¼ 0:005 6 477 ¼ 14:31 in:=min But the actual table feed rates for the selected machine are 11 in./min or 16 in./min, so, being conservative, select f m ¼ table feed rate ¼ 11:00 in:=min Next, assuming 80% of the available power is used for cutting, calculate the depth of cut from equation 24-9: d ¼ DOC ffi

5 0:8 ffi 0:225 in: maximum 1:3 11:00 1:25

Therefore, two passes are needed because (0.375/0.225 ¼ 1.6): 0:375 0:225 ¼ 0:150 in: second pass DOC 2 passes : DOC ¼ 0:225 rough cut DOC ¼ 0:150 finish cut 0:375 total DOC

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Fundamentals of Milling Processes

671

Note that for d ¼ 0.150, the feed per tooth would be only slightly increased to 0.0051 ipt: ft ¼

0:5 0:8 ¼ 0:0051 in:=tooth 1:3 6 477 0:150 1:25

You may want to change ft to improve the surface finish. With a smaller feed per tooth, a better surface finish is usually obtained. However, there are other factors to consider, like machining time. In general (for face, slab, or end milling), if machine power is lacking, the following actions may help. 1. Use a cutter with a positive rake as this can be more efficient than one with a negative rake. 2. Use a cutter with a coarser pitch (fewer teeth). 3. Use a smaller cutter and take several passes (reduce d or DOI).

UP VERSUS DOWN MILLING One of the subtle aspects of milling concerns the direction of rotation of the cutter with respect to the movement of the workpiece. Surfaces can be generated by two distinctly different methods (Figure 24-5). Up milling is the traditional way to mill and is also called conventional milling. The cutter rotates against the direction of feed of the workpiece. In climb or down milling, the cutter rotation is in the same direction as the feed rate. The method of chip formation is completely different in the two cases. In up milling, the chip is very thin at the beginning, where the tooth first contacts the work; then it increases in thickness, becoming a maximum where the tooth leaves the work. The cutter tends to push the work along and lift it upward from the table. This action tends to eliminate any effect of looseness in the feed screw and nut of the milling machine table and results in a smooth cut. However, the action tends to loosen the work from the fixture. Therefore, greater clamping forces must be employed, with the danger of deflecting the part. In addition, the smoothness of the generated surface depends greatly on the sharpness of the cutting edges. In up milling, chips can be carried into the newly machined surface, causing the surface finish to be poorer (rougher) than in down milling and causing damage to the insert. In down milling, maximum chip thickness occurs close to the point at which the tooth contacts the work. Because the relative motion tends to pull the workpiece into the cutter, any possibility of looseness in the table feed screw must be eliminated if down milling is to be used. It should never be attempted on machines that are not

Climb cut (down milling)

Blade takes thickness at entry

Ns

Depth of cut

Feed

Blade takes thick chips here and ease out of cut here

Ns

(Down milling) climb cut

Cutting forces pull workpiece toward cutter

Feed Thin chip at entry with abrupt exit

Ns

Feed

Depth of cut

(Up milling) conventional cut

FIGURE 24-5 Climb cut or down milling versus conventional cut or up milling for slab or face or end milling.

Peripherial or slab milling

Blade begins cutting, taking thin chip here, and abruptly leaves cut here Conventional cut (up milling)

Cutting forces oppose feed

Face or end milling

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90⬚

D Exit

FIGURE 24-6 Conventional face milling (left) with cutting force diagram for Fc (right) showing the interrupted nature of the process. (From Metal Cutting Principles, 2nd ed., Ingersoll Cutting Tool Company)

* C Cutting force – Fc

C Feed

C24

B

* D

*B

A Ns

0⬚ A 360

180⬚

* 0⬚

90⬚

180⬚

270⬚

Rotation 0⬚ to 360⬚ of cutter

270⬚

designed for this type of milling. Virtually all modern milling machines are capable of down milling, and it is a most favorable application for carbide cutting edges. Because the material yields in approximately a tangential direction at the end of the tooth engagement, there is less tendency (than when up milling is used) for the machined surface to show toothmarks, and the cutting process is smoother, with less chatter. Another advantage of down milling is that the cutting force tends to hold the work against the machine table, permitting lower clamping forces. However, the fact that the cutter teeth strike against the surface of the work at the beginning of each chip can be a disadvantage if the workpiece has a hard surface, as castings sometimes do. This may cause the teeth to dull rapidly. Metals that readily work-harden should be down milled, and many toolmakers recommend that down milling should always be the first choice.

MILLING SURFACE FINISH The average surface finishes that can be expected on free-machining materials range from 60 to 150 min. Conditions exist, however, that can produce wide variations on either side of these ranges. For example, some inserts are designed with wiper flats (short parallel surface behind the tool tip). If the feed per revolution (Feed per tooth Number of teeth) of the cutter is smaller than the length of the wiper flat (the land on the tool), then the surface finish on the workpiece will be generated by the highest insert. In finishing cuts, keeping the depth of cut small will limit the axial cutting force, reducing vibrations and producing a superior finish. See Chapter 37 for discussions on measuring surface finish. Milling is an interrupted cutting process. The individual teeth enter and leave the cut and subject the tool to impact loading, cyclic heating, and cycle cutting forces. As shown in Figure 24-6, in upmilling the cutting force, Fc, builds rapidly as the tool enters the work at A and progresses to B, peaks as the blade crosses the direction of feed at C, decreases to D, and then drops to zero abruptly upon exit. Down milling produces impact forces upon tool entry. The diagram does not indicate the impulse loads caused by impacts. The interrupted-cut phenomenon explains in large part why milling cutter teeth are designed to have small positive or negative rakes, particularly when the tool material is carbide or ceramic. These brittle materials tend to be very strong in compression, and negative rake results in the cutting edges being placed in compression by the cutting forces rather than tension. Cutters made from high-speed steel (HSS) are made with positive rakes, in the main, but must be run at lower speeds. Positive rake tends to lift the workpiece, while negative rakes compress the workpiece and allow heavier cuts to be made. Table 24-2 summarizes some additional milling problems.

& 24.3 MILLING TOOLS AND CUTTERS Most milling work today is done with face mills and end mills. The face mills use indexable carbide insert tooling, while the end mills are either solid HSS or insert tooling (Figure 24-7). Basically, mills are shank-type cutters having teeth on the circumferential surface and one end. They can thus be used for facing, profiling, and end milling. The teeth may be either straight or helical, but the latter is more common. Small end mills have straight shanks, whereas taper shanks are used on larger sizes (Figure 24-8).

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TABLE 24-2 Problem Chatter (vibration)

673

Milling Tools and Cutters

Probable Causes of Milling Problems Probable Cause

Cures

1. Lack of rigidity in machine, fixtures, arbor, or workpiece 2. Cutting load too great 3. Dull cutter 4. Poor lubrication 5. Straight-tooth cutter 6. Radial relief too great 7. Rubbing, insufficient clearance

Use larger arbors. Change rpm (cutting speed). Decrease feed per tooth or number of teeth in contact with work. Sharpen or replace inserts. Flood coolant. Use helical cutter.

Loss of accuracy (cannot hold size)

1. High cutting load causing deflection 2. Chip packing, between teeth 3. Chips not cleaned away before mounting new piece of work

Decrease number of teeth in contact with work or feed per tooth. Adjust cutting fluid to wash chips out of teeth.

Cutter rapidly dulls

1. Cutting load too great 2. Insufficient coolant

Decrease feed per tooth or number of teeth in contact. Add blending oil to coolant.

Poor surface finish

1. 2. 3. 4.

Cutter digs in (hogs into work)

1. Radial relief too great 2. Rake angle too large 3. Improper speed

Work bumishing

1. 2. 3. 4.

Cutter burns

1. Not enough lubricant 2. Speed too high

Add sulfur-based oil. Reduce cutting speed. Flood coolant.

Teeth breaking

1. Feed too high 2. Depth of cut too large

Decrease feed per tooth. Use cutter with more teeth. Reduce table feed rate.

Feed too high Tool dull Speed too low Not enough cutter teeth

Cut is too light Tool edge worn Insufficient radial relief Land too wide

Check tool angles.

Check to see if all teeth are set at same height.

Check to see that workpiece is not deflecting and is securely clamped. Enlarge feed per tooth. Sharpen cutter.

Adapted from Cutting Tool Engineering, October 1990, p. 90, by Peter Liebhold, museum specialist, Division of Engineering and Industry, the Smithsonian Institute, Washington, DC.

D1

D1

C aa

C1 C1 aa

D D 2.75

FIGURE 24-7 Solid end mills are often coated. Insert tooling end mills come in a variety of sizes and are mounted on taper shanks. (Courtesy J T. Black)

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Milling D6 B

Face mill

ø1 1/2" 5/8" 0.38" aa

H

D

B

B

B

Taper

Taper

Taper

G

C C

FIGURE 24-8 Face mills come in many different designs using many different insert geometries and different mounting arbors.

D D

Stub arbor mounting

C

Lock screw

D Lock screw

Plain end mills have multiple teeth that extend only about halfway toward the center on the end. They are used in milling slots, profiling, and facing narrow surfaces. Two-lip mills have two straight or helical teeth that extend to the center. Thus, they may be sunk into material, like a drill, and then fed lengthwise to form a groove, a slot, or a pocket. Shell end mills are solid multiple-tooth cutters, similar to plain end mills but without a shank. The center of the face is recessed to receive a screw head or nut for mounting the cutter on a separate shank or a stub arbor. One shank can hold any of several cutters and thus provides great economy for larger-sized end mills. Hollow end mills are tubular in cross section, with teeth only on the end but having internal clearance. They are used primarily on automatic screw machines for sizing cylindrical stock, producing a short cylindrical surface of accurate diameter. Face mills have a center hole so that they can be arbor mounted. Face-milling cutters are widely used in both horizontal- and vertical-spindle machine tools and come in a wide variety of sizes (diameters and heights) and geometries (round, square, triangular, etc.), as shown in Figure 24-8. The insert can usually be indexed four times and must be well supported. Either the power or the rigidity of the machine tool will be the limiting factor, although sometimes setup can be the limiting factor. The typical tooth geometry for a slab mill is shown in Figure 24-9. Here, a positive rake angle is shown. Another common type of arbor-mounted milling cutter is called a side mill because it cuts on the ends and sides of the cutters. Figure 24-10 shows the geometry of a staggered-tooth side-milling cutter.

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Milling Tools and Cutters

675

Clearance angle Relier angle

Positive redial rake angle Heel

Face

Fillet or gullet

FIGURE 24-9 The geometry of a slab milling cutter showing the main cutting angles.

Radial relief angle Peripheral cutting edge Tooth face Clearance surface Land Axial relief angle Heel

Flute

Clearance surface

Tooth

Offset

Radial rake angle (positive shown)

Chip space

Fillet

Concavity

Root radius or gullet Solid profile milling cutter (staggered tooth side shown)

Lip

Lip angle

ft ⫻ R ⫽ feed/mV

ft Cutter

D

fa ⫽ hi

D Ad

fa

D

d d=DOC fm

hi

FIGURE 24-10 The staggered tooth side mill can cut on both the ends and sides of the cutter so it can cut slots or grooves.

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Staggered-tooth milling cutters are narrow cylindrical cutters having staggered teeth, and with alternate teeth having opposite helix angles. They are ground to cut only on the periphery, but each tooth also has chip clearance ground on the protruding side. These cutters have a free cutting action that makes them particularly effective in milling deep slots. Staggered-tooth cutters are really special side-milling cutters, which are similar to plain milling cutters except that the teeth extend radially part way across one or both ends of the cylinder toward the center. The teeth may be either straight or helical. Frequently, these cutters are relatively narrow, being disk-like in shape. Two or more side-milling cutters often are spaced on an arbor to straddle the workpiece (called straddle milling), and two or more parallel surfaces are machined at once. The side-milling cutter can cut on sides and ends of the teeth, so it can cut slots or grooves. However, only a few teeth are engaged at any one point in time, causing heaving torsional vibrations. The average chip thickness, hi, will be less than the feed per tooth, ft. The actual feed per tooth, fa, will be less than feed per tooth selected, Ft, qffiffiffi according to fa ¼ hi Dd. See Figure 24-10. For example, a thickness (hi) of 0.004 in. corresponds to 0.012 in. feed per tooth in most side- and face-milling operations. If the radial depth of cut, d, is very small compared to the cutter diameter, D, use this formula: rffiffiffiffiffi D Feed per tooth ¼ f t ¼ 0:004 ðiptÞ d For calculating the table feed, use half the number of inserts in a full side and face mill to arrive at the effective number of teeth. Thus, Table feed rate (ipm) ¼ rpm Number of effective teeth feed per tooth. In Figure 24-11, insert-tooth side mills are arranged in a gang-milling setup to cut three slots in the workpiece simultaneously. Thus, the desired part geometry is repeatedly produced by the setup as the position of the cutters is fixed. However, in side- and face-milling operations, only a few teeth are engaged at any point in time, resulting in heavy torsional vibrations detrimental to the resulting machined product. A flywheel can solve this problem and in many cases be the key to improved productivity. For the gang milling as shown in Figure 24-11, the diameter of the flywheel should be as large as possible. (The moment of inertia increases with the square of the radius.) The best position of the flywheel is inboard on the arbor at A, but depending on the setup, this may not be possible, so then position B should be chosen. It is important that the distance between the cutters and flywheel be as small as possible. A flywheel can be built up from a number of carbon steel disks, each having a center hole and keyway to fit the arbor, so the weight can be easily varied. Interlocking slotting cutters consist of two cutters similar to side mills but made to operate as a unit for milling slots. The two cutters are adjusted to the desired width by inserting shims between them.

Spacers

Slots for spindle drive keys

Arbor

Shank with #50 taper

A

Insert V cutter tooth

Arbor nut Drive key

Arbor bearing surface for outboard support

B

FIGURE 24-11 Arbor (two views) used on a horizontal-spindle milling machine on left. On right, a gang-milling setup showing three side-milling cutters mounted on an arbor (A) with an outboard flywheel (B).

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Milling Tools and Cutters

Solid helical plain milling cutter

Helical rake angle (l.h. helix shown)

Helical teeth

Chips

FIGURE 24-12 The chips are formed progressively by the teeth of a plain helical-tooth milling cutter during up milling.

Feed

677

Face width

Chip

ed

Fe

1 3 Slitting saws are thin, plain milling cutters, usually from 32 to 16 in. thick, which have their sides slightly ‘‘dished’’ to provide clearance and prevent binding. They usually have more teeth per unit of diameter than ordinary plain milling cutters and are used for milling deep narrow slots and cutting-off operations. Another method of classification for face and end mill cutters relates to the direction of rotation. A right-hand cutter must rotate counterclockwise when viewed from the front end of the machine spindle. Similarly, a left-hand cutter must rotate clockwise. All other cutters can be reversed on the arbor to change them from one hand to the other. Positive rake angles are used on general-purpose HSS milling cutters. Negative rake angles are commonly used on carbide- and ceramic-tipped cutters employed in mass-production milling in order to obtain the greater strength and cooling capacity. TiN coating of these tools is quite common, resulting in significant increases in tool life. Plain milling cutters used for plain or slab milling have straight or helical teeth on the periphery and are used for milling flat surfaces. Helical mills (Figure 24-12) engage the work gradually, and usually more than one tooth cuts at a given time. This reduces shock and chattering tendencies and promotes a smoother surface. Consequently, this type of cutter usually is preferred over one with straight teeth. Angle milling cutters are made in two types: single angle and double angle. Angle cutters are used for milling slots of various angles or for milling the edges of workpieces to a desired angle. Single-angle cutters have teeth on the conical surface, usually at an angle of 45 to 60 degrees to the plane face. Double-angle cutters have V-shaped teeth, with both conical surfaces at an angle to the end faces but not necessarily at the same angle. The V-angle usually is 45, 60, or 90 degrees. Form milling cutters have the teeth ground to a special shape—usually an irregular contour—to produce a surface having a desired transverse contour. They must be sharpened by grinding only the tooth face, thereby retaining the original contour as long as the plane of the face remains unchanged with respect to the axis of rotation. Convex, concave, corner-rounding, and gear-tooth cutters are common examples (Figure 24-13). Solid HSS cutters of simple shape and reasonably small size are usually more economical in initial cost than inserted-blade cutters. However, inserted-blade cutters may be lowest in overall cost on large production jobs. Form-relieved cutters can be cost effective where intricately shaped cuts are needed. Solid or carbide insert tool cutters may need large volumes to be cost-justified by high-production requirements.

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Milling Radial rake angle (positive shown)

Radial relief angle

Tooth face

Axial relief angle

Offset Fillet Flute Tooth Solid form relieved milling cutter

FIGURE 24-13 Solid form relieved milling cutter, which would be mounted on an arbor in a horizontal milling machine.

Most larger-sized milling cutters are of the insert-tooth type. In such insert-tooth milling cutters, cutter body is made of steel, with the teeth made of high-speed steel, carbides, or TiN carbides, fastened to the body by various methods. An insert-tooth cutter uses indexable carbide or ceramic inserts, as shown in Figure 24-9. This type of construction reduces the amount of costly material that is required and can be used for any type of cutter, but it is most often used with face mills. T-slot cutters are integral-shank cutters with teeth on the periphery and both sides. They are used for milling the wide groove of a T-slot. To use them, the vertical groove must first be made with a slotting mill or an end mill to provide clearance for the shank. Because the T-slot cutter cuts on five surfaces simultaneously, it must be fed with care. Woodruff keyseat cutters are made for the single purpose of milling the semicylindrical seats required in shafts for Woodruff keys. They come in standard sizes corresponding to Woodruff key sizes. Those less than 2 in. in diameter have integral shanks; the larger sizes may be arbor mounted. Occasionally, fly cutters may be used for face milling or boring. Both operations may be done with a single tool at one setup. A single-point cutting tool is attached to a special shank, usually with provision for adjusting the effective radius of the cutting tool with respect to the axis of rotation. The cutting edge can be made in any desired shape and, because it is a single-point tool, is very easy to grind.

& 24.4 MACHINES FOR MILLING The four most common types of manually controlled milling machines are listed here in order of increasing power (and therefore metal removal capability): 1. Ram-type milling machines. 2. Column-and-knee-type milling machines: a. Horizontal spindle. b. Vertical spindle. 3. Fixed-bed-type milling machines. 4. Planer-type milling machines. Milling machines whose motions are electronically controlled are listed in order of increasing production capacity and decreasing flexibility: 1. Manual data input milling machines. 2. Programmable CNC milling machines. 3. Machining centers (tool changer and pallet exchange capability). 4. Flexible manufacturing cell and flexible manufacturing system. 5. Transfer lines.

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Machines for Milling

679

BASIC MILLING MACHINE CONSTRUCTION Most basic milling machines are of column-and-knee construction, employing the components and motions shown in Figure 24-14. The column, mounted on the base, is the main supporting frame for all the other parts and contains the spindle with its driving mechanism. This construction provides controlled motion of the worktable in three mutually perpendicular directions: (1) through the knee, moving vertically on ways on the front of the column; (2) through the saddle, moving transversely on ways on the knee; and (3) through the table, moving longitudinally on ways on the saddle. All these motions can be imparted by either manual or powered means. In most cases, a powered rapid traverse is provided in addition to the regular feed rates for use in setting up work and in returning the table at the end of a cut. The ram-type milling machine is one of the most versatile and popular milling machines, using the knee-and-column design. Ram-type machines have a head equipped with a motor-stopped pulley and belt drive as well as a spindle. The ram, mounted on horizontal ways at the top of the column, supports the head and permits positioning of the spindle with respect to the table. Ram-type milling machines are normally 10 hp or less and are suitable for light-duty milling, drilling, reaming, and so on (Figure 24-14). Milling machines having only the three mutually perpendicular table motions are called plain column-and-knee milling machines. These are available with both horizontal and vertical spindles (Figure 24-14). On the older, horizontal-spindle-type machines, an adjustable overarm provides an outboard bearing support for the end of the cutter arbor, which is shown in Figure 24-11 and 24-14. These machines are well suited for slab, side, or straddle milling. In some vertical-spindle machines, the spindle can be fed up and down, either by power or by hand. Vertical-spindle machines are especially well suited for face and end milling operations. They also are very useful for drilling and boring, particularly where holes must be spaced accurately in a horizontal plane, because of the controlled table motion. Turret-type column-and-knee milling machines have dual heads that can be swiveled about a horizontal axis on the end of a horizontally adjustable ram. This permits milling to be done horizontally, vertically, or at any angle. This added flexibility is advantageous when a variety of work has to be done, as in tool and die or experimental shops. They are available with either plain or universal tables.

Table

Column

Spindle

Overarm

Saddle

Base

Elevating screw

Knee

Ram-type miller

Horizontal spindle Vertical spindle ram-type

FIGURE 24-14 Major components of a plain column-and-knee-type milling machine, which can have horizontal spindle (shown on the left) or a turret type machine with a vertical spindle (shown on the right). The workpiece and workholder on the table can be translated in X, Y, and Z directions with respect to the tool.

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Bed-type vertical spindle

FIGURE 24-15 Bed-type vertical-spindle heavy-duty production machine tools for milling usually have three axes of motion.

Universal column-and-knee milling machines differ from plain column-and-knee machines in that the table is mounted on a housing that can be swiveled in a horizontal plane, thereby increasing its flexibility. Helices, as found in twist drills, milling cutters, and helical gear teeth, can be milled on universal machines.

BED-TYPE MILLING MACHINES In production manufacturing operations, ruggedness and the capability of making heavy cuts are of more importance than versatility. Bed-type milling machines (Figure 24-15) are made for these conditions. The table is mounted directly on the bed and has only longitudinal motion. The spindle head can be moved vertically in order to set up the machine for a given operation. Normally, once the setup is completed, the spindle head is clamped in position and no further motion of it occurs during machining. However, on some machines, vertical motion of the spindle occurs during each cycle. After such milling machines are set up, little skill is required to operate them, permitting faster learning time for the operators. Some machines of this type are equipped with automatic controls so that all the operator has to do is load and unload workpieces into the fixture and set the machine into operation. For stand-alone machines, a fixture can be located at each end of the table so that one workpiece can be loaded while another is being machined. Bed-type milling machines with single spindles are sometimes called simplex milling machines; they are made with both horizontal and vertical spindles. Bed-type machines also are made in duplex and triplex types, having two or three spindles respectively, permitting the simultaneous milling of two or three surfaces at a single pass.

PLANER-TYPE MILLING MACHINES Planer-type milling machines (Figure 24-16) utilize several milling heads, which can remove large amounts of metal while permitting the table and workpiece to feed quite slowly. Often only a single pass of the workpiece past the cutters is required. Through the use of different types of milling heads and cutters, a wide variety of surfaces can be machined with a single setup of the workpiece. This is an advantage when heavy workpieces are involved.

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Machines for Milling

681

FIGURE 24-16 Large planer-type milling machines shares its basic design with that of a planer with the planing tool replaced by a powered milling head (not shown). (American Machinist Special Report to Fundamentals of Milling, J Jablonowski, Feb, 1978)

ROTARY-TABLE MILLING MACHINES Some types of face milling in mass-production manufacturing are often done on rotarytable milling machines. Roughing and finishing cuts can be made in succession as the workpieces are moved past the several milling cutters while held in fixtures on the rotating table. The operator can load and unload the work without stopping the machine.

PROFILERS AND DUPLICATORS Milling machines that can duplicate external or internal geometries in two dimensions are called profilers or tracer-controlled machines. A tracing probe follows a two-dimensional pattern or template and, through electronic or hydraulic air-actuated mechanisms, controls the cutting spindles in two mutually perpendicular directions. All hydraulic tracers work basically the same way, in that they utilize a stylus connected to a precision servomechanism for each axis of control. The servos are connected to hydraulic actuators on the machine slides. As the stylus traces a template, the servos control the motion of the slides so that the milling cutter duplicates the template shape onto the workpiece. Duplicators produce forms in three dimensions and are widely used to machine molds and dies. Sometimes these machines are called die-sinking machines. They are used extensively in the aerospace industry to machine parts from wrought plate or bar stock as substitutes for forgings when the small number of parts required would make the cost of forging dies uneconomical. Many of these kinds of jobs are now done on NC and CNC-type machines; their applications are discussed in Chapter 39.

MILLING MACHINE SELECTION When purchasing or using a milling machine, consider the following issues: 1. Spindle orientation and rpm. 2. Machine capability (accuracy and precision).

