SHEET METAL FORMING PROCESSES AND DIE DESIGN VUKOTA BOLJANOVIC, Ph.D. Doctor in Technical Science Industrial Press New York Library of Congress Cataloging-in-Publication Data Boljanovic, Vukota. Sheet metal forming processes and die design / Vukota Boljanovic. p. cm. Includes bibliographical references and index. ISBN 0-8311-3182-9, ISBN-13 978-0-8311-3182-1 1. Sheet-metal work. 2. Dies (Metal-working) I. Title. TS250.B815 2004 671.8´23—dc22 2004042151 Industrial Press Inc. 200 Madison Avenue New York, New York 10016 Copyright © 2004. Printed in the United States of America. All rights reserved. This book or parts thereof may not be reproduced, stored in a retrieval system or transmitted in any form without the permission of the publisher. 10 9 8 7 6 5 4 3 2 LIST OF TABLES 1.1 Value of the modulus of elasticity, measured in different directions 6 3.1 Relative amount of penetration of the upper blade into the material 26 4.1 Values for clearances as a percentage of the thickness of the materials 32 4.2 Absolute values for clearance for some stock materials 33 4.3 Absolute values for mandn 36 4.4 Number of lines iin an alternate multi-line layout 38 4.5 Value for specific pressure p(MPa) 41 4.6 Values for ddepending on type and thickness of the materials 43 5.1 Value of Specific Pressure p (MPa) 52 5.2 Values of the coefficient c 58 5.3 Values of coefficient 62 5.4 Values of Springback Factor Ks 65 6.1 Numbers of draws (n) for a cylindrical cup draw 72 6.2 Optimal ratio mfor drawing a cylindrical cup without flange 73 6.3 Coefficient of friction l 75 6.4 Percent value added for trimming (e) 77 6.5 Values of material added for trimming (∆c) 79 6.6 Lubricants commonly used in deep drawing process 83 7.1 Median values of nosing ratio msfor different materials 91 7.2 Values of expanding ratio Kfor enlarging portions of drawing shell 94 7.3 Values of die corner radius R , for dimpling 94 m 7.4 Values of the ratio of the hole diameter to the flange diameter m 96 7.5 Values of specific pressure p(MPa) 100 7.6 Maximum values for drawing ratio m, drawing height h, and minimum drawing radius R 100 m 9.1 Value of Dimension hDepending on Material Thickness T 111 9.2 Value of Constant