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3. Machine capacity (size of workpieces). 4. Horsepower available at spindle (usually 70% of machine horsepower). 5. Automatic tool changing. The choice of spindle orientation, horizontal or vertical, depends on the parts to be machined. Relatively flat parts are usually done on vertical machines. Cubic parts are usually done on a horizontal machine, where chips tend to fall free of the part. Operations like slotting and side milling are best done on horizontal machines with outboard supports for the arbor. Use the largest-diameter arbor possible to reduce twist and deflection due to cutting forces. Machine capability refers to the tolerances, while capacity refers to the size of parts and the power available. As with all tooling applications, the tolerances that can be maintained in milling depend on the rigidity of the workpiece, the accuracy and rigidity of the machine spindle, the precision and accuracy of the workholding device, and the quality of the cutting tool itself. Milling produces forces that contribute to chatter and vibration because of the intermittent cutting action. Soft materials tend to adhere to the cutter teeth and make it more difficult to hold tolerances. Materials such as cast iron and aluminum are easy to mill. Within these criteria, properly maintained cutters used in rigid spindles on properly fixtured workpieces can expect to machine within tolerances with surface flatness tolerances of 0.001 in./ft. Such tolerances are also possible on ‘‘slotting’’ operations with milling cutters, but 10.001 in. to 10.002 in. is more probable. Flatness specifications are more difficult to maintain in steel and easier to maintain in some types of aluminum, cast iron, and other nonferrous material. Part size is the primary factor in selecting the machine size, but the length of the tooling as mounted in the spindle must be considered. Horsepower required at the spindle depends on the MRR and the materials (unit horsepower, HPs). Remember, coated inserts allow the MRR (the cutting speed) to be increased and available power may be exceeded. Finally, the capacity of the tool changers on machining centers is limited by the number, size, and weight of the tools—especially if large-diameter tools are being employed. These often have to be stored in every other space in the storage mechanism.

ACCESSORIES FOR MILLING MACHINES The usefulness of ordinary milling machines can be greatly extended by employing various accessories or attachments. Here are some examples. A horizontal milling machine can be equipped with a vertical milling attachment to permit vertical milling to be done. Ordinarily, heavy cuts cannot be made with such an attachment. The universal milling attachment (Figure 24-17) is similar to the vertical attachment but can be swiveled about both the axis of the milling machine spindle and a second, perpendicular axis to permit milling to be done at any angle. The universal dividing head is by far the most widely used milling machine accessory, providing a means for holding and indexing work through any desired arc of

Universal milling attachment

Spindle Index plate Crank

FIGURE 24-17 End milling a helical groove on a horizontalspindle milling machine using a universal dividing head and a universal milling attachment. (Courtesy of Cincinnati Milacron, Inc., Cincinnati, OH)

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Key Words

683

rotation. The work may be mounted between centers (Figure 24-17) or held in a chuck that is mounted in the spindle hole of the dividing head. The spindle can be tiled from about 5 degrees below horizontal to beyond the vertical position. Basically, a dividing head is a rugged, accurate, 40:1 worm-gear reduction unit. The spindle of the dividing head is rotated one revolution by turning the input crank 40 turns. An index plate mounted beneath the crank contains a number of holes, arranged in concentric circles and equally spaced, with each circle having a different number of holes. A plunger pin on the crank handle can be adjusted to engage the holes of any circle. This permits the crank to be turned an accurate, fractional part of a complete circle as represented by the increment between any two holes of a given circle on the index plate. Utilizing the 40:1 gear ratio and the proper hole circle on the index plate, the spindle can be rotated a precise amount by the application of either of the following rules: Number of turns of crank ¼

40 Cuts per revolution of work

¼

40 Holes index circle Cuts per revolution of work

Holes to be indexed

If the first rule is used, an index circle must be selected that has the proper number of holes to be divisible by the denominator of any resulting fractional portion of a turn of the crank. In using the second rule, the number of holes in the index circle must be such that the numerator of the fraction is an even multiple of the denominator. For example, if 24 cuts are to be taken about the circumference of a workpiece, the number of turns of the crank required would be 123. An index circle having 12 holes could be used with one full turn plus eight additional holes. The second rule would give the same result. Adjustable sector arms are provided on the index plate that can be set to a desired number of holes, less than a full turn, so that fractional turns can be made readily without the necessity for counting holes each time. Dividing heads are made having ratios other than 40:1.The ratio should be checked before using. Because each full turn of the crank on a standard dividing head represents 360/40, or 9 degrees of rotation of the spindle, indexing to a fraction of a degree can be obtained. Indexing can be done in three ways. Plain indexing is done solely by the use of the 40:1 ratio in the dividing head. In compound indexing, the index plate is moved forward or backward a number of hole spaces each time the crank handle is advanced. For differential indexing, the spindle and the index plate are connected by suitable gearing so that as the spindle is turned by means of the crank, the index plate is rotated a proportional amount. The dividing head can also be connected to the feed screw of the milling machine table by means of gearing. This procedure is used to provide a definite rotation of the workpiece with respect to the longitudinal movement of the table, as in cutting helical gears. This procedure is illustrated in Chapter 42, which is on the Web.

& KEY WORDS angle milling cutter arbor-mounted milling cutter bed-type milling machine climb (down) milling column-and-knee milling machine compound indexing conventional (up) milling cutting time die-sinking machine differential indexing down (climb) milling duplicator end milling face milling

face milling cutter fly cutter form milling cutter gang milling helical mill hollow end mill interlocking slotting cutter insert-tooth milling cutter interrupted cutting left-hand cutter machining center metal removal rate (MRR) milling milling cutters milling machines peripheral milling

plain column-and-knee milling machine plain end mill plain indexing plain milling cutter planer-type milling machine profiler ram-type milling machine right-hand cutter rotary-table milling machine sector arm shell end mill side-milling cutter slab milling

slitting saw staggered-tooth milling cutter straddle milling T-slot cutter turret-type column-andknee milling machine two-lip mill universal column-and-knee milling machine universal dividing head universal milling attachment up (conventional) milling Woodruff keyseat cutter

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CHAPTER 24

Milling

& REVIEW QUESTIONS 1. Suppose you wanted to machine a cast iron with BHN of 275. The process to be used is face milling and an HSS cutter is going to be used. What feed and speed values would you select? 2. Calculate the spindle rpm and table feed (ipm) for a facemilling machine after the speed and feed per tooth are selected. Use Figure 24-3 for reference. 3. Why must the number of teeth on the cutter be known when calculating milling machine table feed, in in./min? 4. Why is the question of up or down milling more critical in horizontal slab milling than in vertical-spindle (end or face) milling? 5. For producing flat surfaces in mass-production machining, how does face milling basically differ from peripheral milling? 6. Milling has a higher metal removal rate than planing. Why? 7. Which type of milling (up or down) is being done in Figures 24-1b, 24-1d, and 24-2b? 8. Why does down milling dull the cutter more rapidly than up milling when machining sand castings? 9. What parameters do you need to specify in order to calculate MRR in milling? 10. In Figure 24-2b, the tool material is carbide. What would you change in the process?

11. What is the advantage of a helical-tooth cutter over a straight-tooth cutter for slab milling? 12. What would the cutting force diagram for Fc look like if the cutter were performing climb milling? 13. Could the stub arbor–mounted face mill shown in Figure 24-9 be used to machine a T-slot? Why or why not? 14. In a typical solid arbor milling cutter shown in Figure 24-10, why are the teeth staggered? (Review the discussion of dynamics in Chapter 20.) 15. Make some sketches to show how you would you set up a plain column-and-knee milling machine to make it suitable for milling the top and sides of a large block. 16. Make some sketches to show how you would set up a horizontal milling machine to cut both sides of a block of metal simultaneously. 17. Explain how controlled movements of the work in three mutually perpendicular directions are obtained in columnand-knee-type milling machines. 18. What is the basic principle of a universal dividing head? 19. What is the purpose of the hole-circle plate on a universal dividing head?

& PROBLEMS 1. You have selected a feed per tooth and a cutting speed for a face milling process, using Table 24-1. Reasonable values for feed and speed are 0.010 in. per tooth and 200 sfpm. The cutter is 8 in. in diameter, as shown in Figure 24-9. Compute the input values for the machine tool. 2. How much time will be required for a milling machine to face mill an AISI 1020 steel surface BHN 150, that is 12 in. long and 5 in. wide, using a 6-in.-diameter, eight-tooth tungsten carbide inserted-tooth face mill cutter? Select values of feed per tooth and cutting speed from Table 24-1. 3. If the depth of cut is 0.35 in., what is the metal removal rate in Problem 2? 4. Estimate the power required for the operation of Problem 3. Do not forget to consider Figure 24-7. 5. Examine the pinion shown in Chapter 1. The slot on the left end must be produced by machining. Provide a process plan (a description [sketch] of how the part would set up in the machine for machining the slot and the details regarding cutting tools, such as material, sizes, and so on). Specify (select) the type of milling machine, the cutting parameters, and any other information needed to make this component. 6. A gray cast iron surface 6 in. wide and 18 in. long may be machined on either a vertical milling machine, using an 8-in.diameter face mill having eight inserted HSS teeth, or on a horizontal milling machine using an HSS slab mill with eight teeth on a 4-in. diameter. Which machine has the faster cutting time? 7. An operation is to be performed to machine three grooves on a number of parts shown in Figure 24-11. Setup time is 40 min on a shaper (not shown) and 30 min on the horizontal milling

machine. The direct time to machine each piece on the shaper is 14 min and on the miller is 6 min. Labor costs $10/hr. The charge for the use of the shaper is $10/hr and for the milling machine $20/hr. What is the breakeven quantity, below which the shaper is more economical than the mill? 8. In Figure 24-12, the feed is 0.006 in. per tooth. The cutter is rotating at an rpm that will produce the desired surface cutting speed of 125 sfpm. The cutter diameter is 3 in. The depth of cut is 0.5 in. The block is 2 in. wide. a. What is the feed rate, in inches per minute, of the milling machine table? b. What is the MRR for this situation? c. What is horsepower (HP) consumed by this process, assuming an 80% efficiency and a HPs value for this material of 1.8? 9. Suppose you want to do the job described in Problem 6 by slab milling. You have selected a 6-in.-diameter cutter with eight TiN-coated carbide teeth. The cutting speed will be 500 sfpm and the feed per tooth will be 0.010 in. per tooth. Determine the input parameters for the machine (rpm of arbor and table feed), then calculate the Tm and MRR. Compare these answers with what you got for slab milling the block with HSS teeth. 10. The Bridgeport vertical-spindle milling machine is perhaps the single most popular machine tool. Virtually every factory (or shop) that does machining has one or more of these type machines. Go to your nearest machine shop and find a Bridgeport, make a sketch to show how it works, and explain what makes it so popular.

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Case Study

Chapter 24

685

CASE STUDY

HSS versus Tungsten Carbide Milling

T

he KC Machine works, which does job shop machining, has received an order to make 40 duplicate pieces, made of AISI 4140 steel, which will require 1 hr/piece of actual cutting time if a high-speedsteel (M1) milling cutter is used. Abigail Langley, a new machinist, says the cutting time could be reduced significantly if the company would purchase a suitable tungsten carbide milling cutter. Brandon Kannan, the foreman for the milling area, says he does not believe that Abigail’s estimate is realistic, and he is not going to spend $450 (the current price from the vendor) of the

TABLE CS-24

company’s money on a carbide cutter that probably would not be used again. The machine hour rate, including labor for the shop is $40/hr. Abigail and Brandon have come to you, the manufacturing engineer (MfE) of the plant, for a decision on whether or not to buy the cutter, which is readily available from a local supplier. What factors should you consider in this situation? How much faster could the carbide cutter cut compared to the HSS cutter? See Table 24-1, in Chapter NaN. Based on your best guess as to the savings in actual cutting time per piece, who do you think is correct: Abigail or Brandon?

Representative Cutting Data

Material

Forces

Work

Tool

Back Rake (deg.)

Feed (ipt)

Width (in.)

Velocity (fpm)

Cutting (lb)

Thrust (lb)

AISI4140

HSS

0.0104

0.100

100

360

190

AISI4140

Carbide

0.011

0.15

540

540

156

AISI4145

Carbide

0.015

0.25

560

1190

560

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CHAPTER 25 SAWING, BROACHING, AND OTHER MACHINING PROCESSES 25.1 INTRODUCTION 25.2 INTRODUCTION TO SAWING Saw Blades Circular Saws Types of Sawing Machines Power Hacksaws Bandsawing Machines Cutting Fluids Feeds and Speeds Circular-Blade Sawing Machines Bandsawing Operations

25.3 INTRODUCTION TO BROACHING 25.4 FUNDAMENTALS OF BROACHING The Advantages and Limitations of Broaching Broach Design (The Cutting Tool) Broaching Speeds, Accuracy, and Finish Broaching Materials and Construction Sharpening Broaches

25.5 BROACHING MACHINES 25.6 INTRODUCTION TO SHAPING AND PLANING Machine Tools for Shaping Planing Machines Workholding and Setup on Planers 25.7 INTRODUCTION TO FILING Filing Machines Case Study: Cost Estimating––Planing vs. Milling

& 25.1 INTRODUCTION While milling, drilling, and turning make up the bulk of the machining processes, there are many other chipmaking (metal removal) processes. This chapter will cover sawing, broaching, shaping, planing, and filing.

& 25.2 INTRODUCTION TO SAWING Sawing is a basic machining process in which chips are produced by a succession of small cutting edges, or teeth, arranged in a narrow line on a saw ‘‘blade.’’ As shown in Figure 25-1, each tooth forms a chip progressively as it passes through the workpiece. The chips are contained within the spaces between successive teeth until the teeth pass from the work. Because sections of considerable size can be severed from the workpiece with the removal of only a small amount of the material in the form of chips, sawing is probably the most economical of the basic machining processes with respect to the waste of material and power consumption, and in many cases with respect to labor. In recent years, vast improvements have been made in saw blades (design and materials) and sawing machines, resulting in improved accuracy and precision of the process. Most sawing is done to sever bar stock and shapes into desired lengths for use in other operations. There are many cases in which sawing is used to produce desired shapes. Frequently, and especially for producing only a few parts, contour sawing may be more economical than any other machining process.

SAW BLADES Saw blades are made in three basic configurations. The first type, commonly called a hacksaw blade, is straight, relatively rigid, and of limited length, with teeth on one edge. The second type, called a bandsaw, is sufficiently flexible that a long length can be formed into a continuous band with teeth on one edge. The third form is a rigid disk having teeth on the periphery; these are called circular saws or cold saws. Figure 25-2 gives the standard nomenclature for the widely used saw blades. All saw blades have certain common and basic features: (1) material, (2) tooth form, (3) tooth spacing, (4) tooth set, and (5) blade thickness or gage. Small hacksaw blades are usually made entirely of tungsten or molybdenum high-speed steel. Blades

686

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687

Introduction to Sawing

Gage Saw blade Gravity

Slot or kerf

Saw blade body

Feed

Cutting speed V Feed

Kerf Work

Chips Side view of workpiece

FIGURE 25-1

End view

Formation of chips in sawing.

for power-operated hacksaws are often made with teeth cut from a strip of high-speed steel that has been electron-beam-welded to the heavy main portion of the blade, which is made from a tougher and cheaper alloy steel (see Figure 25-2). Bandsaw blades are frequently made with this same type of construction but with the main portion of the blade made of relatively thin, high-tensile-strength alloy steel to provide the required flexibility. Bandsaw blades are also available with tungsten carbide teeth and TiN coatings. The three most common tooth forms are regular, skip tooth, and hook. Tooth spacing, abbreviated as TPI for teeth per inch, is very important in all sawing because it determines three factors. First, it controls the size of the teeth. From the viewpoint of strength, large teeth are desirable. Second, tooth spacing determines the space (gullet) available to contain the chip that is formed. The chip cannot drop from this space until it emerges from the slot cut in the workpiece, called the kerf. Tooth set refers to the manner in which the teeth are offset from the centerline in order to make the kerf wider than the gage (the thickness of the back) of the blade. This allows the saw to move more freely in the kerf, reducing rubbing, friction, and heating. The kerf–gullet space must be such that there is no crowding of the chip. Chips should not become wedged between the teeth and not drop out of the gullet when the saw emerges from the cut. Thirdly, tooth spacing determines how many teeth will engage the workpiece at any point during the cut. This is very important in cutting thin material, such as tubing. At least two teeth should be in contact with the work at all times. If the teeth are too coarse, only one tooth rests on the work at a given time, permitting the saw to rock, and the teeth may be stripped from the saw. Raker-tooth saws are used in cutting most steel and iron. Straight-set teeth are used for sawing brass, copper, and plastics. Saws with wave-set teeth are used primarily for cutting thin sheets and thin-walled tubing. The gage or blade thickness of nearly all hand hacksaw blades is 0.025 in. Saw blades for power hacksaws vary in thickness from 0.050 to 0.100 in. Hand hacksaw blades come in two standard lengths, 10 and 12 in. All are 12 in. wide. Blades for power hacksaws vary in length from 12 to 24 in. and in width from 1 to 2 in. Wider and thicker blades are desirable for heavy-duty work. As a general rule, in hacksawing the blade should be at least twice as long as the maximum length of cut that is to be made. Bandsaw blades are available in straight, raker, wave, or combination sets. In order to reduce the noise from high-speed bandsawing, it is becoming increasingly common to use blades that have more than one pitch, size of teeth, and type of set. Blade width is very important in bandsawing because it determines the minimum radius that 1 to 12 in., although wider blades can be can be cut. The most common widths are from 16

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CHAPTER 25

MATRIX MODIFIED MIX-TOOTH The best all-purpose welded-edge blade for sawing varying sizes, shapes, and cross sections. Cobalt-tough for cutting wide range of materials. Welded to length and coil stock.

Sawing, Broaching, and Other Machining Processes

M-42 COBALT WELDED-EDGE

M-2 HIGH-SPEED WELDED-EDGE

HARD BACK CARBON

For high-production cutting of solids, superalloys, tool steels, high-temperature alloys. Welded to length and coil stock.

The original and widely used welded-edge band blade for general-purpose sawing. Welded to length and coil stock.

Hardened back provides greater beam strength for more accurate sawing. Welded to length and coil stock.

FLEXIBLE BACK CARBON Recommended for contour saws running over 3000 SFPM. Welded to length and coil stock.

Standard design has zero rake for general purpose sawing

Gage

Common Tooth Sets

Gullet depth

Tooth face

Tooth back clearance angle

Tooth rake angle Tooth back

Tooth gullet

Tooth spacing on pitch

Skip-tooth blade clears chips, cuts nonferrous, nonmetalics

Side clearance angle

Set.

Side clearance

Width

Body

Hook tooth with 10° rake for large sections good for cast iron

Variable pitch can change by section or individually, improves blade life

Raker set has a straight tooth between one left and one right

Wavy set for thin sections has progressive set, both directions

Variable pitch with 5° rake is more aggressive, smooth cutting

Straight set—left, then right—is for better finish

Cluster set has only a few straight teeth

FIGURE 25-2

Variable pitch with 10° rake sheds chips better

Bandsaw blade designs and nomenclature (above). Tooth set patterns (left) and tooth designs (right).

obtained. Because wider blades are stronger, select the widest blade possible. However, cutting small radii requires a narrower and weaker blade. Bandsaw blades come in tooth spacings from 2 to 32 teeth per inch (TPI). Hand hacksaw blades have 14 to 32 TPI. In order to make it easier to start a cut, some hand hacksaw blades are made with a short section at the forward end having teeth of a special form with negative rake angles. Tooth spacing for power hacksaw blades ranges from 4 to 18 TPI.

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Introduction to Sawing

689

Bevel tooth Bevel tooth

Bevel tooth

Circular saw blade

Workpiece

FIGURE 25-3 Circular sawing a structural shape on the left. Examples of circular saws (left to right): an insert tooth, a segmental tooth, and an integral-tooth circular saw blades on the right. (Courtesy J T. Black)

CIRCULAR SAWS Circular saws for cutting metal are often called cold saws to distinguish them from friction-type disk saws. Friction saws do not make chips but rather heat the metal to the melting temperature at the point of metal removal. Cold saws cut rapidly and produce chips like a milling cutter while producing surfaces that are comparable in smoothness and accuracy with surfaces made by slitting saws in a milling machine or by a cutoff tool in a lathe. Disk, or circular, saws necessarily differ somewhat from straight-blade forms. The sizes up to about 18 in. in diameter have an integral-tooth design with teeth cut directly into the disk (Figure 25-3). Larger saws use either segmented or inserted teeth. The teeth are made of high-speed steel or tungsten carbide. The remainder of the disk is made of ordinary, less expensive, and tougher steel. Segmental blades are composed of segments mounted around the periphery of the disk, usually fitted with a tongue and groove and fastened by means of screws or rivets. Each segment contains several teeth. If a single tooth is broken, only one segment needs be replaced to restore the saw to an operating condition. As shown in Figure 25-3, circular saw teeth are usually beveled. A common tooth form has every other tooth beveled on both sides; that is, the first tooth is beveled on the left side, the second tooth on both sides, the third tooth on the right side, the fourth tooth on the left side, and so forth. Another method is to bevel the opposite sides of successive teeth. Beveling is done to produce a smoother cut. Precision circular saws made from carbide, which are becoming available, are very thin (0.03 in.) and have high cutting-off accuracy, around 0.00008 in., with negligible burrs.

TYPES OF SAWING MACHINES Metal-sawing machines may be classified as follows: 1. Reciprocating saw a. Manual hacksaw. b. Power hacksaw (Figure 25-4). c. Abrasive disc. 2. Bandsaw (Figure 25-5) a. Vertical cutoff. b. Horizontal cutoff.

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Sawing, Broaching, and Other Machining Processes

c. Combination cutoff and contour. d. Friction. 3. Circular saw (Figure 25-6) a. Cold saw. b. Steel friction disk.

POWER HACKSAWS As the name implies, power hacksaws are machines that mechanically reciprocate a large hacksaw blade (Figure 25-4). These machines consist of a bed, a workholding frame, a power mechanism for reciprocating the saw frame, and some type of feeding mechanism. Because of the inherent inefficiency of cutting in only one stroke direction, they have often been replaced by more efficient, horizontal bandsawing machines.

BANDSAWING MACHINES The earliest metalcutting bandsawing machines were direct adaptations from woodcutting bandsaws. Modern machines of this type are much more sophisticated and versatile and have been developed specifically for metal cutting. To a large degree they were made possible by the development of vastly better and more flexible bandsaw blades and simple flash-welding equipment, which can weld the two ends of a strip of bandsaw blade together to form a band of any desired length. Three basic types of bandsawing machines are in common use. Horizontal metal-cutting bandsawing machines were developed to combine the flexibility of reciprocating power hacksaws and the continuous cutting action of vertical

Hydraulic or gravity pressure

Work vise

Eccentric drive on

cti

g

ttin

e dir

Cu

Tough alloy steel back

High-speed steel teeth

Electric welded

FIGURE 25-4 The power hacksaw blade reciprocates and uses gravity or hydraulic pressure to feed the saw through the workpiece. (Courtesy J T. Black)

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Introduction to Sawing

691

bandsaws. These heavy-duty automatic bandsaws feed the saw vertically by a hydraulic mechanism and have automatic stock feed that can be set to feed the stock laterally any desired distance after a cut is completed and automatically clamp it for the next cut. Such machines can be arranged to hold, clamp, and cut several bars of material simultaneously. Computer numerical control (CNC) bandsaws are available with automatic storage and retrieval systems for the bar stock. Smaller and less expensive types have swing-frame construction, with the bandsaw head mounted in a pivot on the rear of the machine. Feed is accomplished by gravity through rotation of the head about the pivot point. Because of their continuous cutting action, horizontal bandsawing machines are very efficient (Figure 25-5). Vertical, cutoff bandsawing machines are designed primarily for cutoff work on single stationary workpieces that can be held on a table. On many machines the blade mechanism can be tilted to about 45 degrees, as shown in Figure 25-6, to permit cutting at an angle. They usually have automatic power feed of the blade into the work, automatic stops, and provision for supplying coolant. Combination cutoff and contour bandsawing machines (Figure 25-7) can be used not only for cutoff work but also for contour sawing. They are widely used for cutting irregular shapes in connection with making dies and the production of small numbers of parts and are often equipped with rotary tables. Additional features on these machines include a table that pivots so that it can be tilted to any angle up to 45 degrees. Usually, these machines have a small flash butt welder on the vertical column, so that a straight length of bandsaw blade can be welded quickly into a continuous band. A small grinding wheel is located beneath the welder so that the flash can be ground from the weld to provide a smooth joint that will pass through the saw guides. This welding and grinding unit makes it possible to cut internal openings in a part by first drilling a hole, inserting one end of the saw blade through the hole, and then butt welding the two ends together. When the cut is finished, the band is again cut apart and removed from the opening. The cutting speed of the saw blade can be varied continuously over a wide range to provide correct operating conditions for any material. A method of power feeding the work is provided, sometimes gravity-actuated. Contour-sawing machines are made in a wide range of sizes, the principal size dimension being the throat depth. Sizes from 12 to 72 in. are available. The speeds available on most machines range from about 50 to 2000 ft/min. Modern horizontal bandsaws are accurate to 0.002 in. per vertical inch of cut but have feeding accuracy of only 0.005 in., subject to the size of the stock and the feed rate. Repeatability from one feed to the next may be 0.010 to 0.020 in. CNC-controlled sawing centers with microprocessor controls have opened up new automation aspects for sawing. Such control systems can improve accuracy to within 0.005 in. over entire cuts by controlling saw speed, blade feed pressure, and feed rate. Special bandsawing machines are available with very high speed ranges, up to 14,000 ft/min. These are known as friction bandsawing machines. Material is not cut by chip formation. Instead, the friction between the rapidly moving saw blade and the work is sufficient to raise the temperature of the material at the end of the kerf to or just below the melting point, where its strength is very low. The saw blade then pulls the molten, or weakened, material out of the kerf. Consequently, the blades do not need to be sharp; they frequently have no teeth—only occasional notches in the blade to aid in removing the metal. Almost any material, including ceramics, can be cut by friction sawing. Because only a small portion of the blade is in contact with the work for an instant and then is cooled by its passage through the air, it remains cool. Usually, the major portion of the work, away from contact with the saw blade, also remains quite cool. The metal adjacent to the kerf is heat affected, recast, and sometimes harder than the bulk metal. It is also a very rapid method for trimming the flash from sheet metal parts, castings, and forgings.

CUTTING FLUIDS Cutting fluids should be used for all bandsawing, with the exception that cast iron is always cut dry. Commercially available oils or light cutting oils will give good results in

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Sawing, Broaching, and Other Machining Processes

Band saws Heavy-duty automatic machines

Materials • All machinable metals • Single piece, bundled or stacked materials • Superalloys

• Vertical or horizontal

Heavy-duty automatic machines • Vertical or horizontal

• Moderate to difficult Machinability steels • High-temperature alloys • Superalloys • Tool steels • Solids or heavy Wall tubing (For Example: Inconel, waspalloy, hastelloy, D grade steels, stainless steels etc.)

Light- to mediumduty machines • Manual or automatic • Vertical or horizontal • Contour and cutoff machine

Standard lightduty machines Vertical or horizontal

FIGURE 25-5 Bandsawing machines can be vertical or horizontal, contour and cutoff types.

Contour machines + friction saws up to 15,000 sfpm

• Easy to moderate machinability steels • Solids, shapes, pipe, tubing, structurals • Bundled or stacked materials (For Example: Alloy steels, carbon steels, structural steels, tools steels, stainless steels, air hardening, die steels, etc.) • Nonmetalic materials • Mild steels (easy machinability) • Nonferrous metals • Low-alloys steels (For Example: Aluminum, cast iron, cold rolled, annealed alloy steels, low carbon steels, wood, plastic)

cutting ferrous materials. Beeswax or paraffin are common lubricants for cutting aluminum and aluminum alloys.

FEEDS AND SPEEDS Because of the many different types of feed involved in bandsawing, it is not practical to provide tabular feed or pressure data. Under general conditions, however, an even

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SECTION 25.2

FIGURE 25-6 Vertical bandsaw setup to cut pipe. Inset shows the head tilted to 45 degrees.

Introduction to Sawing

pre-programming of 3 functions: 1: Cut-off length 2: Number of cuts to be made 3: Angle of cut (left, right of vertical) and vertical

Saw blade Flash welder

Blade

FIGURE 25-7 Contour bandsawing on vertical bandsawing machine, shown in inset. (Courtesy J T. Black)

693

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Sawing, Broaching, and Other Machining Processes

pressure, without forcing the work, gives best results. A nicely curled chip usually indicates an ideal feed pressure. Burned or discolored chips indicate excessive pressure, which can cause tooth breakage and premature wear. Most bandsaws provide recommended cutting speed information right on the machine, depending on the material being sawed. In general, HSS blades are run at 200 to 300 ft/min when cutting 1-in.-thick, low- and medium-carbon steels. For high-carbon steels, alloy steels, and tool and die steels, the range is from 150 to 225 ft/min, and most stainless steels are cut at 100 to 125 ft/min.

CIRCULAR-BLADE SAWING MACHINES Machines employing rotating circular or cold saw blades are used exclusively for cutoff work. These range from small, simple types, in which the saw is fed manually, to very large saws having power feed and built-in coolant systems, commonly used for cutting off hot-rolled shapes as they come from a rolling mill. In some cases, friction saws are used for this purpose, having disks up to 6 ft in diameter and operating at surface speeds up to 25,000 ft/min. Steel sections up to 24 in. can be cut in less than 1 min by this technique. Although technically not a sawing operation, cutoff work up to about 6 in. is often done utilizing thin, abrasive disks. The equipment used is the same as for sawing. It has the advantage that very hard materials that would be very difficult to saw can be cut readily. A thin rubber- or resinoid-bonded abrasive wheel is used. Usually, a somewhat smoother surface is produced.

BANDSAWING OPERATION The extremely sharptooth points and edges on new saw blades requires a brief ‘‘breaking-in’’ period before operating at full feed pressure. Recommended procedure is to reduce the feed pressure by 50% for the first 50 to 100 in.2 of material cut and then gradually increase the feed rate to full. Figure 25-8 provides some solutions to the typical problems incurred in bandsawing.

& 25.3 INTRODUCTION TO BROACHING The process of broaching is one of the most productive of the basic machining processes. The machine tool is called a broaching machine or a broach, and the cutting tool is also called the broach. Figure 25-9 shows the basic shape of a conventional pull broach and the machine tool used to pull the cutting tool through the workpiece. The details of the tool are shown in Figure 25-10. In this figure, P is the pitch, ns is the number of semiroughing teeth, and nf is the number of finishing teeth where the rise per tooth gets smaller from rough to finish. The feed per tooth in broaching is the change in height of successive teeth. This is called the rise per tooth (RPT, or tr). Broaching looks similar to sawing except that the saw makes many passes through the cut, whereas the broach produces a finished part in one pass. The heart of this process lies in the broaching tool, in which roughing, semifinishing, and finishing teeth are combined into one tool, as shown in Figure 25-11. Broaching is unique in that it is the only one of the basic machining processes in which feed, which determines the chip thickness, is built into the cutting tool. The machined surface is always the inverse of the profile of the broach, and, in most cases, it is produced with a single linear stroke of the tool across the workpiece (or the workpiece across the broach). Broaching competes economically with milling and boring and is capable of producing precision-machined surfaces. The broach finishes an entire surface in a single pass. Broaches are used in production to finish holes, splines, and flat surfaces. Typical workpieces include small to medium-sized castings, forgings, screw-machine parts, and stampings. This rise per tooth (RPT), also known as step or the feed per tooth, determines the amount of material removed, see Figure 25-11. No feeding of the broaching tool is required. The frontal contour of the teeth determines the shape of the resulting machined surface. As the result of these conditions being built into the tool, no complex motion of the tool relative to the workpiece is required and the need for highly skilled machine operators is minimized.

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Fundamentals of Broaching

695

PROBLEM

PREMATURE BLADE BREAKAGE Straight Break Indicates fatigue

PREMATURE DULLING OF TEETH Material

Material

INACCURATE CUT BAND LEADING IN CUT

BHIP WELDING TEETH FRACTURE Back of tooth indicates work spinning in clamps

FIGURE 25-8 Bandsawing problems, causes, and solutions.

IRREGULAR BREAK Indicates material movement

Figure 25-9 shows a pull broach in a vertical pull-down broaching machine. The pull end of the broach is passed through the part, and a key mates to the slot. The broach is pulled through the part. The broach is retracted (pulled up) out of the part. The part is transferred from the left fixture to the right fixture. One finished part is completed in every manufacturing cycle.

& 25.4 FUNDAMENTALS OF BROACHING In broaching, the tool (or work) is translated past the work (or tool) with a single stroke of velocity, V. The feed is provided by a gradual increase in height of successive teeth. The rise per tooth varies depending on whether the tooth is for roughing (tr), semifinishing (ts), or final sizing or finishing (tf). In a typical broach there are three to five semifinishing and

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TEETH STRIPPING WEAR ON BACK OF BLADES

ROUGH CUT Washboard surface Vibration and or chatter

WEAR LINES, LOSS OF SET

TWISTED BLADE Profile sawing

BLADE WEAR

FIGURE 25-8

(Continued)

Teeth bleed

finishing teeth specified. The number of roughing teeth must be determined so that broach length, which is needed to estimate the cutting time, can be calculated. Other lengths needed for a typical pull broach are shown in Figure 25-10. The chip breakers in the first section of roughing teeth may be extended to more teeth if the cut is heavy or material difficult to machine. The distance between the teeth, called the pitch P, is important because it determines the tooth construction and strengths and the number of teeth actually cutting at a given instant. It is preferable that at least two teeth be in contact with the workpiece at any instant. The pitch or distance between teeth is pffiffiffiffiffiffiffi ð25-1Þ P ffi 35 Lw where length of cut usually equals Lw, as shown in Figure 25-11.

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Fundamentals of Broaching

Rear pilot

Notched tail

SECTION 25.4

See Figure 25-11 for detail.

(a)

Cutting motion Rear pilot

Chip breakers Slot

Pull end D2

1 D 6 1 Section A–A'

(b)

Pull end

Slot

FIGURE 25-9 Vertical pulldown broaching machine shown with parts in position ready for the two broaches to be inserted. An extra part is shown lying at the front of the machine. (Courtesy J T. Black)

Root diameter

Pull broach

Cutting motion

C25

Root diameter

Front pilot

tooth rise only in this section

Roughing teeth

D1 Shank length Ls

P(nr + ns + nf)

A

Notched tail

A⬘ SemiFinishing Follower finishing end teeth teeth LRP

Total broach length P– D– L– R– α– γ– RPT –

pitch of teeth depth of teeth (0.4P) land behind cutting edge (0.25P) radius of gullet (.25P) hook angle or rake angle backoff angle or clearance angle rise per tooth (chip load), tr

FIGURE 25-10 (a) Photo of pull broach. (b) Basic shape and nomenclature for a conventional pull (hole) broach. Section A–A’ shows the cross section of a tooth. P ¼ pitch; nr ¼ number of roughing teeth; ns ¼ number of semifinishing teeth; nf ¼ number of finishing teeth.

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Sawing, Broaching, and Other Machining Processes Lw

P α γ

tr

RPT

RPT or tr

L

Workpiece + gullet D

Flank

+ R

+

Chip

Body of broach

Body of broach traveling at speed V

FIGURE 25-11 The feed in broaching depends on the rise per tooth, tr (RPT). The sum of the RPT gives the depth of cut, DOC. P ¼ pitch of teeth; D ¼ depth of teeth (0.4P); L ¼ land behind cutting edge (0.25P); R ¼ radius of gullet (0.25P); a ¼ hook angle or rake angle; g ¼ back-off angle or clearance angle.

The number of roughing teeth is DOC ns ts nf tf nr ¼ tr

ð25-2Þ

where DOC is the total amount of metal to be removed and tr is the rise per tooth. The overall length of the broach for a pull broach is ð25-3Þ LB ¼ nr þ ns þ nf P þ Ls þ LRP The length of stroke is L ¼ LB Lw, in inches, if the broach moves past work or Lw þ LB if the work moves past the broach. The cutting time is Tm ¼

L 12V

ð25-4Þ

where V is the cutting speed, in surface feet per minute (sfpm). The metal removal rate (MRR) depends on the number of teeth (roughing) contacting the work. MRR ðper toothÞ ¼ 12tr WV in:3 =min per roughing tooth

ð25-5Þ

where W is the width of broach tooth. The number of roughing teeth in contact with part n ffi Lw =P for a broach longer than the part. MRR ðfor processÞ ¼ 12tr WV in:3 =min

ð25-6Þ

where n is usually rounded off to the next-largest whole number. The pull broach must be strong enough so that it will not be pulled apart. The strength of a pull broach is determined by its minimum cross section, which occurs either at the root of the first tooth or at the pull end: allowable pull ¼

area of minimum section YS of broach material factor of safety

ð25-7Þ

where YS is yield strength. The push broach must be strong enough so that it will not buckle. If the length-todiameter ratio, L/Dr, is greater than 25, the broach must be considered a long column that can buckle if overloaded. Let L ¼ Length from push end to first tooth, Dr ¼ Root diameter at 0.5L, and S ¼ Factor of safety: allowable load ¼

13:5 106 D4r SL2

For L/Dr less than 25, the normal broach loads are not critical.

ð25-8Þ

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Fundamentals of Broaching

699

Calculation of the total push or pull load depends on the number of teeth engaged, n, estimated from Lw/P; the width of the cut, W; the RPT per tooth engaged, tr; and the shear strength of the metal being machined. The force necessary to operate a broach depends on the material being broached, the conditions of the tool, and the nature of the process. An empirical constant is required in the force calculation to account for the large amount of rubbing (friction) between the tool, the chips captured in the tooth gullet, and the workpiece. Let FCB be the broach pull force in pounds: F CB ffi 5t s ntr W

ð25-9Þ

where t s is flow stress. Here, t s found in Chapter 20, depends on the hardness for the metal. This force estimate can be used to estimate the horsepower needed for the broaching machine.

THE ADVANTAGES AND LIMITATIONS OF BROACHING Because of the features built into a broach, it is a simple and rapid method of machining. There is a close relationship among the contour of the surface to be produced, the amount of material that must be removed, and the design of the broach. For example, the total depth of the material to be removed cannot exceed the total step provided in the broach, and the step of each tooth must be sufficient to provide proper chip thickness for the type of material to be machined. Consequently, either a special broach must be made for each job, or the workpiece must be designed so that a standard broach can be used. Broaching is widely used and particularly well suited for mass production because the volume can easily justify the cost of the broaching tool, which can be easily $15,000 to $30,000 per tool. It is also used for certain simple and standardized shapes, such as keyways, where inexpensive standard broaches can be used. Broaching was originally developed for machining internal keyways. However, its obvious advantages quickly led to its development for mass-production machining of various surfaces, such as flat, interior or exterior, cylindrical or semicylindrical, and many irregular surfaces. Because there are few limitations as to the contour form that broach teeth may have, there is almost no limitation in the shape of surfaces that can be produced by broaching. The only physical limitations are that there must be no obstruction to interfere with the passage of the entire tool over the surface to be machined and that the workpiece must be strong enough to withstand the forces involved. In internal broaching, a hole must exist in the workpiece into which the broach may enter. Such a hole can be made by drilling, boring, or coring. Broaching usually produces better accuracy and finish than can be obtained by drilling, boring, or reaming. Although the relative motion between the broaching tool and the work usually is a single linear one, a rotational motion can be added to permit the broaching of spiral splines or gun-barrel rifling.

BROACH DESIGN (THE CUTTING TOOL) Broaches commonly are classified by the following design features: Purpose

Motion

Construction

Function

Single

Push

Solid

Roughing

Combination

Pull

Built-up

Sizing

Stationary

Burnishing

Figure 25-10 shows the principal components of a pull broach and the shape and arrangement of the teeth. Each tooth is essentially a single-edge cutting tool, arranged much like the teeth on a saw except for the step, which determines the depth cut by each tooth. The rise per tooth, which determines the chip load, varies from about 0.006 in. for roughing teeth in machining free-cutting steel to a minimum 0.001 in. for finishing teeth. Typically, the RPT is 0.003 to 0.006 in. in surface broaching and 0.0012 to 0.0025 in. on

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the diameter for internal broaching. The exact amount depends on several factors. Toolarge cuts impose undue stresses on the teeth and the work; too-small cuts result in rubbing rather than cutting action. The strength and ductility of the metal being cut are the primary factors. Where it is desirable for each tooth to take a deep cut, as in broaching castings or forgings that have a hard, abrasive surface layer, rotor-cut or jump-cut tooth design may be used (Figure 25-12). In this design, two or three teeth in succession have the same diameter, or height, but each tooth of the group is notched or cut away so that it cuts only a portion of the circumference or width. This permits deeper but narrower cuts by each tooth without increasing the total load per tooth. This tooth design also reduces the forces and the power requirements. Chip-breaker notches are also used on round broaches to break up the chips (Figure 25-12b).

Rotor teeth

Finishing teeth

Cut direction

(a) Rotor- or jump-tooth broach design.

(b) Round, push-type broach with chip-breaking notches on alternate teeth except at the finishing end. Relieved portion of cutting edge

Series of 4 teeth of same height

Top view

End view

Side view

(c) Notched tooth, flat broach.

(d) Progressive surface broach.

FIGURE 25-12 Methods to decrease force or break up chip rings in broaches: (a) rotor or jump tooth; (b) notched tooth, round; (c) notched tooth, flat design (overlapping teeth permit large RPTs without increasing chip load); (d) progressive tooth design for flat broach. (Federal Broach Holdings LLC 1961 Sullivan Drive, Harrison, MI 48625)

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Fundamentals of Broaching

701

A similar idea can be used for flat surfaces. Tooth loads and cutting forces also can be reduced by using the double-cut construction, shown in Figure 25-12c. Four consecutive teeth get progressively wider. The teeth remove metal over only a portion of their width until the fourth tooth completes the cut. Another technique for reducing tooth loads utilizes the principle illustrated in Figure 25-12d. Employed primarily for broaching wide, flat surfaces, the first few teeth in progressive broaches completely machine the center, while succeeding teeth are offset in two groups to complete the remainder of the surface. Rotor, double-cut, and progressive designs require the broach to be made longer than if normal teeth were used, and they therefore can be used only on a machine having adequate stroke length. The cutting edges of the teeth on surface broaches may be either normal to the direction of motion or at an angle of from 5 to 20 degrees. The latter, shear-cut broaches, provide smoother cutting action with less tendency to vibrate. Other shapes that can be broached are shown in Figure 25-13 along with push- or pull-type broaches used for the job. The pitch of the teeth and the gullet between them must be sufficient to provide ample room for the chips. All chips produced by a given tooth during its passage over the full length of the workpiece must be contained in the space between successive teeth. At the same time, it is desirable to have the pitch sufficiently small so that at least two or three teeth are cutting at all times. The hook determines the primary rake angle and is a function of the material being cut. It is 15 to 20 degrees for steel and 6 to 8 degrees for cast iron. Back-off or end clearance angles are from 1 to 3 degrees to prevent rubbing. Most of the metal removal is done by the roughing teeth. Semifinishing teeth provide surface smoothness, whereas finishing teeth produce exact size. On a new broach, all the finishing teeth are usually the same size. As the first finishing teeth become worn, those behind continue the sizing function. On some round broaches, burnishing teeth are provided for finishing. These teeth have no cutting edges but are rounded disks of hard steel or carbide that are from 0.001 to 0.003 in. larger than the size of the hole.

1 Pull-type broach for sizing width and depth of slot in one operation.

2 Push-type 8 point or star-shaped broach.

3 Pull-type single-pass keyway broach with threaded type pull end. 4 Pull-type broach for producing four inverted keyways.

5 Push-type cut-and-finish keyway broach will cut an internal keyway, deburr the keyway, and finish the bore– all in one pass.

6 Push-type broach to cut two keyways 180 degrees apart in one pass. Can also be made in 3 and 4 Keyway style.

7 Push-type "D" hole broach. Can also be made in "double D" style.

8 Push-type rectangular broach (shear angle) for sizing rectangular hole, radar wave guide flanges, etc.

FIGURE 25-13 Examples of push- or pull-type broaches. (Courtesy The duMONT Company, LLC is the sole owner of rights to this material and it does not infringe upon the copyright or other rights of anyone.)

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The resulting rubbing action smoothes and sizes the hole. They are used primarily on cast iron and nonferrous metals. The pull end of a broach provides a means of quickly attaching the broach to the pulling mechanism. The front pilot aligns the broach in the hole before it begins to cut, and the rear pilot keeps the tool square with the finished hole as it leaves the workpiece. Shank length must be sufficient to permit the broach to pass through the workpiece and be attached to the puller before the first roughing tooth engages the work. If a broach is to be used on a vertical machine that has a tool-handling mechanism, a tail is necessary. A broach should not be used to remove a greater depth of metal than that for which it is designed—the sum of the steps of all the teeth. In designing workpieces, a minimum of 0.020 in. should be provided on surfaces that are to be broached, and about 0.025 in. is the practical maximum.

BROACHING SPEEDS, ACCURACY, AND FINISH Depending on the metal being cut, cutting speeds for broaching range from low (25 to 20 sfpm) to high while completing the surface in a single stroke, so the productivity is high. A complete cycle usually requires only from 5 to 30 s, with most of that time being taken up by the return stroke, broach handling, and workpiece loading and unloading. Such cutting conditions facilitate cooling and lubrication and result in very low tool wear rates, which reduce the necessity for frequent resharpening and prolong the life of the expensive broaching tool. For a given cutting speed and material, the force required to pull or push a broach is a function of the tooth width, the step, and the number of teeth cutting. Consequently, it is necessary to design or specify a broach within the stroke length and power limitations of the machine on which it is to be used. The average machining precision is typically 0.001-in.-(0.02-mm) tolerance with surface finish 120 to 60 root mean square (RMS) or better. Burrs are minimal on the exit side of cuts.

BROACHING MATERIALS AND CONSTRUCTION Because of the low cutting speeds employed, most broaches are made of alloy or highspeed tool steel. Carbide-tipped broaches are seldom used for machining steel parts or forgings because the cutting edges tend to chip on the first stroke, probably due to a lack of rigidity in the combination of machine tool and cutting tool. TiN coating of highspeed-steel (HSS) broaches is becoming more common, greatly prolonging the life of broaches. When they are used in continuous mass-production machines, particularly in surface broaching of cast iron, tungsten carbide teeth may be used, permitting the broach to be used for long periods of time without resharpening. Internal broaches are usually solid but may be made of shells mounted on an arbor (Figure 25-14). When the broach (or a section of it) is subject to rapid wear, a single shell can be replaced. This will be much cheaper than replacing an entire solid broach. Shell construction, however, is initially more expensive than a solid broach of comparable size. Small-surface broaches may be of solid construction, but larger ones usually use modular construction (Figure 25-15). Building in sections makes the broach easier and cheaper to construct and sharpen. It also often provides some degree of interchangeability of the sections for different parts, bringing down the tool cost significantly.

SHARPENING BROACHES Most broaches are resharpened by grinding the hook faces of the teeth. The lands of internal broaches must not be reground because this would change the size of the

FIGURE 25-14 Shell construction for a pull broach. (Courtesy J T. Black)

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SECTION 25.5

Broaching Machines

703

FIGURE 25-15 A modularly constructed broach is cheaper to build and can be sharpened in sections. (Courtesy J T. Black)

broach. Lands of flat-surface broaches are sometimes ground, in which case all of them must be ground to maintain their proper relationship.

& 25.5 BROACHING MACHINES Because all the factors that determine the shape of the machined surface and that determine all cutting conditions except speed are built into the broaching tool, broaching machines are relatively simple. Their primary functions are to impart plain reciprocating motion to the broach and to provide a means for handling the broach automatically. Most broaching machines are driven hydraulically, although mechanical drive is used in a few special types. The major classification relates to whether the motion of the broach is vertical or horizontal, as given in Table 25-1. The choice between vertical and horizontal machines is determined primarily by the length of the stroke required and the available floor space. Vertical machines seldom have strokes greater than 60 in. because of height limitations. Horizontal machines can have almost any length of stroke, but they require greater floor space. The most

TABLE 25-1

Broaching Machines

Vertical Push-broaching

Arbor press with guided ram, 5- to 50-ton capacity Internal broaching

Pull-down

Double-ram design most common Long changeover times

Pull-up

Ram above table pulling broach up Machines with multiple rams common

Surface

No handling of broach Multiple slides

Horizontal

Short cycle times

Pull

Longer strokes and broaches Basically vertical machines laid on side

Surface

Broaches stationary, work moves on conveyor Work held in fixtures Conveyor chain holds fixtures

Continuous Rotary

Rotary broach stationary, work translates beneath tool Work held in fixtures

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common machine is the vertical pull-down machine shown in Figure 25-9. The worktable, usually having a spherical-seated workholder, sits below the broach elevator, with a pulling mechanism below the table. When the elevator raises the broach above the table, the work can be placed into position. The elevator then lowers the pilot end of the broach through the hole in the workpiece, where it is engaged by the puller. The elevator then releases the upper end of the broach, and it is pulled through the workpiece. The workpieces are removed from the table, and the broach is raised upward to be engaged by the elevator mechanism. In some machines with two rams, one broach is being pulled down while the work is being unloaded and the broach raised at the other station. The part is being broached in two passes, first on the left, then on the right. Most broaching machines are found in job shops, have long setup or changeover times and relatively large footprints. In Chapter 29, a rotary broach is shown, designed for manufacturing cells and featuring rapid changeover and a narrow footprint.

& 25.6 INTRODUCTION TO SHAPING AND PLANING The processes of shaping and planing are among the oldest single-point machining processes. Shaping has largely been replaced by milling and broaching as a production process, while planing still has applications in producing long flat cuts, like those in the ways of machine tools. From a consideration of the relative motions between the tool and the workpiece, shaping and planing both use a straight-line cutting motion with a singlepoint cutting tool to generate a flat surface. In shaping, the workpiece is fed at right angles to the cutting motion between successive strokes of the tool, as shown in Figure 25-16, where fc is the feed per stroke, V is the cutting speed, and d is the depth of cut (DOC). (In planing, discussed next, the workpiece is reciprocated and the tool is fed at right angles to the cutting motion.) For either shaping or planing, the tool is held in a clapper box, which prevents the cutting edge from being damaged on the return stroke of the tool. In addition to plain flat surfaces, the shapes most commonly produced on the shaper and planer are those illustrated in Figure 25-17. Relatively skilled workers are required to operate shapers and planers, and most of the shapes that can be produced on them can also be made by much more productive processes, such as milling, broaching, or grinding. Consequently, except for certain special types, planers that will do only planing have become obsolete. Today, shapers are used mainly in tool and die work, in very low volume production, or in the manufacture of gear teeth. In shaping, the cutting tool is held in the tool post located in the ram, which reciprocates over the work with a forward stroke, cutting at velocity V and a quick return stroke at velocity, VR. The rpm of drive crank (Ns) drives the ram and determines the velocity of the operation (see Figure 25-16d). The stroke ratio is Rs ¼

Cutting stroke angle 200 5 ¼ ¼ 360 9 360

ð25-10Þ

The tool is advancing 55% of the time. The number of strokes per minute is determined by the rpm of the drive crank. Feed is in inches per stroke and is at right angles to the cutting direction. As in other machining processes, speed and feed are selected by the operator. The length of cut, L, is the length of the workpiece. The length of stroke, l, must be greater than the length of the workpiece. Because velocity is position variant, let l ¼ twice the length of the block being cut, or 2L. The cutting velocity, V, is assumed to be twice the average forward velocity, V, of the ram. The general relationship between cutting speed and rpm is pDN s ft=min ð25-11Þ V¼ 12Rs where D is the diameter of the rotating bull wheel in inches. For shaping, the cutting speed is 2IN s ft=min ð25-12Þ V¼ 12Rs

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SECTION 25.6 Cutting tool

Introduction to Shaping and Planing

V fc

705

Unfinished surface Machined

Tool

C25

V

surface

Chip e

In

t

te

fe

rm

ed

itt

en

t

n di

vis

el

eh

c pie

rk Wo

L W Single-point tool process

(a) Basic geometry for shaping and planing

Velocity Ram

Column

Velocity diagram

V Cut 0

V

Return

Displacement

Tool head VR Tool post Ram

Vise holding workpiece

V

Advance

Table

200° Ns rpm of bull wheel

Base Return 160° Speed

Bull wheel

Feed Motion (b)

(c)

Shaper (quickreturn) mechanism for driving tool past work

FIGURE 25-16 Basics of shaping and planing. (a) The cutting speed, V, and feed per stroke fc. (b) Block diagram of the machine tool. (c) The ram of the shaper carries the cutting tool at cutting velocity V and reciprocates at velocity VR by the rotation of a bull wheel turning at rpm ns.

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Grooves

FIGURE 25-17 Types of surfaces commonly machined by shaping and planing.

T-slot

Dovetails

Flats and angles

Once a cutting speed is selected, the rpm of the machine can be calculated. Tables for suggested feed values, fc, are in inches per stroke (or cycle), and recommended depths of cut are also available. The maximum depth of cut is based on the horsepower available to form the chips. This calculation requires that the metal removal rate (MRR) be known. The MRR is the volume of metal removed per unit time MRR ¼

LWd 3 in: =min Tm

ð25-13Þ

where W is the width of block being cut and L is the length of block being cut, so Volume of cut ¼ WLd where d is the depth of cut and Tm is the time in minutes to cut that volume. In general, Tm is the total length of the cut divided by the feed rate. For shaping, Tm is the width of the block divided by the feed rate fc of the tool moving across the width. Thus, for shaping, W ð25-14Þ Tm ¼ Ns f c Also, Tm ¼

S Ns

ð25-15Þ

where Ns the number of strokes for the job is for a surface of width W.

MACHINE TOOLS FOR SHAPING Shapers, as machine tools, are usually classified according to their general design features as follows: 1. Horizontal: a. Push-cut. b. Pull-cut or draw-cut shaper. 2. Vertical: a. Regular or slotters. b. Keyseaters. 3. Special. They are also classified as to the type of drive employed: mechanical drive or hydraulic drive. Most shapers are of the horizontal push-cut type (Figure 25-18), where cutting occurs as the ram pushes the tool across the work. On horizontal push-cut shapers, the work is usually held in a heavy vise mounted on the top side of the table. Shaper vises have a very heavy movable jaw, because the vise must often be turned so that the cutting forces are directed against this jaw. In clamping the workpiece in a shaper vise, care must be exercised to make sure that it rests solidly against the bottom of the vise (on parallel bars) so that it will not be

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SECTION 25.6

Tool slide

Introduction to Shaping and Planing

707

Ram

Clapper box Tool post Vise Column Table

FIGURE 25-18 The most widely used shaper is the horizontal push-cut machine tool, shown here with no tool in the tool post. (Courtesy J T. Black)

Saddle Cross rail

Base

deflected by the cutting force and so that it is held securely yet not distorted by the clamping pressure. Most shaping is done with simple high-speed-steel or carbide-tipped cutting tool bits held in a heavy, forged tool holder. Although shapers are versatile tools, the precision of the work done on them is greatly dependent on the operator. Feed dials on shapers are nearly always graduated in 0.001-in. divisions, and work is seldom done to greater precision than this. A tolerance of 0.002 to 0.003 in. is desirable on parts that are to be machined on a shaper, because this gives some provision for variations due to clamping, possible looseness or deflection of the table, and deflection of the tool and ram during cutting.

PLANING MACHINES Planing can be used to produce horizontal, vertical, or inclined flat surfaces on large workpieces (too large for shapers). However, planing is much less efficient than other basic machining processes, such as milling, that will produce such surfaces. Consequently, planing and planers have largely been replaced by planer milling machines or machines that can do both milling and planing. Figure 25-19 shows the basic components and motions of planers—the most common designs being double-housing and single-housing types. In most planing, the action is opposite to that of shaping. The work is moved past one or more stationary singlepoint cutting tools. Because a large and heavy workpiece and table must be reciprocated at relatively low speeds, several tool heads are provided, often with multiple tools in each head. In addition, many planers are provided with tool heads arranged so that cuts occur on both directions of the table movement. However, because only singlepoint cutting tools are used and the cutting speeds are quite low, planers are low in productivity as compared with some other types of machine tools. The double housing has a closed-housing structure, spanning the reciprocating worktable, with a cross rail supported at each end on a vertical column and carrying two tool heads. An additional tool head is usually mounted on each column, so that four tools (or four sets) can cut during each stroke of the table. The closed-frame structure of this type of planer limits the size of the work that can be machined. Open-side planers have the cross rail supported on a single column. This design provides unrestricted access to one side of the table to permit wider workpieces to be accommodated. Some

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Sawing, Broaching, and Other Machining Processes

Housing Tool heads

Tool heads Housing

Column

Tool head

Table

Table

t

Cu Bed

t

Cu

n

r etu

R

Bed

n

tur

Re

Cross rail

Tool head

Tool head Feed Motion

Feed Motion (a)

Block diagram showing the basic components of a double-housing planer

FIGURE 25-19 Schematic of planers: (a) double-housing planer with multiple tool heads (4) and a large reciprocating table; (b) singlehousing or open-sided planer; (c) interchangeable multiple tool holder for use in planers. (Courtesy J T. Black)

(b)

Block diagram of an open-side planer

(c) Four planer tools in tool holder

open-side planers are convertible, in that a second column can be attached to the bed when desired so as to provide added support for the cross rail.

WORKHOLDING AND SETUP ON PLANERS Workpieces in planers are usually large and heavy. They must be securely clamped to resist large cutting forces and the high-inertia forces that result from the rapid velocity changes at the ends of the strokes. Special stops are provided at each end of the workpiece to prevent the work from shifting. Considerable time is usually required to set up the planer, thus reducing the time the machine is available for producing chips. Sometimes special setup plates are used for quick setup of the workpiece. Another procedure is to use two tables. Work is set up on one table while another workpiece is being machined on the other. The tables can be fastened together for machining long workpieces. The large workpieces can usually support heavy cutting forces, so large depths of cut are recommended, which decrease the cutting time. Consequently, planer tools are usually quite massive and can sustain the large cutting forces. Usually, the main shank of

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SECTION 25.7

Introduction to Filing

709

the tools is made of plain-carbon steel, with tips made from high-speed steel or carbide. Chip breakers should be used to avoid long and dangerous chips in ductile materials. Theoretically, planers have about the same precision as shapers. The feed and other dimension-controlling dials are usually graduated in 0.001-in. divisions. However, because larger and heavier workpieces are usually involved, with much longer beds and tables, the working tolerances for planer work are somewhat greater than for shaping.

& 25.7 INTRODUCTION TO FILING Basically, the metal-removing action in filing is the same as in sawing, in that chips are removed by cutting teeth that are arranged in succession along the same plane on the surface of a tool, called a file. There are two differences: (1) the chips are very small, and therefore the cutting action is slow and easily controlled, and (2) the cutting teeth are much wider. Consequently, fine and accurate work can be done. Files are classified according to the following: 1. The type, or cut, of the teeth. 2. The degree of coarseness of the teeth. 3. Construction: a. Single solid units for hand use or in die-filing machines. b. Band segments, for use in band-filing machines. c. Disks, for use in disk-filing machines. Four types of cuts are available as shown in Figure 25-20. Single-cut files have rows of parallel teeth that extend across the entire width of the file at the angle of from 65 to 85 degrees. Double-cut files have two series of parallel teeth that extend across the width of the file. One series is cut at an angle of 40 to 45 degrees. The other series is coarser and is cut at an opposite angle that varies from about 10 to 80 degrees. A vixen-cut file has a series of parallel curved teeth, each extending across the file face. On a rasp-cut file, each tooth is short and is raised out of the surface by means of a punch. The coarseness of files is designated by the following terms, arranged in order of increasing coarseness: dead smooth, smooth, second cut, bastard, coarse, and rough. There is also a series of finer Swiss pattern files, designated by numbers from 00 to 8. Files are available in a number of cross-sectional shapes: flat, round, square, triangular, and half-round. Flat files can be obtained with no teeth on one or both narrow edges, known as safe edges. Safe edges prevent material from being removed from a surface that is normal to the one being filed. Most files for hand filing are from 10 to 14 in. in length and have a pointed tang at one end on which a wood or metal handle can be fitted for easy grasping.

FIGURE 25-20 Four types of teeth (cuts) used in files. Left to right: single, double, rasp, and curved (vixen).

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FILING MACHINES An experienced operator can do very accurate work by hand filing, but it can be a difficult task. Therefore, three types of filing machines have been developed that permit quite accurate results to be obtained rapidly and with much less effort. Die-filing machines hold and reciprocate a file that extends upward through the worktable. The file rides against a roller guide at its upper end, and cutting occurs on the downward stroke; therefore, the cutting force tends to hold the work against the table. The table can be tilted to any desired angle. Such machines operate at from 300 to 500 strokes per minute, and the resulting surface tends to be at a uniform angle with respect to the table. Quite accurate work can be done. Because of the reciprocating action, approximately 50% of the operating time is nonproductive. Band-filing machines provide continuous cutting action. Most band filing is done on contour bandsawing machines by means of a special band file that is substituted for the usual bandsaw blade. The principle of a band file is shown in Figure 25-21. Rigid, straight

(a)

(b)

FIGURE 25-21 Band file segments (a) are joined together to form a continuous band (b) which runs on a band-filing machine (c). (Courtesy of DoAll Co.)

(c)

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Key Words

711

FIGURE 25-22 Disk-type filing machine and some of the available types of disk files.

file segments, about 3 in. long, are riveted to a flexible steel band near their leading ends. One end of the steel band contains a slot that can be hooked over a pin in the other end to form a continuous band. As the band passes over the drive and idler wheels of the machine, it flexes so that the ends of adjacent file segments move apart. When the band becomes straight, the ends of adjacent segments move together and interlock to form a continuous straight file. Where the file passes through the worktable, it is guided and supported by a grooved guide, which provides the necessary support to resist the pressure of the work against the file. Band files are available in most of the standard cuts and in several widths and shapes. Operating speeds range from about 50 to 250 ft/min. Although band filing is considerably more rapid than can be done on a die-filing machine, it usually is not quite as accurate. Frequently, band filing may be followed by some finish filing on a die-filing machine. Some disk-filing machines have files in the form of disks (Figure 25-22). These are even simpler than die-filing machines and provide continuous cutting action. However, it is difficult to obtain accurate results by their use.

& KEY WORDS back-off angle band-filing machine bandsaw blade thickness broach broaching burnishing teeth circular saw cold saw combination cutoff and contour bandsawing die-filing machine disk-filing machine disk saw double-cut construction double-cut file

end clearance angle feed feed per tooth file filing finishing teeth friction bandsawing front pilot gullet hacksaw hook horizontal metal-cutting bandsawing horizontal push-cut shaper jump-cut tooth design kerf

planing progressive broach pull broach push broach raker-tooth saw rasp-cut file rear pilot reciprocating saw rise per tooth (RPT) rotor-cut tooth design roughing teeth safe edges saw blades sawing semifinishing teeth shank length

shaping shear-cut broach shells single-cut file step straight-set teeth surface broach tooth form tooth set tooth spacing vertical, cutoff bandsawing vixen-cut file wave-set teeth

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Sawing, Broaching, and Other Machining Processes

& REVIEW QUESTIONS 1. Why is sawing one of the most efficient of the chip-forming processes? 2. Explain why tooth spacing (pitch) is important in sawing. 3. What is the tooth gullet used for on a saw blade? 4. Explain what is meant by the ‘‘set’’ of the teeth on a saw blade. 5. How is tooth set related to saw kerf? 6. Why can a bandsaw blade not be hardened throughout the entire width of the band? 7. What are the advantages of using circular saws? 8. Why have bandsawing machines largely replaced reciprocating saws? 9. Explain how the hole in Figure 25-7 is made on a contour bandsawing machine. 10. How would you calculate or estimate Tm for a horizontal bandsaw cutting a 3-in. round of 1040 steel. 11. What is the disadvantage of using gravity to feed a saw in cutting round bar stock? 12. What is unique about the broaching process compared to the other basic machining processes? 13. Can a thick saw blade be used as a broach? Why or why not? 14. Broaching machines are simpler in a basic design than most other machine tools. Why is this? 15. Why is broaching particularly well suited for mass production? 16. In designing a broach, what would be the first thing you have to calculate? 17. Why is it necessary to relate the design of a broach to the specific workpiece that is to be machined? 18. What two methods can be utilized to reduce the force and power requirements for a particular broaching cut?

19. For a given job, how would a broach having rotor-tooth design compare in length with one having regular, full-width teeth? 20. Why are the pitch and radius of the gullet between teeth on a broach of importance? 21. Why are broaching speeds usually relatively low, as compared with other machining operations? 22. What are the advantages of shell-type broach construction? 23. Why are most broaches made from alloy or high-speed steel rather than from tungsten carbide? 24. What are the advantages of TiN-coated broaching tools? 25. For mass-production operations, which process is preferred, pull-up broaching or pull-down broaching? 26. What is the difference between the roughing teeth and the finishing teeth in a typical pull broach? 27. The sides of a square, blind hole must be machined all the way to the bottom (who designed this part?). The hole is first drilled to full depth, then the bottom is milled flat. Is it possible to machine the hole square by broaching? Why or why not? 28. The interior, flat surfaces of socket wrenches, which have one ‘‘closed’’ end, often are finished to size by broaching. By examining one of these, determine what design modification was incorporated to make broaching possible. 29. How is feed per stroke in shaping related to feed per tooth in milling? 30. What are some ways to improve the efficiency of a planer? Do any of these apply to the shaper? 31. To what extent is filing different from sawing? 32. What is a safe edge on a file? 33. How does a rasp-cut file differ from other types of files? 34. How does the process of shaping differ from planing?

& PROBLEMS 1. A surface 12 in. long is to be machined with a flat, solid broach that has a rise per tooth of 0.0047 in. What is the minimum cross-sectional area that must be provided in the chip gullet between adjacent teeth? 2. The pitch of the teeth on a simple surface broach can be determined by equation 25-1. If a broach is to remove 0.25 in. of material from a gray iron casting that is 3 in. wide and 17.75 in. long, and if each tooth has a rise per tooth of 0.004 in., what will be the length of the roughing section of the broach? 3. Estimate the (approximate maximum) horsepower needed to accomplish the operation described in Problem 2 at a cutting speed of 10 m/min. (Hint: First find the HP used per tooth and determine the maximum number of teeth engaged at any time. What are those units?) 4. Estimate the approximate force acting in the forward direction during cutting for the conditions stated in Problems 2 and 3. 5. In cutting a 6-in.-long slot in a piece of AISI 1020 cold-rolled steel that is 1 in. thick, the material is fed to a bandsaw blade with teeth having a pitch of 1.27 mm (20 pitch) at the rate of 0.0001 in. per tooth. Estimate the cutting time for the cut. 6. The strength of a pull broach is determined by its minimum cross section, which usually occurs either at the root of the first

7.

8. 9. 10.

11.

tooth or at the pull end. Suppose the minimum root diameter is Dr, the pull end diameter is Dp, and the width of the pull slot is W. Write an equation for the allowable pull, in psi, using 200,000 as the yield strength for the broach material. Suppose you want to shape a block of metal 7 in. wide and 4 in. long (L ¼ 4 in.) using a shaper as set up in Figure 25-16. You have determined for this metal that the cutting speed should be 25 sfpm, the depth of cut needed here for roughing is 0.25 in., and the feed will be 0.1 in. per stroke. Determine the approximate crank rpm, and then estimate the cutting time and the MRR. Could you have saved any time in Problem 7 by cutting the block in the 7-in. direction? Redo with L ¼ 7 and W ¼ 4 in. Derive the equation for shaping cutting speed (equation 25-12). How many strokes per minute would be required to obtain a cutting speed of 36.6 m/min (120 ft/min) on a typical mechanical drive shaper if a 254-mm (10-in.) stroke is used? How much time would be required to shape a flat surface 254 mm (1 in.) wide and 203 mm (8 in.) long on a hydraulic drive shaper, using a cutting speed of 45.7 m (150 ft) per minute, a feed of 0.51 mm (0.020 in.) per stroke, and an overrun of 12.7 mm (12 in.) at each end of the cut?

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Case Study 12. What is the metal removal rate in Problem 11 if the depth of cut is 6.35 mm (14 in.)? 13. Suppose you decide to mill the flat surface described in Problems 11 and 12. The work will be done on a vertical milling machine using a 1.25-in.-diameter end mill (four teeth) (HSS) cutting at 150 sfpm with a feed per tooth of 0.005 in. per tooth cutting at d ¼ 0.25 in. Compare the milling time and MRR to that of shaping. 14. A planer has a 10-hp motor, and 75% of the motor output is available at the cutting tool. The specific power for cutting

Chapter 25

713

cast iron metal is 0.03 W/mm3, or 0.67 hp/in.3/min. What is the maximum depth of cut that can be taken in shaping a surface in this material if the surface is 305 305 mm (12 12), the feed is 0.25 in. per stroke, and the cutting speed is 54.9 mm/ min (180 ft/min)? 15. Calculate the Tm for planing the block of cast iron in Problem 14, and then estimate Tm for milling the same surface. You will have to determine which milling process to use and select speeds and feeds for an HSS cutter.

CASE STUDY

Cost Estimating—Planing vs. Milling Carol and Wally work in the K9Rice factory as manufacturing engineers. There are two machine tools available for a job the company is bidding on. One is a 48 48 10 double housing planer that originally costs $80,000, is depreciated over a 20-yr period, and is operated about 6000 hr/yr. The charge for the use of the machine is $5/hr, and labor and overhead in addition are charged at $40/hr. The other machine is a large vertical spindle CNC milling machine that costs $165,000 new, depreciated over a 20-yr time period also. The charge for

the use of the CNC milling machine is $16/hr, and labor and overhead in addition are charged at $62/hr. They have estimated that it will take 8 hr to machine the workpiece under consideration on the planer and 4 hr on the CNC milling machine. The cutting tools consumed cost $6/piece for the planer and $50/piece for the mill. Both machines need about the same amount of time to setup. Purchasing has not issued an order (quantity not yet decided), so they need to determine the BEQ so they know which machine to use when the order is placed.

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CHAPTER 26 ABRASIVE MACHINING PROCESSES 26.1 INTRODUCTION 26.2 ABRASIVES Abrasive Grain Size and Geometry 26.3 GRINDING WHEEL STRUCTURE AND GRADE G Ratio Bonding Materials for Grinding Wheels Abrasive Machining Versus Conventional Grinding Versus Low-Stress Grinding Truing and Dressing

26.4 GRINDING WHEEL IDENTIFICATION Grinding Wheel Geometry Grinding Operations Balancing Grinding Wheels Safety in Grinding Use of Cutting Fluids in Grinding 26.5 GRINDING MACHINES Cylindrical Grinding Centerless Grinding Surface Grinding Machines Disk-Grinding Machines Tool and Cutter Grinders

Mounted Wheels and Points Coated Abrasives 26.6 HONING 26.7 SUPERFINISHING Lapping 26.8 FREE ABRASIVES Ultrasonic Machining Water-Jet Cutting and Abrasive Water-Jet Machining Abrasive Jet Machining 26.9 DESIGN CONSIDERATIONS IN GRINDING Case Study: Process Planning for the MfE

& 26.1 INTRODUCTION Abrasive machining is a material removal process that involves the interaction of abrasive grits with the workpiece at high cutting speeds and shallow penetration depths. The chips that are formed resemble those formed by other machining processes. Unquestionably, abrasive machining is the oldest of the basic machining processes. Museums abound with examples of utensils, tools, and weapons that ancient peoples produced by rubbing hard stones against softer materials to abrade away unwanted portions, leaving desired shapes. For centuries, only natural abrasives were available for grinding, while other more modern basic machining processes were developed using superior cutting materials. However, the development of manufactured abrasives and a better fundamental understanding of the abrasive machining process have resulted in placing abrasive machining and its variations among the most important of all the basic machining processes. The results that can be obtained by abrasive machining range from the finest and smoothest surfaces produced by any machining process, in which very little material is removed, to rough, coarse surfaces that accompany high material removal rates. The abrasive particles may be (1) free; (2) mounted in resin on a belt (called coated product); or, most commonly (3) close packed into wheels or stones, with abrasive grits held together by bonding material (called bonded product or a grinding wheel). Figure 26-1 shows a surface grinding process using a grinding wheel. The depth of cut d is determined by the infeed and is usually very small, 0.002 to 0.005 in., so the arc of contact (and the chips) is small. The table reciprocates back and forth beneath the rotating wheel. The work feeds into the wheel in the cross-feed direction. After the work is clear of the wheel, the wheel is lowered and another pass is made, again removing a couple of thousandths of inches of metal. The metal removal process is basically the same in all abrasive machining processes but with important differences due to spacing of active grains (grains in contact with work) and the rigidity and degree of fixation of the grains. Table 26-1 summarizes the primary abrasive processes. The term abrasive machining applied to one particular form of the grinding process is unfortunate, because all these process are machining with abrasives.

714

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SECTION 26.2

715

Abrasives

Infeed ft

Abrasive grinding wheel

Grinding wheel Grinding fluid Air scraper

ed

Cross fe Worktable

Vs

Vs

Workpiece surface

d Confining baffle Vw (Down grinding)

Table motion

Tr a

(Up grinding)

ve

rs

e

Side view

Oblique view

FIGURE 26-1 Schematic of surface grinding, showing infeed and cross-feed motions along with cutting speeds, VS, and workpiece velocity, VW.

Compared to machining, abrasive machining processes have three unique characteristics. First, each cutting edge is very small, and many of these edges can cut simultaneously. When suitable machine tools are employed, very fine cuts are possible, and fine surfaces and close dimensional control can be obtained. Second, because extremely hard abrasive grits, including diamonds, are employed as cutting tool materials, very hard materials, such as hardened steel, glass, carbides, and ceramics, can readily be machined. As a result, the abrasive machining processes are not only important as manufacturing processes, they are indeed essential. Many of our modern products, such as modern machine tools, automobiles, space vehicles, and aircraft, could not be manufactured without these processes. Third, in grinding, you have no control over the actual tool geometry (rake angles, cutting edge radius) or all the cutting parameters (depth of cut). As a result of these parameters and variables, grinding is a complex process. To get a handle on the complexity, Table 26-2 presents the primary grinding parameters, grouped by their independence or dependence. Independent variables are those that are controllable (by the machine operator) while the dependent variables are the resultant effects of those inputs. Not listed in the table is workpiece hardness, which has a significant effect on all the resulting effects. Workpiece hardness will be an input factor but it is not usually controllable.

& 26.2 ABRASIVES An abrasive is a hard material that can cut or abrade other substances. Natural abrasives have existed from the earliest times. For example, sandstone was used by ancient

TABLE 26-1

Abrasive Machining Processes

Process

Particle Mounting

Features

Grinding

Bonded

Uses wheels, accurate sizing, finishing, low MRR; can be done at high speeds (>12,000 sfpm)

Creep feed grinding

Bonded open, soft

Uses wheels with long cutting arc, very slow feed rate, and large depth of cut

Abrasive machining

Bonded

High MRR, to obtain desired shapes and approximate sizes

Snagging

Bonded belted

High MRR, rough rapid technique to clean up and deburr castings, forgings

Honing

Bonded

‘‘Stones’’ containing fine abrasives; primarily a hole-finishing process

Lapping Abrasive water-jet

Free Free in jet

Fine particles embedded in soft metal or cloth; primarily a surface-finishing process Water jets with velocities up to 3000 sfpm carry abrasive particles (silica and garnet)

Ultrasonic

Free in liquid

Vibrating tool impacts abrasives at high velocity

Abrasive flow

Free in gel

Abrasives in gel flow over surface-edge finishing

Abrasive jet

Free in

A focused jet of abrasives in an inert gas at high velocity

sfpm ¼ surface feet per minute; this is the cutting velocity.

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Abrasive Machining Processes

TABLE 26-2

Grinding Parameters

Independent Parameters/Controllable

Dependent Variables/Resulting Effects

Grinding wheel selection Abrasive type Grain size Hardness grade Openness of structure Bonding media

Forces per unit width of wheel Normal Tangential Surface finish Material removal rate (MRR) Wheel wear (G, or grinding ratio)

Dressing of wheel Type of dressing tool Feed and depth of cut Sharpness of dressing tool

Thermal effects Wheel surface changes Chemical effects Horsepower

Machine settings Wheel speed Infeed rate (depth of cut) Cross-feed rate Workpiece speed Rigidity of setup Type and quality of machine Grinding fluid Type Cleanliness Method of application

peoples to sharpen tools and weapons. Early grinding wheels were cut from slabs of sandstone, but because they were not uniform in structure throughout, they wore unevenly and did not produce consistent results. Emery, a mixture of alumina (Al2O3) and magnetite (Fe3O4), is another natural abrasive still in use today and is used on coated paper and cloth (emery paper). Corundum (natural Al2O3) and diamonds are other naturally occurring abrasive materials. Today, the only natural abrasives that have commercial importance are quartz, sand, garnets, and diamonds. For example, quartz is used primarily in coated abrasives and in air blasting, but artificial abrasives are also making inroads in these applications. The development of artificial abrasives having known uniform properties has permitted abrasive processes to become precision manufacturing processes. Hardness, the ability to resist penetration, is the key property for an abrasive. Table 26-3 lists the primary abrasives and their approximate Knoop hardness (kg/mm2). The particles must be able to decompose at elevated temperatures. Two other properties are significant in abrasive grits—attrition and friability. Attrition refers to the abrasive wear action of the grits resulting in dulled edges, grit flattening,

TABLE 26-3

Knoop Hardness Values for Common Abrasives Temperature of Decomposition in Oxygen ( C)

Abrasive Material

Year of Discovery

Hardness (Knoop)

Quartz

?

320

Aluminum oxide

1893

1600–2100

1700–2400

Softer and tougher than silicon carbide; used on steel, iron, brass, silicon

Carbide

1891

2200–2800

1500–2000

Used for brass, bronze, aluminum, and stainless and cast iron

Borazon [cubic boron nitride stainless (CBN)] Diamond (synthetic)

1957

4200–5400

1200–1400

1955

6000–9000

700–800

For grinding hard, tough tool steels, stainless steel, cobalt and nickel based, superalloys, and hard coatings Used to grind nonferrous materials, tungsten carbide, and ceramics

Comments and Uses Sand blasting

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SECTION 26.2

Abrasives

717

FIGURE 26-2 Loose abrasive grains at high magnification, showing their irregular, sharp cutting edges. (Courtesy of Norton Abrasives/Saint Gobain)

and wheel glazing. Friability refers to the fracture of the grits and is the opposite of toughness. In grinding, it is important that grits be able to fracture to expose new, sharp edges. Artificial abrasives date from 1891, when E. G. Acheson, while attempting to produce precious gems, discovered how to make silicon carbide (SiC). Silicon carbide is made by charging an electric furnace with silica sand, petroleum coke, salt, and sawdust. By passing large amounts of current through the charge, a temperature of greater than 4000 F is maintained for several hours, and a solid mass of silicon carbide crystals results. After the furnace has cooled, the mass of crystals is removed, crushed, and graded (sorted) into various desired sizes. As can be seen in Figure 26-2, the resulting grits, or grains, are irregular in shape, with cutting edges having every possible rake angle. Silicon carbide crystals are very hard (Knoop 2480), friable, and rather brittle. This limits their use. Silicon carbide is sold under the trade names Carborundum and Crystolon. Aluminum oxide (Al2O3) is the most widely used artificial abrasive. Also produced in an arc furnace from bauxite, iron filings, and small amounts of coke, it contains aluminum hydroxide, ferric oxide, silica, and other impurities. The mass of aluminum oxide that is formed is crushed, and the particles are graded to size. Common trade names for aluminum oxide abrasives are Alundum and Aloxite. Although aluminum oxide is softer (Knoop 2100) than silicon carbide, it is considerably tougher. Consequently, it is a better general-purpose abrasive. Diamonds are the hardest of all materials. Those that are used for abrasives are either natural, off-color stones (called garnets) that are not suitable for gems, or small, synthetic stones that are produced specifically for abrasive purposes. Manufactured stones appear to be somewhat more friable and thus tend to cut faster and cooler. They do not perform as satisfactorily in metal-bonded wheels. Diamond abrasive wheels are used extensively for sharpening carbide and ceramic cutting tools. Diamonds also are used for truing and dressing other types of abrasive wheels. Diamonds are usually used only when cheaper abrasives will not produce the desired results. Garnets are used primarily in the form of very finely crushed and graded powders for fine polishing. Cubic boron nitride (CBN) is not found in nature. It is produced by a combination of intensive heat and pressure in the presence of a catalyst. CBN is extremely hard, registering at 4700 on the Knoop scale. It is the second-hardest substance created by nature or manufactured and is often referred to, along with diamonds, as a superabrasive. Hardness, however, is not everything. CBN far surpasses diamond in the important characteristic of thermal resistance. At temperatures of 650 C, at which diamond may begin to revert to plain carbon

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CHAPTER 26

Abrasive Machining Processes

dioxide, CBN continues to maintain its hardness and chemical integrity. When the temperature of 1400 C is reached, CBN changes from its cubic form to a hexagonal form and loses hardness. CBN can be used successfully in grinding iron, steel, and alloys of iron, nickel-based alloys, and other materials. CBN works very effectively (long wheel life, high G ratio, good surface quality, no burn or chatter, low scrap rate, and overall increase in parts/shift) on hardened materials (Rc 50 or higher). It can also be used for soft steel in selected situations. CBN does well at conventional grinding speeds (6,000 to 12,000 ft/min), resulting in lower total grinding in conventional equipment. CBN can also perform well at high grinding speeds (12,000 ft/min and higher) and will enhance the benefits from future machine tools. CBN can solve difficult-to-grind jobs, but it also generates cost benefits in many production grinding operations despite its higher cost. CBN is manufactured by the General Electric Company under the trade name of Borazon.

ABRASIVE GRAIN SIZE AND GEOMETRY To enhance the process capability of grinding, abrasive grains are sorted into sizes by mechanical sieving machines. The number of openings per linear inch in a sieve (or screen) through which most of the particles of a particular size can pass determines the grain size (Figure 26-3). A no. 24 grit would pass through a standard screen having 24 openings per inch but would not pass through one having 30 openings per inch. These numbers have since been specified in terms of millimeters and micrometers (see ANSI B74.12 for details). Commercial practice commonly designates grain sizes from 4 to 24, inclusive, as coarse; 30 to 60, inclusive, as medium; and 70 to 600, inclusive, as fine. Grains smaller than 220 are usually termed powders. Silicon carbide is obtainable in grit sizes ranging from 2 to 240 and aluminum oxide in sizes from 4 to 240. Superabrasive grit sizes normally range from 120 grit for CBN to 400 grit for diamond. Sizes from 240 to 600 are designated as flour sizes. These are used primarily for lapping, or in fine-honing stones for fine finishing tasks. The grain size is closely related to the surface finish and metal removal rate. In grinding wheels and belts, coarse grains cut faster (higher removal rate) while fine grains provide better finish, as shown in Figure 26-4. The grain diameter can be estimated from the screen number (S), which corresponds to the number of openings per inch. The mean diameter of the grain (g) is related to the screen number by g ffi 0.7/S. Regardless of the size of the grain, only a small percentage (2 to 5%) of the surface of the grain is operative at any one time. That is, the depth of cut for an individual grain (the actual feed per grit) with respect to the grain diameter is very small. Thus, the chips are small. As the grain diameter decreases, the number of active grains per unit area increases and the cuts become finer because grain size is the controlling factor for surface finish (roughness). Of course, the MRR also decreases. The grain shape is also important, because it determines the tool geometry— that is, the back rake angle and the clearance angle at the cutting edge of the grit 1⬙

Screen

8 openings

FIGURE 26-3 Typical screens for sifting abrasives into sizes. The larger the screen number (of opening per linear inch), the smaller the grain size. (Courtesy of Carborundum Company)

Grains

Screen no. 8 Grain size 8

Screen no. 24 Grain size 24

Screen no. 60 Grain size 60

14:26:55

Page 719

SECTION 26.2

MRR decreasing

Surface finish Improving

12

24

36

48

60

Abrasives

719

Metal removal rate

07/01/2011

Surface finish, µin.

C26

80

FIGURE 26-4 MRR and surface finish versus grit size.

Grit size decreasing

(Figure 26-5). In the figure, g is the clearance angle, u is the wedge angle, and a is the rake angle. The cavities between the grits provide space for the chips, as shown in Figure 26-6. The volume of the cavities must be greater than the volume of the chips generated during the cut. Obviously, there is no specific rake angle but rather a distribution of angles. Thus a grinding wheel can present to the surface rake angles in the range of þ45 to 60 degrees or greater. Grits with large negative rake angles or rounded cutting edges do not form chips but will rub or plow a groove in the surface (Figure 26-7). Thus abrasive machining is a mixture of cutting, plowing, and rubbing, with the percentage of each being highly dependent on the geometry of the grit. As the grits are continuously abraded, fractured, or dislodged from the bond, new grits are exposed and the mixture of cutting, plowing, and rubbing is changing continuously. A high percentage of the

–α

+α Chip

θ

θ γ

γ

Workpiece

Abrasive particle

Cavity full of chips

Open cavity (void)

Workpiece

FIGURE 26-5 The rake angle of abrasive particles can be positive, zero, or negative.

Binder forming bond post

Partly filled cavity

Cavity

FIGURE 26-6 The cavities or voids between the grains must be large enough to hold all the chips during the cut.

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CHAPTER 26

Abrasive Machining Processes Cutting

Side view of grain

Grinding chip

Workpiece Plowing (no chips)

End view Side flow of grain

Side view

Rubbing (no chip)

Loaded workpiece material

FIGURE 26-7 The grits interact with the surface in three ways: cutting, plowing, and rubbing.

Loading

Attritious wear of grit

energy used for rubbing and plowing goes into the workpiece, but when chips are found, 95 to 98% of the energy (the heat) goes into the chip. Figure 26-8 shows a scanning electron microscope (SEM) micrograph of a ground surface with a plowing track. In grinding, the chips are small but are formed by the same basic mechanism of compression and shear as discussed in Chapter 20 for regular metal cutting. Figure 26-9 shows steel chips from a grinding process at high magnification. They show the same structure as chips from other machining processes. Chips flying in the air from a grinding process often have sufficient heat energy to burn or melt in the atmosphere. Sparks observed during grinding steel with no cutting fluid are really burning chips. The feeds and depths of cut in grinding are small while the cutting speeds are high, resulting in high specific horsepower numbers. Because cutting is obviously more efficient than plowing or rubbing, grain fracture and grain pullout are natural

M

O

T

P

FIGURE 26-8 SEM micrograph of a ground steel surface showing a plowed track (T) in the middle and a machined track (M) above. The grit fractured, leaving a portion of the grit in the surface (X), a prow formation (P), and a groove (G) where the fractured portion was pushed farther across the surface. The area marked (O) is an oil deposit. (Courtesy J T. Black)

X G

10 mm

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SECTION 26.3

Grinding Wheel Structure and Grade

721

FIGURE 26-9 SEM micrograph of stainless steel chips from a grinding process. The tops (T) of the chips have the typical shearfront-lamella structure while the bottoms (B) are smooth where they slide over the grit 4800. (Courtesy J T. Black)

phenomena used to keep the grains sharp. As the grains become dull, cutting forces increase, and there is an increased tendency for the grains to fracture or break free from the bonding material.

& 26.3 GRINDING WHEEL STRUCTURE AND GRADE Grinding, wherein the abrasives are bonded together into a wheel, is the most common abrasive machining process. The performance of grinding wheels is greatly affected by the bonding material and the spatial arrangement of the particles’ grits. The spacing of the abrasive particles with respect to each other is called structure. Close-packed grains have dense structure; open structure means widely spaced grains. Open-structure wheels have larger chip cavities but fewer cutting edges per unit area (Figure 26-10a). The fracturing of the grits is controlled by the bond strength, which is known as the grade. Thus, grade is a measure of how strongly the grains are held in the wheel. It is really dependent on two factors: the strength of the bonding materials and the amount of the bonding agent connecting the grains. The latter factor is illustrated in Figure 26-10b. Abrasive wheels are really porous. The grains are held together with ‘‘posts’’ of bonding material. If these posts are large in cross section, the force required to break a grain

(a)

FIGURE 26-10 Meaning of terms structure and grade for grinding wheels. (a) The structure of a grinding wheel depends on the spacing of the grits. (b) The grade of a grinding wheel depends on the amount of bonding agent (posts) holding abrasive grains in the wheel.

Open spacing

Medium spacing

Dense spacing

Weak "posts" Open spacing

Medium strength "posts" Open spacing

Strong "posts" Open spacing

(b)

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CHAPTER 26

Abrasive Machining Processes

free from the wheel is greater than when the posts are small. If a high dislodging force is required, the bond is said to be hard. If only a small force is required, the bond is said to be soft. Wheels are commonly referred to as hard or soft, referring to the net strength of the bond, resulting from both the strength of the bonding material and its disposition between the grains.

G RATIO The loss of grains from the wheel means that the wheel is changing size. The grinding ratio, or G ratio, is defined as the cubic inches of stock removed divided by the cubic inches of wheel lost. In conventional grinding, the G ratio is in the range 20:1 to 80:1. The G ratio is a measure of grinding production and reflects the amount of work a wheel can do during its useful life. As the wheel loses material, it must be reset or repositioned to maintain workpiece size. A typical vitrified grinding wheel will consist of 50 vol% abrasive particles, 10 vol% bond, and 40 vol% cavities; that is, the wheels have porosity. The manner in which the wheel performs is influenced by the following factors: 1. The mean force required to dislodge a grain from the surface (the grade of the wheel). 2. The cavity size and distribution of the porosity (the structure). 3. The mean spacing of active grains in the wheel surface (grain size and structure). 4. The properties of the grain (hardness, attrition, and friability). 5. The geometry of the cutting edges of the grains (rake angles and cutting-edge radius compared to depth of cut). 6. The process parameters (speeds, feeds, cutting fluids) and type of grinding (surface, or cylindrical). It is easy to see why grinding is a complex process, difficult to control.

BONDING MATERIALS FOR GRINDING WHEELS Bonding material is a very important factor to be considered in selecting a grinding wheel. It determines the strength of the wheel, thus establishing the maximum operating speed. It determines the elastic behavior or deflection of the grits in the wheel during grinding. The wheel can be hard or rigid, or it can be flexible. Finally, the bond determines the force required to dislodge an abrasive particle from the wheel and thus plays a major role in the cutting action. Bond materials are formulated so that the ratio of bond wear matches the rate of wear of the abrasive grits. Bonding materials in common use are the following: 1. Vitrified bonds are composed of clays and other ceramic substances. The abrasive particles are mixed with the wet clays so that each grain is coated. Wheels are formed from the mix, usually by pressing, and then dried. They are then fired in a kiln, which results in the bonding material becoming hard and strong, having properties similar to glass. Vitrified wheels are porous, strong, rigid, and unaffected by oils, water, or temperature over the ranges usually encountered in metal cutting. The operating speed range in most cases is 5500 to 6500 ft/min, but some wheels now operate at surface speeds up to 16,000 ft/min. 2. Resinoid bonds, or phenolic resins, can be used. Because plastics can be compounded to have a wide range of properties, such wheels can be obtained to cover a variety of work conditions. They have, to a considerable extent, replaced shellac and rubber wheels. Composite materials are being used in rubber-bonded or resinoidbonded wheels that are to have some degree of flexibility or are to receive considerable abuse and side loading. Various natural and synthetic fabrics and fibers, glass fibers, and nonferrous wire mesh are used for this purpose. 3. Silicate bond wheels use silicate of soda (waterglass) as the bond material. The wheels are formed and then baked at about 500 F for a day or more. Because they are more brittle and not so strong as vitrified wheels, the abrasive grains are released

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SECTION 26.3

Grinding Wheel Structure and Grade

723

more readily. Consequently, they machine at lower surface temperatures than vitrified wheels and are useful in grinding tools when heat must be kept to a minimum. 4. Shellac-bonded wheels are made by mixing the abrasive grains with shellac in a heated mixture, pressing or rolling into the desired shapes, and baking for several hours at about 300 F. This type of bond is used primarily for strong, thin wheels having some elasticity. They tend to produce a high polish and thus have been used in grinding such parts as camshafts and mill rolls. 5. Rubber bonding is used to produce wheels that can operate at high speeds but must have a considerable degree of flexibility so as to resist side thrust. Rubber, sulfur, and other vulcanizing agents are mixed with the abrasive grains. The mixture is then rolled out into sheets of the desired thickness, and the wheels are cut from these sheets and vulcanized. Rubber-bonded wheels can be operated at speeds up to 16,000 ft/min. They are commonly used for snagging work in foundries and for thin cutoff wheels. 6. Superabrasive bond wheels are either electroplated (single layer of superabrasive plated to outside diameter of a steel blank) or a thin-segmented drum of vitrified CBN surrounds a steel core. The steel core provides dimensional accuracy, and the replaceable segments provide durability, homogeneity, and repeatability while increasing wheel life. The latter type of wheels can use resin, metal, or vitrified bonding. Selection of bond grade and structure (also called abrasive concentration) is critical. For the electroplated wheels, nickel is used to attach a single layer of CBN (or diamond) to the outside diameter (OD) of an accurately ground or turned steel blank. For the vitrified wheel, superabrasives are mixed with bonding media and molded (or preformed and sintered) into segments or a ring. The ring is mounted on a split steel body. Porosity is varied (to alter structure) by varying preform pressure or by using ‘‘pore-forming’’ additives to the bond material that are vaporized during the sintering cycle. The steel-cored segmented design can rotate at 40,000 sfpm (200 m/s) whereas a plain vitrified wheel may burst at 20,000 fpm.

ABRASIVE MACHINING VERSUS CONVENTIONAL GRINDING VERSUS LOW-STRESS GRINDING The condition wherein very rapid metal removal can be achieved by grinding is the one to which some have applied the term abrasive machining. The metal removal rates are compared with, or exceed, those obtainable by milling or turning or broaching, and the size tolerances are comparable. It is obviously just a special type of grinding, using abrasive grains as cutting tools, as do all other types of abrasive machining. Abrasive grinding done in an aggressive way can produce sufficient localized plastic deformation and heat in the surface so as to develop tensile residual stresses, layers of overtempered martensite (in steels), and even microcracks, because this process is quite abusive. See Figure 26-11 for a discussion of residual stresses produced by various surface-grinding processes. Conventional grinding can be replaced by procedures that develop lower surface stresses when service failures due to fatigue or stress corrosion are possible. This is accomplished by employing softer grades of grinding wheels, reducing the grinding speeds and infeed rates, using chemically active cutting fluids (e.g., highly sulfurized oil or KNO2 in water), as outlined in the table of grinding conditions in Figure 26-11. These procedures may require the addition of a variable-speed drive to the grinding machine. Generally, only about 0.005 to 0.010 in. of surface stock needs to be finish ground in this way, as the depth of the surface damage due to conventional grinding or abusive grinding is 0.005 to 0.007 in. High-strength steels, high-temperature nickel, and cobalt-based alloys and titanium alloys are particularly sensitive to surface deformation and cracking problems from grinding. Other postprocessing processes, such as polishing, honing, and chemical milling plus peening, can be used to remove the deformed layers in critically stressed parts. It is strongly recommended, however, that testing programs be used

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CHAPTER 26

Abrasive Machining Processes

Residual stress, KSI

Tension

+100

Compression

C26

+80 +60 +40

Conventional grinding (CG)

+20 0 –20 –40

Grinding conditions

Abusive grinding (AG)

Mild or low-stress grinding (LSG)

–60

Abusive AG

Conventional CG

Wheel

A46MV

A46KV

A46HV or A60IV

Wheel speed ft/min

6,000–18,000

4,500–6,500

2500–3000

Down feed in./pass

.002–.004

.001–.003

.0002–.005

Cross feed in./pass

.004–.060

.040–.060

.040–.060

Table speed ft/min

40–100

40–100

40–100

Fluid

Dry

Sol oil (1 : 20)

Sulfurized oil

0 .002 .004 .006 .008 .010 Depth below the surface (in.)

Low-stress LSG

FIGURE 26-11 Typical residual stress distributions produced by surface grinding with different grinding conditions for abusive, conventional, and low-stress grinding. Material is 4340 steel. (From M. Field and M. P. Kahles, ‘‘Surface Integrity in Grinding,’’ in New Developments in Grinding, Carnegie-Mellon University Press, Pittsburgh, 1972, p. 666)

along with service experience on critical parts before these procedures are employed in production. In the casting and forging industries, the term often used for abrasive machining is snagging. Snagging is a type of rough manual grinding that is done to remove fins, gates, risers, and rough spots from castings or flash from forgings, preparatory to further machining. The primary objective is to remove substantial amounts of metal rapidly without much regard for accuracy, so this is a form of abrasive machining except that pedestal-type or swing grinders ordinarily are used. Portable electric or hand air grinders are also used for this purpose and for miscellaneous grinding in connection with welding.

TRUING AND DRESSING Grinding wheels lose their geometry during use. Truing restores the original shape. A single-point diamond tool can be used to true the wheel while fracturing abrasive grains to expose new grains and new cutting edges on worn, glazed grains (Figure 26-12). Truing can also be accomplished by grinding the grinding wheel with a controlled-path or Before truing

A

Rotary disk

Rotary cup

After truing

B

Grinding wheels lose geometry during use and need truing

Before truing

Diamond roll

FIGURE 26-12

C

Cross feed

15° drag angle

D

Single-point diamond nib dressing tool After truing

Diamond block

Truing methods for restoring grinding geometry include nibs, rolls, disks, cups, and blocks.

Infeed for dressing tool about 0.001 in. per pass

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SECTION 26.3

Grinding Wheel Structure and Grade

725

Dressing stick pushed into the wheel at constant force or constant infeed rate

After truing

Grinding wheel

FIGURE 26-13

After dressing

Schematic arrangement of stick dressing versus truing.

powered rotary device using conventional abrasive wheels. The precision in generating a trued wheel surface by these methods is poorer than by the method described earlier. As the wheel is used, there is a tendency for the wheel to become loaded (metal chips become lodged in the cavities between the grains). Also, the grains dull or glaze (grits wear, flatten, and polish). Unless the wheel is cleaned and sharpened (or dressed), the wheel will not cut as well and will tend to plow and rub more. Figure 26-13 shows an arrangement for stick dressing a grinding wheel. The dulled grains cause the cutting forces on the grains to increase, ideally resulting in the grains’ fracturing or being pulled out of the bond, thus providing a continuous exposure of sharp cutting edges. Such a continuous action ordinarily will not occur for light feeds and depths of cut. For heavier cuts, grinding wheels do become somewhat self-dressing, but the workpiece may become overheated and turn a bluish temper color (this is called burn) before the wheel reaches a fully dressed condition. A burned surface, the consequence of an oxide layer formation, results in the scrapping of several workpieces before parts of good quality are ground. Resin-bonded wheels can be trued by grinding with hard ceramics such as tungsten carbide. The procedure for truing and dressing a CBN wheel in a surface grinder might be as follows: use 0.0002-in. downfeed per pass and cross feed slightly more than half the wheel thickness at moderate table speeds. The wheel speed is the same as the grinding speed. The grinding power will gradually increase, as the wheel is getting dull, while being trued. When the power exceeds normal power drawn during workpiece grinding, stop the truing operation. Dress the wheel face open using a J-grade stick, with abrasive one grit size smaller than CBN. Continue the truing. Repeat this cycle until the wheel is completely trued. Modern grinding machines are equipped so that the wheel can be dressed and/or trued continuously or intermittently while grinding continues. A common way to do this is by crush dressing (Figure 26-14). Crush dressing consists of forcing a hard roll

Crushtrue® roll Crush roll Grinding wheel Grinding wheel

FIGURE 26-14 Continuous crush roll dressing and truing of a grinding wheel (form—truing and dressing throughout the process rather than between cycles) performing plunge-cut grinding on a cylinder held between centers.

Vs Infeed Vw Workpiece Workcenters Centertype form grinding

Workpiece

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Abrasive Machining Processes

(tungsten carbide or high speed steel) having the same contour as the part to be ground against the grinding wheel while it is revolving—usually quite slowly. A water-based coolant is used to flood the dressing zone at 5 to 10 gal/min. The crushing action fractures and dislodges some of the abrasive grains, exposing fresh sharp edges, allowing free cutting for faster infeed rates. This procedure is usually employed to produce and maintain a special contour to the abrasive wheel. This is also called wheel profiling. Crush dressing is a very rapid method of dressing grinding wheels, and because it fractures abrasive grains, it results in free cutting and somewhat cooler grinding. The resulting surfaces may be slightly rougher than when diamond dressing is used.

& 26.4 GRINDING WHEEL IDENTIFICATION Most grinding wheels are identified by a standard marking system that has been established by the American National Standards Institute (ANSI). This system is illustrated and explained in Figure 26-15. The first and last symbols in the marking are left to the discretion of the manufacturer.

GRINDING WHEEL GEOMETRY The shape and size of the wheel are critical selection factors. Obviously, the shape must permit proper contact between the wheel and all of the surface that must be ground. Grinding wheel shapes have been standardized, and eight of the most commonly used types are shown in Figure 26-16. Types 1, 2, and 5 are used primarily for grinding external or internal cylindrical surfaces and for plain surface grinding. Type 2 can be mounted for grinding on either the periphery or the side of the wheel. Type 4 is used with tapered safety flanges so that if the wheel breaks during rough grinding, such as snagging, these flanges will prevent the pieces of the wheel from flying and causing damage. Type 6, the straight cup, is used primarily for surface grinding but can also be used for certain types of offhand grinding. The flaring-cup type of wheel is used for tool grinding. Dish-type wheels are used for grinding tools and saws. Type 1, the straight grinding wheels, can be obtained with a variety of standard faces. Some of these are shown in Figure 26-17. The size of the wheel to be used is determined primarily by the spindle rpm values available on the grinding machine and the proper cutting speed for the wheel, as dictated by the type of bond. For most grinding operations the cutting speed is about 2500 to 6500 ft/min. Different types and grades of bond often justify considerable deviation from these speeds. For certain types of work using special wheels and machines, as in thread grinding and ‘‘abrasive machining,’’ much higher speeds are used.

GRINDING OPERATIONS The operation for which the abrasive wheel is intended will also influence the wheel shape and size. The major use categories are the following: 1. Cutting off: for slicing and slotting parts; use thin wheel, organic bond. 2. Cylindrical between centers: grinding outside diameters of cylindrical workpieces. 3. Cylindrical, centerless: grinding outside diameters with work rotated by regulating wheel. 4. Internal cylindrical: grinding bores and large holes. 5. Snagging: removing large amounts of metal without regard to surface finish or tolerances. 6. Surface grinding: grinding flat workpieces. 7. Tool grinding: for grinding cutting edges on tools such as drills, milling cutters, taps, reamers, and single-point high-speed-steel tools. 8. Offhand grinding: work or the grinding tool is handheld. In many cases, the classification of processes coincides with the classification of machines that do the process. Other factors that will influence the choice of wheel to be

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Page 727

SECTION 26.4 Sequence Prefix

1

2

3

4

5

6

Abrasive Type

Abrasive (Grain) Size

Grade

Structure

Bond Type

Manufacturer's Record

51 – A – 36 – L – 5 – V Dense MANUFACTURER'S SYMBOL INDICATING EXACT KIND OF ABRASIVE. (USE OPTIONAL) A TFA 3A 2A FA HA JA LA 13A 36A WA EA ZT YA C GC RC CA BA DA

Coarse Medium 30 8 36 10 46 12 54 14 60 16 20 24

Regular Aluminum Oxide Treated Aluminum Oxide

Special Aluminum Oxide

White Aluminum Oxide Extruded Aluminum Oxide Zirconia — 25% Special Blend Silicon Carbide Green Silicon Carbide Mixture Silicon Carbide Mixture S/C and A/O

Fine 70 80 90 100 120 150 180

Very Fine 220 240 280 320 400 500 600

727

Grinding Wheel Identification

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Etc. (Use optional)

– 23 MANUFACTURER'S PRIVATE MARKING TO IDENTIFY WHEEL. (USE OPTIONAL) B BF E O R RF S V

Resinoid Resinoid Reinforced Shellac Oxychloride Rubber Rubber Reinforced Silicate Vitrified

Open Soft

Medium

Hard

A B C D E F G H I J K L M N O P Q R ST U V W XY Z Grade Scale

Standard bonded-abrasive wheel-marching system (ANSI Standard B74.13-1977)

1

2

3

4

5

6

7

8

Abrasive Type

Abrasive (Grain) Size

Grade

Concentration

Bond Type

Bond Modification

Depth of Abrasive

Manufacturer's Record

D

120

N

100

B

77

1/8

Sequence Prefix

M MANUFACTURER'S SYMBOL INDICATING EXACT KIND OF ABRASIVE. (USE OPTIONAL)

Diamond—D CBN—B

Coarse to 8 10 12 14 16 20 24

30 36 46 54 60 70 80

90 100 120 150 180 220 240

Very Fine 280 320 400 500 600 etc.

Working depth of abrasive section in inches or millimeters. inches illustrated.

Manufacturer's designation: may be number or symbol

B V M

Resin Vitrified Metal

Manufacturer's identification symbol (use optional) Manufacturer's Notation of Special Bond Type or Modification

Hardness

A B C D E F G H I J K L M N O P Q R ST U V W XY Z Soft ............................................................................................. Hard Wheel-marching system for diamond and cubic boron nitride wheels (ANSI Standard B74.13-1977).

FIGURE 26-15

Standard marking systems for grinding wheels (ANSI standard B74. 13-1977).

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CHAPTER 26

Abrasive Machining Processes D W T

T

H 5. Cylinder

1. Straight

D

D P

W

F T

E

T E

H

H 6. Straight cup

2. Recessed one side

D W

D P

F

K

E

T

T E

G H P 3. Recessed two sides D J

H J 7. Flaring cup D K

U

U

T

FIGURE 26-16 Standard grinding wheel shapes commonly used. (Courtesy of Carborundum Company)

T

E H J

H J 4. Tapered

8. Dish

selected include the workpiece material, the amount of stock to be removed, the shape of the workpiece, and the accuracy and surface finish desired. Workpiece material has a great impact on choice of the wheel. Hard, high-strength metals (tool steels, alloy steels) are generally ground with aluminum oxide wheels or cubic boron nitride wheels. Silicon carbide and CBN are employed in grinding brittle materials (cast iron and ceramics) as well as softer, low-strength metals such as aluminum, brass, copper, and bronze. Diamonds have taken over the cutting of tungsten carbides, and CBN is used for precision grinding of tool and die steel, alloy steels, stainless steel, and other very hard materials. There are so many factors that affect the cutting action that there are no hard-and-fast rules with regard to abrasive selection.

1⬙ 8

1⬙ 8 90°

45°

65° A

B

60° C

D

R

FIGURE 26-17 Standard face contours for straight grinding wheels. (Courtesy of Carborundum Company)

60°

60°

1⬙ 8

23°

23°

3T 10 T

T J

K

60°

E

R I

45°

60°

R

R= 1⬙ 8

45°

T

L

T 45°

R

F R=

45° 65°

G T 2

R=

65°

H T 4

R=

T 6

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SECTION 26.4

Grinding Wheel Identification

729

Selection of grain size is determined by whether coarse or fine cutting and finish are desired. Coarse grains take larger depths of cut and cut more rapidly. Hard wheels with fine grains leave smaller tracks and therefore are usually selected for finishing cuts. If there is a tendency for the work material to load the wheel, larger grains with a more open structure may be used for finishing.

BALANCING GRINDING WHEELS Because of the high rotation speeds involved, grinding wheels must never be used unless they are in good balance. A slight imbalance will produce vibrations that will cause waviness in the work surface. It may cause a wheel to break, with the probability of serious damage and injury. The wheel should be mounted with proper bushings so that it fits snugly on the spindle of the machine. Rings of blotting paper should be placed between the wheel and the flanges to ensure that the clamping pressure is evenly distributed. Most grinding wheels will run in good balance if they are mounted properly and trued. Most machines have provision for compensating for a small amount of wheel imbalance by attaching weights to one mounting flange. Some have provision for semiautomatic balancing with weights that are permanently attached to the machine spindle.

SAFETY IN GRINDING Because the rotational speeds are quite high, and the strength of grinding wheels is usually much less than that of the materials being ground, serious accidents occur much too frequently in connection with the use of grinding wheels. Virtually all such accidents could be avoided and are due to one or a combination of four causes. First, grinding wheels are occasionally operated at unsafe and improper speeds. All grinding wheels are clearly marked with the maximum rpm value at which they should be rotated. They are all tested to considerably above the designated rpm and are safe at the specified speed unless abused. They should never, under any condition, be operated above the rated speed. Second, a very common form of abuse, frequently accidental, is dropping the wheel or striking it against a hard object. This can cause a crack (which may not be readily visible), resulting in subsequent failure of the wheel while rotating at high speed under load. If a wheel is dropped or struck against a hard object, it should be discarded and never used unless tested at above the rated speed in a properly designed test stand. A third common cause of grinding wheel failure is improper use, such as grinding against the side of a wheel that was designed for grinding only on its periphery. The fourth and most common cause of injury from grinding is the absence of a proper safety guard over the wheel and/or over the eyes or face of the operator. The frequency with which operators will remove safety guards from grinding equipment or fail to use safety goggles or face shields is amazing and inexcusable.

USE OF CUTTING FLUIDS IN GRINDING Because grinding involves cutting, the selection and use of a cutting fluid is governed by the basic principles discussed in Chapter 21. If a fluid is used, it should be applied in sufficient quantities and in a manner that will ensure that the chips are washed away, not trapped between the wheel and the work. This is of particular importance in grinding horizontal surfaces. In hardened steel, the use of a fluid can help to prevent fine microcracks that result from highly localized heating. The air scraper shown in Figure 26-18 permits the cutting fluid (lubricant) to get onto the face of the wheel. Metal air scrapers disrupt the airflow. Upper and lower nozzles cool the grinding zone, while a high-pressure scrubber helps deter loading of the wheel. Much snagging and off-hand grinding is done dry. On some types of material, dry grinding produces a better finish than can be obtained by wet grinding. Grinding fluids strongly influence the performance of CBN wheels. Straight, sulfurized, or sulfochlorinated oils can enhance performance considerably when used with straight oils.

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CHAPTER 26

Abrasive Machining Processes Air scrapers Highpressure scrubber Tangential jet

Side plates

CBN wheel

Workpiece

Normal jet

FIGURE 26-18 Coolant delivery system for optimum CBN grinding. (M. P. Hitchiner, ‘‘Production Grinding with CBN,’’ Machining Technology, Vol. 2, no. 2, 1991)

Lower nozzle

& 26.5 GRINDING MACHINES Grinding machines commonly are classified according to the type of surface they produce. Table 26-4 presents such a classification, with further subdivision to indicate characteristic features of different types of machines within each classification. Grinding on TABLE 26-4

Classification of Grinding Machines

Type of Machine

Type of Surface

Specific Types or Features

Cylindrical external

External surface on rotating, usually cylindrical parts

Work rotated between centers Centerless Centerless Chucking Tool post Crankshaft, cam, etc.

Cylindrical internal

Internal diameters of holes

Chucking Planetary (work stationary) Centerless

Surface conventional

Flat surfaces

Reciprocating table or rotating table Horizontal or vertical spindle

Creep feed

Deep slots, profiles in hard steels, carbides, and ceramics using CBN and diamond

Tool grinders

Tool angles and geometries

Other

Special or any of the above

Rigid, chatter-free, creep feed rate Continuous dressing Heavy coolant flows NC or CNC control Variable speed wheel Universal Special Disk, contour, thread, flexible shaft, swing frame, snag, pedestal, bench

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SECTION 26.5

Infeed depth or downfeed

Wheelhead

Lon g

itud

inal

731

Wheelhead column

Infeed controlled by handwheel (manual)

Grinding area

Grinding Machines

se ver

ns Tra

Infeed

C26

reci

pro

Cross slide with guide

cati

on

Downfeed

FIGURE 26-19 Horizontalspindle surface grinder, with insets showing movements of wheelhead.

Infeed infinitely variable in 0.0001 to 0.0025⬙ increments controlled automatically

Machine table

all machines is done in three ways. In the first, the depth of cut (dt) is obtained by infeed—moving the wheel down into the work or the work up into the wheel (Figure 26-19). The desired surface is then produced by traversing the wheel across (cross feed) the workpiece, or vice versa. In the second method, known as plunge-cut grinding, the basic movement is of the wheel being fed radially into the work while the latter revolves on centers. It is similar to form cutting on a lathe; usually a formed grinding wheel is used (Figure 26-14). In the third method, the work is fed very slowly past the wheel and the total downfeed or depth (d) is accomplished in a single pass (Figure 26-20). This is called creep feed grinding (CFG). (Table 26-5 compares CFG to conventional and high-speed grinding for CBN applications.) The CFG method, often done in the surface grinding mode, is markedly different from conventional surface grinding. The depth of cut is increased 1,000 to 10,000 times, and the work feed ratio is decreased in the same proportion; hence the name creep feed grinding. The long arc of contact between the wheel and the work increases the cutting forces and the power required. Therefore, the machine tools to perform this type of grinding must be specially designed with high static and dynamic stability, stick-slipfree ways, adequate damping, increased horsepower, infinitely variable spindle speed, variable but extremely consistent table feed (especially in the low ranges), high-pressure cooling systems, integrated devices for dressing the grinding wheels, and specially designed (soft with open structure) grinding wheels. The process is mainly applied when grinding deep slots with straight parallel sides or when grinding complex profiles in difficult-to-grind materials. The process is capable of producing extreme precision at relatively high metal removal rates (MRRs). Because the process can operate at relatively low surface temperatures, the surface integrity of the metals being ground is good. However, in CFG, the grinding wheels must maintain their initial profile much longer, so continuous dressing is used that is form-truing and dressing the grinding wheel throughout the process rather than between cycles. Continuous crush dressing

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Abrasive Machining Processes Diamond-coated roll in process dressing Grinding wheel continually compensates downward to maintain size

+

Hard wheel

Soft wheel

Same feed

Cutting fluid jet

High-volume cutting fluid

d Fast

Vw

ft Short cutting arc approx. 4.4 mm

Total depth of cut

Conventional grinding

Slow Long cutting arc

Approx. 20 mm; chip cavity almost full

Creep feed grinding

FIGURE 26-20 Conventional grinding contrasted to creep feed grinding. Note that crush roll dressing is used here; see Figure 26-14.

results in higher MRRs, improved dimensional accuracy and form tolerance, reduced grinding forces (and power), and reduced thermal effects while sacrificing wheel wear. Creep feed grinding eliminates preparatory operations such as milling or broaching, because profiles are ground into the solid workpiece. This can result in significant savings in unit part costs. Grinding machines that are used for precision work have certain important characteristics that permit them to produce parts having close dimensional tolerances. They are constructed very accurately, with heavy, rigid frames to ensure permanency of alignment. Rotating parts are accurately balanced to avoid vibration. Spindles are mounted in very accurate bearings, usually of the preloaded ball-bearing type. Controls are provided so that all movements that determine dimensions of the workpiece can be made with accuracy—usually to 0.001 or 0.00001 in. The abrasive dust that results from grinding must be prevented from entering between moving parts. All ways and bearings must be fully covered or protected by seals. If this is not done, the abrasive dust between moving parts becomes embedded in the softer of the two, causing it to act as lap and abrade the harder of the two surfaces, resulting in permanent loss of accuracy. These special characteristics add considerably to the cost of these machines and require that they be operated by trained personnel. Production-type grinders are more fully automated and have higher metal removal rates and excellent dimensional accuracy. Fine surface finish can be obtained very economically.

TABLE 26-5

Starting Conditions for CBN Grinding

Grinding Variable

Conventional Grinding

Creep Feed Grinding

High-speed Grinding

Wheel speed (fpm)

5000–9000 versus 3000–5000 0.5–5

12000–2500

Table speed (fpm)

5500–9500 versus 4500–6500 vitrified 80–150

Feed (ft) in./pass

0.0005–0.0015

0.100–0.250

250–500

Grinding fluids

10% heavy-duty soluble oil or 3–5% light-duty soluble for light feeds

Sulfurized or sulfochlorinated straight grinding oil applied at 80 to 100 gal/min at 100 psi or more

5–20

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SECTION 26.5

Grinding Machines

733

Wheelhead slideway Grinding wheel (1) Wheelhead (4)

Machine base

Dog (G) Table guideway

Headstock Center (E)

Faceplate (F)

Table

Workpiece (D)

A

1

F

C

G 2

FIGURE 26-21 Cylindrical grinding between centers. A ¼ edge of wheel, B ¼ face of wheel, C ¼ shaft for wheel, D ¼ workpiece, E ¼ centers, F ¼ faceplate, G ¼ dog.

E

Tailstock

B

4

3

D E

Movements 1. Wheel speed 2. Work (rotates) rpm 3. Traverse feed 4. Infeed

CYLINDRICAL GRINDING Center-type cylindrical grinding is commonly used for producing external cylindrical surfaces. Figures 26-14 and Figure 26-21 show the basic principles and motions of this process. The grinding wheel revolves at an ordinary cutting speed, and the workpiece rotates on centers at a much slower speed, usually from 75 to 125 ft/min. The grinding wheel and the workpiece move in opposite directions at their point of contact. The depth of cut is determined by infeed of the wheel or workpiece. Because this motion also determines the finished diameter of the workpiece, accurate control of this movement is required. Provision is made to traverse the workpiece with the wheel, or the work can be reciprocated past the wheel. In very large grinders, the wheel is reciprocated because of the massiveness of the work. For form or plunge grinding, the detail of the wheel is maintained by periodic crush roll dressing. A plain center-type cylindrical grinder is shown in Figure 26-21. On this type the work is mounted between headstock and tailstock centers. Solid dead centers are always used in the tailstock, and provision is usually made so that the headstock center can be operated either dead or alive. High-precision work is usually ground with a dead headstock center, because this eliminates any possibility that the workpiece will run out of round due to any eccentricity in the headstock. The table assembly can be reciprocated—in most cases, by using a hydraulic drive. The speed can be varied, and the length of the movement can be controlled by means of adjustable trip dogs. Infeed is provided by movement of the wheelhead at right angles to the longitudinal axis of the table. The spindle is driven by an electric motor that is also mounted on the wheelhead. If the infeed movement is controlled manually by some type of vernier drive

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Abrasive Machining Processes

to provide control to 0.001 in. or less, the machine is usually equipped with digital readout equipment to show the exact size being produced. Most production-type grinders have automatic infeed with retraction when the desired size has been obtained. Such machines are usually equipped with an automatic diamond wheel-truing device that dresses the wheel and resets the measuring element before grinding is started on each piece. The longitudinal traverse should be about one-fourth to three-fourths of the wheel width for each revolution of the work. For light machines and fine finishes, it should be held to the smaller end of this range. The depth of cut (infeed) varies with the purpose of the grinding operation and the finish desired. When grinding is done to obtain accurate size, infeeds of 0.002 to 0.004 in. are commonly used for roughing cuts. For finishing, the infeed is reduced to 0.00025 to 0.0005 in. The design allowance for grinding should be from 0.005 to 0.010 in. on short parts and on parts that are not to be hardened. On long or large parts and on work that is to be hardened, a grinding allowance of from 0.015 to 0.030 in. is desirable. When grinding is used primarily for metal removal (called abrasive machining), infeeds are much higher, 0.020 to 0.040 in. being common. Continuous downfeed is often used, with rates up to 0.100 in./min being common. Grinding machines are available in which the workpiece is held in a chuck for grinding both external and internal cylindrical surfaces. Chucking-type external grinders are production-type machines for use in rapid grinding of relatively short parts, such as ball-bearing races. Both chucks and collets are used for holding the work, the means dictated by the shape of the workpiece and rapid loading and removal. In chucking-type internal grinding machines, the chuck-held workpiece revolves, and a relatively small, high-speed grinding wheel is rotated on a spindle arranged so that it can be reciprocated in and out of the workpiece. Infeed movement of the wheelhead is normal to the axis of rotation of the work (Figure 26-21).

CENTERLESS GRINDING Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without requiring the workpiece to be mounted between centers or in a chuck. This eliminates the requirement of center holes in some workpieces and the necessity for mounting the workpiece, thereby reducing the cycle time. The principle of centerless external grinding is illustrated in Figure 26-22. Two wheels are used. The larger one operates at regular grinding speeds and does the actual

B 1

A. B. C. D. E.

3

Grinding wheel Grinding face Regulating wheel Workpiece Work rest blade

4

2 5

A C C

A

FIGURE 26-22 Centerless grinding showing the relationship among the grinding wheel, the regulating wheel, and the workpiece in centerless method. (Courtesy of Carborundum Company)

D E

υ Movements υ = Angle of tilt of regulating wheel

1. Grinding wheel speed 2. Work rpm 3. Regulating wheel speed 4. Infeed 5. Traverse feed

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SECTION 26.5

Grinding Machines

735

grinding. The smaller wheel is the regulating wheel. It is mounted at an angle to the plane of the grinding wheel. Revolving at a much slower surface speed—usually 50 to 200 ft/min—the regulating wheel controls the rotation and longitudinal motion of the workpiece and is a usually a plastic- or rubber-bonded wheel with a fairly wide face. The workpiece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel. This axial feed is calculated approximately by the equation F ¼ ND sin f

ð26-1Þ

where F ¼ feed (mm/min or in./min) D ¼ diameter of the regulating wheel (mm or in.) N ¼ revolutions per minute of the regulating wheel f ¼ angle of inclination of the regulating wheel Centerless grinding has several important advantages: 1. It is very rapid; infeed centerless grinding is almost continuous. 2. Very little skill is required of the operator. 3. It can often be made automatic (single-cycle automatic). 4. Where the cutting occurs, the work is fully supported by the work rest and the regulating wheel. This permits heavy cuts to be made. 5. Because there is no distortion of the workpiece, accurate size control is easily achieved. 6. Large grinding wheels can be used, thereby minimizing wheel wear. Thus, centerless grinding is ideally suited to certain types of mass-production operations. The major disadvantages are as follows: 1. Special machines are required that can do no other type of work. 2. The work must be round—no flats, such as keyways, can be present. 3. Its use on work having more than one diameter or on curved parts is limited. 4. In grinding tubes, there is no guarantee that the OD and inside diameter (ID) are concentric. Special centerless grinding machines are available for grinding balls and tapered workpieces. The centerless grinding principle can also be applied to internal grinding, but the external surface of the cylinder must be finished accurately before the internal operation is started. However, it ensures that the internal and external surfaces will be concentric. The operation is easily mechanized for many applications.

SURFACE GRINDING MACHINES Surface grinding machines are used primarily to grind flat surfaces. However formed, irregular surfaces can be produced on some types of surface grinders by use of a formed wheel. There are four basic types of surface grinding machines, differing in the movement of their tables and the orientation of the grinding wheel spindles (Figure 26-23): 1. Horizontal spindle and reciprocating table. 2. Vertical spindle and reciprocating table. 3. Horizontal spindle and rotary table. 4. Vertical spindle and rotary table. The most common type of surface grinding machine has a reciprocating table and horizontal spindle (Figures 26-19). The table can be reciprocated longitudinally either by handwheel or by hydraulic power. The wheelhead is given transverse (cross-feed) motion at the end of each table motion, again either by handwheel or by hydraulic

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Abrasive Machining Processes

C C A D

1

1

E

1

E

A

2 B

2

3

(a)

(b) A. B. C. D. E.

Movements 1. Wheel 2. Infeed 3. Work table traverse

3

B

D Grinding wheel Grinding face Shaft Workpiece Magnetic chuck on table

C

C

A 1 1

D

D

1

A

4

2

3

FIGURE 26-23 Surface grinding: (a) horizontal surface grinding and reciprocating table; (b) vertical spindle with reciprocating table; (c) and (d) both horizontal- and verticalspindle machines can have rotary tables. (Courtesy of Carborundum Company)

B

B

E (c)

2 Movements 1. Wheel 2. Work table rotation 3. Infeed 4. Cross feed

E (d)

3 Movements 1. Wheel 2. Infeed 3. Work table rotation

power feed. Both the longitudinal and transverse motions can be controlled by limit switches. Infeed or downfeed on such grinders is controlled by handwheels or automatically. The size of such machines is determined by the size of the surface that can be ground. In using such machines, the wheel should overtravel the work at both ends of the table reciprocation, so as to prevent the wheel from grinding in one spot while the table is being reversed. The transverse or cross-feed motion should be one-fourth to threefourths of the wheel width between each stroke. Vertical-spindle reciprocating-table surface grinders differ basically from those with horizontal spindles only in that their spindles are vertical and that the wheel diameter must exceed the width of the surface to be ground. Usually, no transverse motion of either the table or the wheelhead is provided. Such machines can produce very flat surfaces. Rotary-table surface grinders can have either vertical or horizontal spindles, but those with horizontal spindles are limited in the type of work they will accommodate and therefore are not used to a great extent. Vertical-spindle rotary-table surface grinders are primarily production-type machines. They frequently have two or more grinding heads, and therefore, both rough grinding and finish grinding are accomplished in one rotation of the workpiece. The work can be held either on a magnetic chuck or in special fixtures attached to the table. By using special rotary feeding mechanisms, machines of this type often are made automatic. Parts are dumped on the rotary feeding table and fed automatically onto workholding devices and moved past the grinding wheels. After they pass the last grinding head, they are automatically unloaded.

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SECTION 26.5

Grinding Machines

737

DISK-GRINDING MACHINES Disk grinders have relatively large side-mounted abrasive disks. The work is held against one side of the disk for grinding. Both single- and double-disk grinders are used; in the latter type, the work is passed between the two disks and is ground on both sides simultaneously. On these machines, the work is always held and fed automatically. On small, single-disk grinders, the work can be held and fed by hand while resting on a supporting table. Although manual disk grinding is not very precise, flat surfaces can be obtained quite rapidly with little or no tooling cost. On specialized, production-type machines, excellent accuracy can be obtained very economically.

TOOL AND CUTTER GRINDERS Simple, single-point tools are often sharpened by hand on bench or pedestal grinders (off-hand grinding). More complex tools, such as milling cutters, reamers, hobs, and single-point tools for production-type operations require more sophisticated grinding machines, commonly called universal tool and cutter grinders. These machines are similar to small universal cylindrical center-type grinders, but they differ in four important respects: 1. The headstock is not motorized. 2. The headstock can be swiveled about a horizontal as well as a vertical axis. 3. The wheelhead can be raised and lowered and can be swiveled through at 360-degree rotation about a vertical axis. 4. All table motions are manual. No power feeds being provided. Specific rake and clearance angles must be created, often repeatedly, on a given tool or on duplicate tools. Tool and cutter grinders have a high degree of flexibility built into them so that the required relationships between the tool and the grinding wheel can be established for almost any type of tool. Although setting up such a grinder is quite complicated and requires a highly skilled worker, after the setup is made for a particular job, the actual grinding is accomplished rather easily. Figure 26-24 shows several typical setups on a tool and cutter grinder. Hand-ground cutting tools are not accurate enough for automated machining processes. Many numerically controlled (NC) machine tools have been sold on the premise that they can position work to very close tolerances—within 0.0001 to 0.0002 in.—only to have the initial workpieces produced by those machines out of tolerance by as much as 0.015 to 0.020 in. In most instances, the culprit was a poorly ground tool. For example, a twist drill with a point ground 0.005 in. off-center can ‘‘walk’’ as much as 0.015 in., thus causing poor hole location. Many companies are turning to computer numerical control (CNC) grinders to handle the regrinding of their cutting tools. A six-axis CNC grinder is

FIGURE 26-24 Three typical setups for grinding single- and multiple-edge tools on a universal tool and cutter grinder. (a) Singlepoint tool is held in a device that permits all possible angles to be ground. (b) Edges of a large hand reamer are being ground. (c) Milling cutter is sharpened with a cupped grinding wheel. (Courtesy J T. Black)

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Abrasive Machining Processes

FIGURE 26-25 Examples of mounted abrasive wheels and points. (Courtesy of Norton Abrasives/Saint Gobain)

capable of restoring the proper tool angles (rake and clearance), concentricity, cutting edges, and dimensional size.

MOUNTED WHEELS AND POINTS Mounted wheels and points are small grinding wheels of various shapes that are permanently attached to metal shanks that can be inserted in the chucks of portable, highspeed electric or air motors. They are operated at speeds up to 100,000 rpm, depending on their diameters, and are used primarily for deburring and finishing in mold and die work. Several types are shown in Figure 26-25.

COATED ABRASIVES Coated abrasives are being used increasingly in finishing both metal and nonmetal products. These are made by gluing abrasive grains onto a cloth or paper backing (Figure 26-26). Synthetic abrasives—aluminum oxide, silicon carbide, aluminum, zirconia, CBN, and diamond—are used most commonly, but some natural abrasives—sand, flint, garnet, and emery—also are employed. Various types of glues are utilized to attach the abrasive grains to the backing, usually compounded to allow the finished product to have some flexibility. Coated abrasives are available in sheets, rolls, endless belts, and disks of various sizes. Some of the available forms are shown in Figure 26-26. Although the cutting action of coated abrasives basically is the same as with grinding wheels, there is one major difference: they have little tendency to be self-sharpened when dull grains are pulled from the backing. Consequently, when the abrasive particles become dull or the belt loaded, the belt must be replaced. Finer grades result in finer first cuts but slower material removal rates. This versatile process is now widely used for rapid stock removal as well as fine surface finishing.

& 26.6 HONING Honing is a stock-removal process that uses fine abrasive stones to remove very small amounts of metal. Cutting speed is much lower than that of grinding. The process is used to size and finish bored holes, remove common errors left by boring (taper, waviness, and tool marks), or remove the tool marks left by grinding. The amount of metal removed is typically about 0.005 in. or less. Although honing is occasionally done by hand, as in finishing the face of a cutting tool, it usually is done with special equipment. Most honing is done on internal cylindrical surfaces, such as automobile cylinder walls. The honing stones are usually held in a honing head, with the stones being held against

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Page 739

SECTION 26.6

739

Honing

Belt composition Grit Size coat

Glue or resin bond Backing—Paper or Cloth (cotton, rayon, polyester)

Grit Size—grade vs

Approx.

Finish (rms)

24 36 50 80 120 150

300 250 140 125 60-80 40-60

μin. " " " " "

Backing

Bonds Name

Make coat

Size coat

Backing

Glue bond Modified glue Resin over glue Resin over resin Waterproof

Glue Mod. glue Glue Resin Resin

Glue Mod. glue Resin Resin Resin

Non WP " " " WP

WP = waterproof

Platen grinder Tension wheel

Platen Fixtured workpieces

Abrasive belt

Drive wheel

FIGURE 26-26 Belt composition for coated abrasives (top). Platen grinder (right) and examples of belts and disks for abrasive machining. (Courtesy J T. Black)

the work with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface (Figure 26-27). The stones are given a complex motion so as to prevent a single grit from repeating its path over the work surface. Rotation is combined with an oscillatory axial motion. For external and flat surfaces, varying oscillatory motions are used. The length of the motions should be such that the stones extend beyond the work surface at the end. A cutting fluid is used in virtually all honing operations. The critical process parameters are rotational speed, Vr, oscillation speed, Vo, the length and position of stroke, and the honing stick pressure. Note that Vc and the inclination angle are both products of Vo and Vr. Virtually all honing is done with stones made by bonding together various fine artificial abrasives. Honing stones differ from grinding wheels in that additional materials, such as sulfur, resin, or wax, are often added to the bonding agent to modify the

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CHAPTER 26

Abrasive Machining Processes Slow rotation + ⌬ Vc Vo

Vr Mandrel Vo

Vr Honing shoe

Oscillation

C26

Honing stone

–Vo Vo = oscillating speed Vr = rotating speed Vc = resulting cutting speed ⌬ = inclined angle Inset

Workpiece

Lay pattern

FIGURE 26-27 Schematic of honing head showing the manner in which the stones are held. The rotary and oscillatory motions combine to produce a cross-hatched lay pattern. Typical values for Vc and Ps are given below.

For:

Honing Parameters

Conventional Abrasives

Diamonds

CBN

High MRR

Vc(m/min)

20–30

40–70

35–90

Ps(N/min2)

1–2

2–8

2–4

Best-quality service

Vc(m/min)

5–30

40–70

20–60

Ps(N/min2)

0.5–1.5

1.0–3.0

1.0–2.0

cutting action. The abrasive grains range in size from 80 to 600 grit. The stones are equally spaced about the periphery of the tool. Reference values for Vc and honing stick pressure, Ps, for various abrasives are shown in Figure 26-27. Single- and multiple-spindle honing machines are available in both horizontal and vertical types. Some are equipped with special sensitive measuring devices that collapse the honing head when the desired size has been reached. For honing single, small, internal cylindrical surfaces, a procedure is often used wherein the workpiece is manually held and reciprocated over a rotating hone. If the volume of work is sufficient, honing is a fairly inexpensive process. A complete honing cycle, including loading and unloading the work, is often less than 1 min. Size control within 0.0003 in. is achieved routinely.

& 26.7 SUPERFINISHING Superfinishing is a variation of honing that is typically used on flat surfaces. The process is: 1. Very light, controlled pressure, 10 to 40 psi. 2. Rapid (more than 400 cycles per minute), short strokes—less than 14 in. 3. Stroke paths controlled so that a single grit never traverses the same path twice. 4. Copious amounts of low-viscosity lubricant-coolant flooded over the work surface. This procedure, illustrated in Figure 26-28, results in surfaces of very uniform, repeatable smoothness. Superfinishing is based on the phenomenon that a lubricant of a given viscosity will establish and maintain a separating, lubricating film between two mating surfaces if their roughness does not exceed a certain value and if a certain critical pressure, holding them apart, is not exceeded. Consequently, as the minute peaks on a surface are cut away by the honing stone, applied with a controlled pressure, a certain degree of smoothness is achieved. The lubricant establishes a continuous film between the stone and the workpiece and separates them so that no further cutting action occurs. Thus, with a given pressure, lubricant, and honing stone, each workpiece is honed to the same degree of smoothness.

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Page 741

SECTION 26.7

Superfinishing

741

Honing stick

Peaks penetrating the lubricating film (a)

FIGURE 26-28 In superfinishing and honing, a film of lubricant is established between the work and the abrasive stone as the work becomes smoother.

Film of lubricant maintained between smoother surface and honing stick. No further abrasive action (b)

Superfinishing is applied to both cylindrical and plane surfaces. The amount of metal removed usually is less than 0.002 in., most of it being the peaks of the surface roughness. Copious amounts of lubricant-coolant maintain the work at a uniform temperature and wash away all abraded metal particles to prevent scratching.

LAPPING Lapping is an abrasive surface finishing process wherein fine abrasive particles are charged (caused to become embedded) into a soft material, called a lap. The material of the lap may range from cloth to cast iron or copper, but it is always softer than the material to be finished, being only a holder for the hard abrasive particles. Lapping is applied to both metals and nonmetals. As the charged lap is rubbed against a surface, the abrasive particles in the surface of the lap remove small amounts of material from the surface to be machined. Thus, the abrasive does the cutting, and the soft lap is not worn away because the abrasive particles become embedded in its surface instead of moving across it. This action always occurs when two materials rub together in the presence of a fine abrasive: the softer one forms a lap, and the harder one is abraded away. In lapping, the abrasive is usually carried between the lap and the work surface in some sort of a vehicle, such as grease, oil, or water. The abrasive particles are from 120 grit up to the finest powder sizes. As a result, only very small amounts of metal are removed, usually considerably less than 0.001 in. Because it is such a slow metal removing process, lapping is used only to remove scratch marks left by grinding or honing or to obtain very flat or smooth surfaces, such as are required on gage blocks or for liquidtight seals where high pressures are involved. Materials of almost any hardness can be lapped. However, it is difficult to lap soft materials because the abrasive tends to become embedded. The most common lap material is fine-grained cast iron. Copper is used quite often and is the common material for lapping diamonds. For lapping hardened metals for metallographic examination, cloth laps are used. Lapping can be done either by hand or by special machines. In hand lapping, the lap is flat, similar to a surface plate. Grooves are usually cut across the surface of a lap to collect the excess abrasive and chips. The work is moved across the surface of the lap, using an irregular, rotary motion, and is turned frequently to obtain a uniform cutting action. In lapping machines for obtaining flat surfaces, workpieces are placed loosely in holders and are held against the rotating lap by means of floating heads. The holders, rotating slowly, move the workpieces in an irregular path. When two parallel surfaces are to be produced, two laps may be employed—one rotating below and the other above the workpieces. Various types of lapping machines are available for lapping round surfaces. A special type of centerless lapping machine is used for lapping small cylindrical parts, such as piston pins and ball-bearing races. Because the demand for surfaces having only a few micrometers of roughness on hardened materials has become quite common, the use of lapping has increased greatly. However, it is a very slow method of removing metal, obviously costly compared with other methods, and should not be specified unless such a surface is absolutely necessary.

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Abrasive Machining Processes

& 26.8 FREE ABRASIVES ULTRASONIC MACHINING Ultrasonic machining (USM), sometimes called ultrasonic impact grinding, employs an ultrasonically vibrating tool to impel the abrasives in a slurry at high velocity against the workpiece. The tool is fed into the part as it vibrates along an axis parallel to the tool feed at an amplitude on the order of several thousandths of an inch and a frequency of 20 kHz. As the tool is fed into the workpiece, a negative of the tool is machined into the workpiece. The cutting action is performed by the abrasives in the slurry, which is continuously flooded under the tool. The slurry is loaded up to 60 wt% with abrasive particles. Lighter abrasive loadings are used to facilitate the flow of the slurry for deep drilling (up to 2 in. deep). Boron carbide, aluminum oxide, and silicon carbide are the most commonly used abrasives in grit sizes ranging from 400 to 2000. The amplitude of the vibration should be set approximately to the size of the grit. The process can use shaped tools to cut virtually any material but is most effective on materials with hardnesses greater than RC 40, including brittle and nonconductive materials such as glass. Figure 26-29 shows a simple schematic of this process. USM uses piezoelectric or magnetostrictive transducers to impart high-frequency vibrations to the tool holder and tool. Abrasive particles in the slurry are accelerated to great speed by the vibrating tool. The tool materials are usually brass, carbide, mild steel, or tool steel and will vary in tool wear depending on their hardness. Wear ratios (workpiece material removed versus tool material lost) from 1:1 (for tool steel) to 100:1 (for glass) are possible. Because of the high number of cyclic loads, the tool must be strong enough to resist fatigue failure. The cut will be oversize by about twice the size of the abrasive particles being used, and holes will be tapered, usually limiting the hole depth-to-diameter ratio to about 3:1. Surface roughness is controlled by the size of the abrasive particles (finer finish with smaller particles). Holes, slots, or shaped cavities can be readily eroded in any hard material—conductive or nonconductive, metallic, ceramic, or composite. Advantages of the process include that it is one of the few machining methods capable of machining glass. Also, it is the safest machining method. Skin is impervious to the process because of its ductility. High-pitched noise can be a problem due to secondary Loop Highfrequency current

Nickel laminations Node Support

Transducer coil

Single phase 115 V 60~

Tool holder

Spindle thrust and feed

From slurry pump

Vibration

Loop Feed

Tool holder Tool Slurry Cooler

Vibration Tool

Abrasive slurry

Loop Fixture

Tool face area

Pump

Workpiece

Workpiece

Vibration stroke of tool Tank

FIGURE 26-29

Sinking a hole in a workpiece with an ultrasonically vibrating tool driving an abrasive slurry.

DETAIL

Working gap

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SECTION 26.8

Free Abrasives

743

vibrations. In addition to machining, ultrasonic energy has also been employed for coining, lapping, deburring, and broaching. Plastics can be welded using ultrasonic energy.

WATER-JET CUTTING AND ABRASIVE WATER-JET MACHINING Water-jet cutting (WJC), also known as water-jet machining or hydrodynamic machining, uses a high-velocity fluid jet impinging on the workpiece to perform a slitting operation (Figure 26-30). Water is ejected from a nozzle orifice at high pressure (up to 60,000 psi). The jet is typically 0.003 to 0.020 in. in diameter and exits the orifice at velocities up to 3000 ft/s. Key process parameters include water pressure, orifice diameter, water flow rate, and working distance (distance between the workpiece and the nozzle). Nozzle materials include synthetic sapphire, due to its machinability and resistance to wear. Tool life on the order of several hundred hours is typical. Mechanisms for tool failure include chipping from contaminants or constriction due to mineral deposits. This emphasizes the need for high levels of filtration prior to pressure intensification. In the past, long-chain polymers were added to the water to make the jet more coherent (i.e., not come out of the jet dispersed). However, with proper nozzle design, a tight, coherent water-jet may be produced without additives. The advantages of WJC include the ability to cut materials without burning or crushing the material being cut. Figure 26-30 shows a comparison (end view) of cutting corrugated boxboard with a mechanical knife and with WJC. The mechanism for material removal is simply the impinging pressure of the water exceeding the compressive strength of the material. This limits the materials that can be cut by the process to leather, plastics, and other soft nonmetals, which is the major disadvantage of the process. Alternative fluids (alcohol, glycerine, cooking oils) have been used in processing meats, baked goods, and frozen foods. Other disadvantages include that the process is noisy and requires operators to have hearing protection. The majority of the metalworking applications for water-jet cutting require the addition of abrasives. This process is known as abrasive water-jet cutting (AWC). A full range of materials, including metals, plastics, rubber, glass, ceramics, and composites, can be machined by AWC. Cutting feed rates vary from 20 in./min for acoustic tile to 50 in./min for epoxies and 500 in./min for paper products. Abrasives are added to the water-jet in a mixing chamber on the downstream side of the water-jet orifice. A single, central water-jet with side feeding of abrasives into a mixing chamber is shown in Figure 26-30. In the mixing chamber, the momentum of the water is transferred to the abrasive particles, and the water and particles are forced out through the AWC nozzle orifice, also called the mixing tube. This design can be made quite compact; however, it also experiences rapid wear in the mixing tube. An alternate configuration is to feed the abrasives from the center of the nozzle with a converging set of angled water-jets imparting momentum to the abrasives. This nozzle design produces better mixing of the water and abrasives as well as increased nozzle life. The inside diameter of the mixing tube is normally from 0.04 to 0.125 in. in diameter. These tubes are normally made of carbide. Generally, the kerf of the cut is about 0.001 in. greater than the nozzle orifice. AWC requires control of additional process parameters over water-jet machining, including abrasive material (density, hardness, shape), abrasive size or grit, abrasive flow rate (pounds per minute), abrasive feed mechanism (pressurized or suction), and AWC nozzle (design, orifice diameter, and material). Typical AWC systems operate under the following conditions: water pressures of 30,000 to 50,000 psi, water orifice diameters from 0.01 to 0.022 in., and working distances of 0.02 to 0.06 in. Working distances are much smaller than in WJC to minimize the dispersion of the abrasive waterjet prior to entering the material. Abrasive materials used include garnet, silica, silicon carbide, or aluminum oxide. Abrasive grit sizes range from 60 to 120 and abrasive flow rates from 0.5 to 3 lb/min. For many applications, the AWC tool is combined with a CNC-controlled X–Y table, which permits contouring and surface engraving. AWC can be used to cut any material through the appropriate choice of the abrasive, water-jet pressure, and feed rate. Table 26-6 gives cutting speeds for various metals. The ability of the abrasive water-jet to cut through thick materials (up to 8 in.) is

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CHAPTER 26

Water inlet

Abrasive Machining Processes

High-pressure intensifier pump and accumulator

Water filtration system (optional)

Booster pump

Swivels

Abrasive hopper

Abrasive cutting head

Abrasive nozzle

High-pressure water to nozzle X–Y Gantry robot motion control system

Abrasive metering valve

Control panel

Abrasive feed line to nozzle

Waste collection tank

Abrasive cutting head Pneumatic control line High-pressure water in

Valve actuator

Nozzle (sapphire) Abrasive in Highvelocity water jet

High-pressure valve body Concentrated abrasive jet slurry

Abrasive tube

Mounting collar

Poppet valve stem

Highpressure water inlet

Jet Work

Kerf

Poppet valve seat

Alignment adjustment screw

Nozzle body extension tube

Orifice assembly

Drain Abrasive mixing chamber

Abrasive feed inlet

Mixing tube

FIGURE 26-30 Schematic of hydrodynamic jet machining. The intensifier elevates the fluid to the desired nozzle pressure, while the accumulator smoothes out the pulses in the fluid jet. Schematic of an abrasive water-jet machining nozzle is shown on the right.

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SECTION 26.8

TABLE 26-6

Free Abrasives

745

Typical Values for Through-cutting Speeds for Simple Water-Jet and Abrasive Water-Jet of Machining Metals and Nonmetals.

Cutting speeds with abrasive waterjet Material

Thickness (in.)

Nozzle speed (in./min)

Edge quality (comments)

Material

Thickness (in.)

Nozzle speed (in./min)

Edge quality (comments)

Aluminum

0.130

20–40

good

Titanium

2.0

0.5–1.0

125 RMS

Aluminum tube

0.220

50

burred

Tool steel

0.250

3–15

125 RMS

Aluminum casting

0.400

15

Tool steel

1.0

2–5

Aluminum

0.500

6–10

Nonmetals

Aluminum

3.0

0.5–5

Acrylic

0.375

15–50

good to fair

Aluminum

4.0

0.2–2

C-glass

0.125

100–200

shape dependent

Brass

0.125

18–20

Carbon/carbon comp.

0.125

50–75

good

Brass

0.500

4–5

Carbon/carbon comp.

0.500

10–20

good

Brass

0.75

0.75–3

striations at 1 þ

Epoxy/glass composite

0.125

100–250

good

Bronze

1.100

1.0

good

Fiberglass

0.100

150–300

good

Copper

0.125

22

good

Fiberglass

0.250

100–150

good

Copper-nickel

0.125

12–14

fair edge

Glass (plate)

0.063

40–150

good

Copper-nickel

2.0

1.5–4.0

fair edge

Glass (plate)

0.75

10–20

125 RMS

Lead

0.25

10–50

good to striated

Graphite/epoxy

0.250

15–70

good to practical

Lead

2.0

3–8

slower ¼ better

Graphite/epoxy

1.0

3–5

good

Magnesium

0.375

5–15

good

Kevlar (steel reinf.)

0.125

30–50

good

Armor plate

0.200

1.5–15

good

Kevlar

0.375/0.580

10–25

good

Carbon steel

0.250

10–12

good

Kevlar

1.0

3–5

good

Carbon steel

0.750

4–8

good to bad edge

Lexan

0.5

10

good

Carbon steel

3.0

0.4

good w. sm. nozzle

Phenolic

0.25–0.50

10–15

good

4130 carbon steel

0.5

3.0

Plexiglass

0.175/0.50

25

Mild steel

7.5

0.017–0.05

Rubber belting

0.300

200

High-strength steel

3.0

0.38

Cast iron

1.5

1.0

good edge

Ceramic matrix composites

Stainless steel

0.1

10–15

good to striated

Toughened zirconia

0.250

1.5

Stainless steel

0.25

4–12

good to striated

SiC fiber in SiC

0.125

1.5

Stainless steel

1.0

1.0

65–150 RMS

0.125

2

15–5 PH stainless

4.0

0.3

striated

Al2O3/CoCrAly (60%/40%)

Inconel 718

1.25

0.5–1.0

good

SiC./TiB2 (15%)

0.250

0.35

Inconel

0.250

8–12

good to striated

Inconel

2–2.5

0.2

good to fair

Titanium

0.025–0.050

5–50

good

Metal matrix composites Mg/B4C (15%)

0.125

35

Al/SiC (15%)

0.500

8–12

good to fair

Titanium

0.500

1–5

65–150 RMS

Al/Al2O3 (15%)

0.250

15–20

good to fair

Material

Thickness (in.)

Nozzle speed (in./min)

Edge quality (comments)

good or small burr

good

fair

Table 2. Cutting speeds with simple waterjet Material

Thickness (in.)

Nozzle speed (in./min)

Edge quality (comments)

ABS plastic

0.087

20–50

100% separation

Lead

0.125

10

good, slight burr

Aluminum

0.050

2–5

burr

Plexiglass

0.118

30–35

fair

Cardboard

0.055

240–600

slits very well

Printed circuit bd

0.050–0.125

50–5

good

Delrin

0.500

2–5

good to stringers

PVC

0.250

10–20

good to fair

Fiberglass

0.100

40–150

good to raggy

Rubber

0.050

2400–3600

good

Formica

0.040

1450

Vinyl

0.040

2000–2400

good

Graphite composite

0.060

25

Wood

0.125

40

fair

Kevlar

0.040–0.250

50–53

fair, some furring

Comment on these tables: In trying to provide data on waterjet and abrasive waterjet cutting we have collected material from diverse sources. But we must note that most of the data presented is not from uniform tests. Also, note that in many cases data was largely absent on such parameters as pump horsepower, waterjet pressure, abrasiveparticle rate of flow or type or size, and standoff distance. So these cutting rates vary widely in value—from laboratory control to shop floor ballpark estimates. Many of the top speeds cited either represent cuts made to illustrate speed alone, without regard to surface quality, or may reflect data from machines with very high power output. (American Machinist, October 1989.)

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Abrasive Machining Processes

attributed to the reentrainment of abrasive particles in the jet by the workpiece material. AWC is particularly suited for composites because the cutting rates are reasonable and they do not delaminate the layered material. In particular, AWC is used in the airplane industry to cut carbon-fiber composite sections of the airplane after autoclaving.

ABRASIVE JET MACHINING One of the least expensive of the nontraditional processes is abrasive jet machining (AJM). AJM removes material by a focused jet of abrasives and is similar in many respects to AWC, with the exception that momentum is transferred to the abrasive particles by a jet of inert gas. Abrasive velocities on the order of 1000 ft/s are possible with AJM. The small mass of the abrasive particles produces a microscale chipping action on the workpiece material. This makes AJM ideal for processing hard, brittle materials, including glass, silicon, tungsten, and ceramics. It is not compatible with soft, elastic materials. Key process parameters include working distance, abrasive flow rate, gas pressure, and abrasive type. Working distance and feed rate are controlled by hand. If necessary, a hard mask can be placed on the workpiece to control dimensions. Abrasives are typically smaller than those used in AWC. Abrasives are typically not recycled, since the abrasives are cheap and are used only on the order of several hundred grams per hour. To minimize particulate contamination of the work environment, a dust-collection hood should be used in concert with the AJM system.

& 26.9 DESIGN CONSIDERATIONS IN GRINDING Almost any shape and size of work can be finished on modern grinding equipment, including flat surfaces, straight or tapered cylinders, irregular external and internal surfaces, cams, anti-friction-bearing races, threads, and gears. For example, the most accurate threads are formed from solid cylindrical blanks on special thread-grinding machines. Gears that must operate without play are hardened and then finish ground to close tolerances. Two important design recommendations are to reduce the area to be ground and to keep all surfaces that are to be ground in the same or parallel planes (Figure 26-31). This is an example of design for manufacturing (DFM). Machine A

B

Original design of base plate

Original design of crankshaft bearing bracket

Grind

B

FIGURE 26-31 Reducing area to be ground and keeping all surface to be ground in the same or parallel planes are two important design recommendations. (From Machine Design, June 1, 1972, p. 87)

Redesigned to reduce weight and grinding time

Redesign eliminated shoulders and made part suitable for grinding in single setup

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Review Questions

747

Abrasive machining can remove scale as well as parent metal. Large allowances of material, needed to permit conventional metal-cutting tools to cut below hard or abrasive inclusions, are not necessary for abrasive machining. An allowance of 0.015 in. is adequate, assuming, of course, that the part is not warped or out of round. This small allowance requirement results in savings in machining time, in material (often 60% less metal is removed), and in shipping of unfinished parts.

& KEY WORDS abrasive abrasive machining abrasive jet machining (AJM) abrasive water-jet cutting (AWC) aluminum oxide attrition bonded product burn centerless grinding center-type cylindrical grinding chucking-type external grinding coated abrasive coated product

corundum creep feed grinding (CFG) crush dressing cubic boron nitride (CBN) cutting design for manufacturing (DFM) diamond disk grinder dressing emery friability G ratio garnet grade grinding hardness

honing honing stone hydrodynamic machining infeed lap lapping off-hand grinding plowing plunge-cut grinding quartz regulating wheel resinoid bond rubber bond rubbing shellac bond silicate bond silicon carbide

snagging structure superadhesive bond superfinishing surface grinding swing grinder truing ultrasonic impact grinding ultrasonic machining (USM) universal tool and cutter grinder vitrified bond water-jet cutting (WJC) water-jet machining

& REVIEW QUESTIONS 1. What are machining processes that use abrasive particles for cutting tools called? 2. What is attrition in an abrasive grit? 3. Why is friability an important grit property? 4. Explain the relationship between grit size and surface finish. 5. Why is aluminum oxide used more frequently than silicon carbide as an abrasive? 6. Why is CBN superior to silicon carbide as an abrasive in some applications? 7. What materials commonly are used as bonding agents in grinding wheels? 8. Why is the grade of a bond in a grinding wheel important? 9. How does grade differ from structure in a grinding wheel? 10. What is crush dressing? 11. How does loading differ from glazing? 12. What is meant by the statement that grinding is a mixture of processes? 13. What is accomplished in dressing a grinding wheel? 14. How does abrasive machining differ from ordinary grinding? 15. What is a grinding ratio or G ratio? 16. How is the feed of the workpiece controlled in centerless grinding? 17. Why is grain spacing important in grinding wheels? 18. Why should a cutting fluid be used in copious quantities when doing wet grinding? 19. How does plunge-cut grinding compare to cylindrical grinding? 20. If grinding machines are placed among other machine tools, what precautions must be taken? 21. What is the purpose of low-stress grinding? 22. How is low-stress grinding done compared to conventional grinding?

23. The number of grains per square inch that actively contact and cut a surface decreases with increasing grain diameter. Why is this so? 24. Why are centerless grinders so popular in industry compared to center-type grinders? 25. Explain how an SEM micrograph is made. Check the Internet or the library to find the answer. 26. Why are vacuum chucks and magnetic chucks widely used in surface grinding but not in milling? 27. How does creep feed grinding differ from conventional surface grinding? 28. Why does a lap not wear, even though it is softer than the material being lapped? 29. How do honing stones differ from grinding wheels? 30. What is meant by ‘‘charging’’ a lap? 31. Why is a honing head permitted to float in a hole that has been bored? 32. How does a coated abrasive differ from an abrasive wheel? 33. Figure out why the bottoms of chips shown in Figure 26-9 are so smooth. The magnification of the micrograph is 4800B. How thick are these chips? 34. What is the inclined angle in honing, and what determines it? 35. What are the common causes of grinding accidents? 36. What other machine tool does a surface grinder resemble? 37. Figure 26-11 showed residual stress distributions produced by surface grinding. What is a residual stress? 38. In grinding, what is infeed versus cross feed? 39. One of the problems with water-jet cutting is that the process is very noisy. Why? 40. In AWC, what keeps the abrasive jet from machining the orifice?

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Abrasive Machining Processes

& PROBLEMS 1. Perhaps you have observed the following wear phenomena: A set of marble or wooden steps shows wear on the treads in the regions where people step when they climb (or descend) the steps. The higher up the steps, the less the wear on the tread. Given that soles of shoes (leather, rubber) are far softer than marble or granite, explain: a. Why and how the steps wear. b. Why the lower steps are more worn than the upper steps. 2. Explain why it is that a small particle of a material can be used to abrade a surface made of the same material (i.e., why

Chapter 26

does the small particle act harder or stronger than the bulk material)? 3. In grinding, both the wheel and workpiece are moving (or rotating). Using the data in Figure 26-11 and assuming that you are doing surface grinding (see Figure 26-1), what are some typical MRR values? How do these compare to MRR values for other machining processes, such as milling? What is the significance of this?

CASE STUDY

Process Planning for the MfE

A

t Lulu’s WarEagle company, Figure CS-26A shows a part design for a small flange to be made out of 1020 steel. Prepare a sequence of operations or process plan to make this part. Note that you need to machine the top, bottom, and all the holes. Specify the machines and the tools you would use, assuming you select bar stock as your raw material. The term ‘‘Co bore’’ in the drawing means counter bore. See Chapter 2 for an example of a process plan. Assume the lot size here is 1200 part/yr made in lots of 120 every month. Now assume that the designer provides you a design like Figure CS-26B and calls for the flange to be made from

cast iron. The casting will probably have a large hole in the center cored during the casting process. What casting process will you use? Prepare another process plan for the cast part. Will the sequence of operations be the same? What about the cost per unit? Which will be lower? 1. The part drawings failed to provide a critical specification in the design. What is it? 2. The drawing failed to show the final geometry (in the top view) correctly. Redraw the part to show these corrections. 3. As the process engineer, how would you advise the design engineer he screwed up—twice?

(5)

8

— drill 8 16 4 holes equally spaced within 0.25 (0.010)

50 (2) Diam.

13 –– 2 Diam. drill and ream

(1 )

13 –– 2 Diam. drill and ream

() 5( )

16

5 –– 8 Diam. 3 — Deep 16

16 3 (–– 16)

ƒ 14

9 (–– 16)

ƒ

32

(1––14 )

Material: 1020 Steel 140 BHN

(1 )

Co bore 5

3 (–– 16)

ƒ 14

9 (–– 16)

ƒ

32

(1––14 )

5

() ( )

5 –– 8 Diam. 3 — Deep 16

Material: Gray iron casting 140 BHN Dimensions in mm (in.)

Dimensions in mm (in.) Figure CS 26a

(5)

— drill 8 16 4 holes equally spaced within 0.25 (0.010)

(25—8 ) Diam.

63

5

(16—5 ) R

Figure CS 26b

FIGURE CS-26 Two different designs of a flange. The design on the right suggested by Doyle et al. in 3rd edition of Manufacturing Process and Materials for Engineers, Prentice-Hall.

Co bore

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CHAPTER 27 WORKHOLDING DEVICES FOR MACHINE TOOLS 27.1 INTRODUCTION 27.2 CONVENTIONAL FIXTURE DESIGN 27.3 TOOL DESIGN STEPS 3-2-1 Location Principle 27.4 CLAMPING CONSIDERATIONS 27.5 CHIP DISPOSAL 27.6 UNLOADING AND LOADING TIME 27.7 EXAMPLE OF JIG DESIGN 27.8 TYPES OF JIGS

27.9 CONVENTIONAL FIXTURES 27.10 MODULAR FIXTURING 27.11 SETUP AND CHANGEOVER Four Stages of SMED Intermediate Jig Concept 27.12 CLAMPS 27.13 OTHER WORKHOLDING DEVICES Assembly Jigs Magnetic Workholders

Electrostatic Workholders Vacuum Chucks 27.14 ECONOMIC JUSTIFICATION OF JIGS AND FIXTURES Jig Cost Example Case Study: Fixture versus No Fixture in Milling

& 27.1 INTRODUCTION Workholding devices, often called jigs and fixtures, are critical components in the manufacturing of interchangeable parts. Workholders hold and locate the work in the machine tool with respect to the cutting tool. For example, Figure 27-1 shows a machining center with a fixture that holds the workpiece in the correct location with respect to the cutting tools. For many machine tools, jigs and fixtures hold the workpieces while providing location with respect to the cutting tools. With workholders, process accuracy and precision (repeatability) can be achieved that otherwise would be impossible with a given combination of cutting tools and machine tools. In this chapter, workholding devices (jigs and fixtures) will be considered as important production tools or adjuncts, with primary attention being directed toward their functional characteristics, their relationship to the machine tools, and the manufacturing processes. In recent years, workholding devices have become more flexible; that is, they are able to (1) hold more than one part and (2) be quickly changed over for different parts. Workholders that can be quickly exchanged are critical elements in lean manufacturing cells, where components are made in families of parts (groups of parts of similar design). Further, being able to change from one device to another quickly to accommodate different parts means smaller lot sizes can be run, which reduces inventory levels in plants. In computer numerical control (CNC) machines (described in Chapter 40), the workholders can be automatic. For example in Figure 27-2, the CNC turning center has two chucks and the workpiece can be automatically transferred from one to the other, so the powered tools on the turrets can work on both ends of the part. These flexibility requirements add significantly to the complexity of conventional jig and fixture design. Let’s begin with a discussion of the basics of jig and fixture design.

& 27.2 CONVENTIONAL FIXTURE DESIGN In the conventional method of fixture design, tool designers rely on their experience and intuition to design simple, single-purpose fixtures for specific machining operations, often using a trial-and-error method until the workholders perform satisfactorily. Of course, these designers should calculate the clamping forces or stress distributions in the fixturing elements to make sure that the loads will not deform the fixtures or the workpieces elastically or plastically. In the design of the workholding devices, two primary functions must be considered: locating and clamping. Locating refers to orienting

749

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CHAPTER 27

Workholding Devices for Machine Tools

FIGURE 27-1 An example of a fixture in a milling machine holding a part.

Upper turret Main spindle

Main spindle Work Lower turret

Powered rotary tools

Spindle orientation can be fixed during cutting

Subspindle Chuck Upper turret

Chuck Finished part out Chuck

Main spindle

Subspindle

Automatic Transfer

Raw material in Lower turret

FIGURE 27-2 CNC turning center with two chucks, turrets for cutting tools, and C-axis control for the main spindle. The C-axis control, on the spindle, can stop it in any orientation so the powered tools can operate on the workpiece.

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SECTION 27.2

Conventional Fixture Design

751

h

e d

g

c

FIGURE 27-3 Drawing of a plate showing locating dimensions (a, b, c, d) versus sizing dimensions (e, f, g, h).

a b

f

and positioning the part in the machine tool with respect to the cutting tools to achieve the required specifications. Clamping refers to holding or maintaining the part in that location during the cutting operations (resisting the cutting forces). Jigs and fixtures are specially designed and built workholding devices that hold the work during machining or assembly operations. In addition, a jig determines a location dimension that is produced by machining or fastening. For example, location dimensions determine the position of a hole on a plate (Figure 27-3). Consider the subject of dimensioning as used in drafting practice. Dimensions are of two types: size and location. Size dimensions denote the size of geometrical shapes—holes, cubes, parallelepipeds, and so on—of which objects are composed. Location dimensions, on the other hand, determine the position or location of these geometrical shapes with respect to each other. Thus a and c in Figure 27-3 are location dimensions, whereas e and g are size dimensions. With location dimensions in mind, one can precisely define a jig as follows: a jig is a special workholding device that, through built-in features, determines location dimensions that are produced by machining or fastening operations. The key requirement of a jig is that it determine a location dimension. Thus, jigs accomplish layout by means of their design. In order to establish location dimensions, jigs may do a number of other things. They frequently guide tools, as in drill jigs, and thus determine the location of a component geometrical shape. However, they do not always guide tools. In the case of welding jigs, component parts are held (located) in a desired relationship with respect to each other while an unguided tool accomplishes the fastening. The guiding of a tool is not a necessary requirement of a jig. Similarly, jigs usually hold the work that is to be machined, fastened, or assembled. However, in certain cases, the work actually supports the jig. Thus, although a jig may incidentally perform other functions, the basic requirement is that, through qualities that are built into it, certain critical dimensions of the workpiece are determined. A fixture is defined as a special workholding device that holds work during machining or assembly operations and establishes size dimensions. The key characteristic is that it is a special workholding device, designed and constructed for a particular part or shape. A general-purpose device, such as a chuck in a lathe or a clamp on a milling machine table, is usually not considered to be a fixture. Thus a fixture has as its specific objective the facilitating of setup, or making the part holding easier. Because many jigs hold the work while determining critical location dimensions, they usually meet all the requirements of a fixture. Alternatively, many fixtures are used in numerical control (NC) machines holding parts where holes are located and drilled according to a program. So the strict definition of jigs and fixtures has been blurred by the changes in technology. In designing workholders, the designer must consider whether the part is a casting, forging, or bar stock. With castings and forgings, variations in shape and size must be accommodated in the design, and usually a machining operation is required to establish a reference surface (called the datum surface) to aid initial fixturing.

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TABLE 27-1

Workholding Devices for Machine Tools

Design Criteria for Workholders

Positive location A fixture must, above all else, hold the workpiece precisely in space to prevent each of 12 kinds of degrees of freedom—linear movement in either direction along the x-, y-, and z-axes and rotational movement in either direction about each axis. Repeatability Identical workpieces should be located by the workholder in precisely the same space on repeated loading and unloading cycles. It should be impossible to load the workpiece incorrectly. This is called ‘‘foolproofing’’ the jig or fixture. Adequate clamping forces The workholder must hold the workpiece immobile against the forces of gravity, centrifugal forces, inertial forces, and cutting forces but not distort the part. Milling and broaching operations, in particular, tend to pull the workpiece out of the fixture, and the designer must calculate these machining forces against the fixture’s holding capacity. The device must be rigid. Reliability The clamping forces must be maintained during machine operation every time the device is used. The mechanism must be easy to maintain and lubricate. Ruggedness Workholders usually receive more punishment during the loading and unloading cycle than during the machining operation. The device must endure impact and abrasion for at least the life of the job. Elements of a device that are subject to damage and wear should be easily replaceable. Design and construction ease Workholders should use standard elements as much as possible to allow the engineer to concentrate on function rather than on construction details. Modular fixtures epitomize this design rule as the entire workholder is made from standard elements, permitting a bolttogether approach for substantial time and cost savings over custom workholders. Low profile Workholder elements must be clear of the cutting-tool path. Designing lugs on the part for clamping can simplify the fixture and allow proper tool clearance. Workpiece accommodation Surface contours of castings or forgings vary from one part to the next. The device should tolerate these variations without sacrificing positive location or other design objectives. Ergonomics and safety Clamps should be selected and positioned to eliminate pinch points and facilitate ease of operation. The workholder elements should not obstruct the loading or unloading of workpieces. In manual operations, the operator should not have to reach past the tool to load or unload parts. A rule sometimes used is that the operator can repeatedly exert a force of 30 to 40 lb to open or close a clamp, but greater forces than this can cause ergonomic problems. Freedom from part distortion Parts being machined can be distorted by gravity, the machining forces, or the clamping forces. Once clamped into the device, the part must be unstressed or, at least, undistorted. Otherwise, the newly machined surfaces take on any distortions caused by the clamping forces. Flexibility The workholding device can locate and restrain more than one type (design) of part. Many different schemes are being proposed to provide workholder flexibility. Modular vise fixturing, programmable clamps using air-activated plungers, part encapsulation with a low-melting-point alloy, and NC clamping machines are some of the more recently developed systems. Despite their flexibilities, these clamping systems have some significant drawbacks. They are expensive, and the individual systems may not integrate well into individual machine tools. (See the discussions of intermediate jig concept and group jigs for additional thoughts on flexibility.)

Even in parts cut from bar stock, allowances must be made for inaccuracies and irregularities produced by the cutoff operations. Table 27-1 provides a summary of design criteria for workholders for careful to review. Obviously, it is impossible to meet all these design criteria for workholders. Compromise is inevitable. Still, it is useful to know the optimal design objectives to illustrate the positioning, holding, and supporting functions that fixtures must fulfill.

& 27.3 TOOL DESIGN STEPS The classical design of a workholder (e.g., a drill jig) involves the following steps: 1. Review the design criteria for workholders in Table 27-1. 2. Analyze the drawing of the workpiece and determine (visualize) the machining operations required to machine it. Note the critical (size and location) dimensions and tolerances. This is called the sequence of operations. 3. Determine the orientations of the workpiece in relation to the cutting tools and the movements of the tools and tables. 4. Perform an analysis to estimate the magnitude and direction of the cutting forces (see Chapter 20). 5. Study the standard devices available for workholders and for the clamping functions. Can an off-the-shelf device be modified? What standard elements can be used? 6. Form a mental picture of the workpiece in position in the workholder in the machine tool with the cutting tools performing the required operation(s). See Figure 27-4 and chapters on machining for more examples. 7. Make a three-dimensional sketch of the workpiece in the workholder in its required position to determine the location of all the elements: clamps, locator buttons, bushings, and so on. Use the 3-2-1 location principle discussed next.

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SECTION 27.4

FIGURE 27-4 Workpiece location is based on the 3-2-1 principle. Three points will define a base surface, two points in a vertical plane will establish an end reference, and one point in a third plane will positively locate most parts.

Clamping Considerations

753

First plane Third plane Workpiece Workpiece Second plane

8. Make a sketch of the workholder and workpiece in the machine tool to show the orientation of these elements with respect to the cutting tool in the machine tool.

3-2-1 LOCATION PRINCIPLE After determining the orientation of the workpiece in the workholder, the next step is to locate it in that position. This location is also used for all similar workpieces. The tool designer must select or design locating devices (supports) that ensure that every workpiece placed in the device occupies the same position with respect to the cutting tools. Thus, when the machining operation is performed, the workpieces are processed identically. This is, of course, the key to making interchangeable parts. In locating the workpiece, the basic 3-2-1 principle of location is used (Figure 27-4). For positive location, the fixture must position the workpiece in each of three perpendicular planes. Positioning processes can vary greatly, but workholder design always begins by defining the first plane of reference with three points. The first plane is supported by three flattened balls that swivel in sockets, providing a self-adapting surface. Once the object is defined in a single plane, supported at three points (like a three-legged stool on a floor), a second plane can be assigned that is perpendicular to the first. To do this, the object is brought up against any two points in the second plane. To continue the example, the stool is slid along the floor until two legs touch a wall. A third plane, perpendicular to each of the other two, is then defined by designating one point on it. As long as an object is in contact with three points on the first plane, two points on the second, and a single point on the third, it is positively located in space. The location points within each plane should be selected as far apart as possible for maximum stability. In practice, it is often necessary to support a workpiece on more points than this 3-2-1 formula dictates. The machining of a large rectangular plate, for example, typically requires support at four or more points. However, any extra points must be established carefully to support the workpiece in a plane defined by three—and only three—points. Appropriate clamping devices are selected so that the clamping forces hold the workpiece in the proper location and resist the effects of the cutting forces, centrifugal forces, and vibrations. If possible, the machining forces should act into the location points, not into the clamps, so that smaller clamps can be used. In reality, the worker often determines clamping force when loading the part into the workholder. Fixtures are usually fastened to the table of the machine tool. Although used primarily on milling and broaching machines, fixtures are also designed and used to hold workpieces for various operations on most of the standard machine tools and machining centers. Black’s 20 principles for fixture design are given in Table 27-2.

& 27.4 CLAMPING CONSIDERATIONS Clamping of the work is closely related to support of the work. Any clamping, of course, induces some stresses into the part that can cause some distortion of the workpiece, usually elastic. If this distortion is measurable, it will cause some inaccuracy in final dimensions of the part, as illustrated in an exaggerated manner in Figure 27-5. The obvious solution is to spread the clamping forces over a sufficient area to reduce the stresses to a level that will not produce appreciable distortion. The clamping forces should direct the work against the points of location and work support. Clamped surfaces often

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Workholding Devices for Machine Tools

TABLE 27-2

20 Principles for Workholder Design

1. Determine the critical surfaces or points for the part, based on the design. 2. Decide on locating points and clamping arrangements. 3. Try to use 3-2-1 location, with 3 assigned to the largest surface. Additional points should be adjustable. 4. Locating points should be visible so that the operator can see if they are clean. Can they be replaced if worn? 5. Provide clamps that are as quick acting and easy to use as is economically justifiable for rapid loading and unloading. 6. Clamps should not require undue effort by the operator to close or to open, nor should they harm hands and fingers during use. 7. Clamps should be integral parts of device. Avoid loose parts that can get lost. 8. Avoid complicated clamping arrangements or combinations that can wear out or malfunction. Keep it simple. 9. Locate clamps opposite locaters (if possible) to avoid deflection/distortion during machining and springback afterward. 10. Take the thrust of the cutting forces on the locaters (if possible), not on the clamps. 11. Arrange the workholder so that the workpiece can easily be loaded and unloaded from the device. 12. Design the workholder so that the part can be loaded only in the correct manner (mistake-proof) and in such a way that the location can be found quickly (visually). 13. Consistent with strength and r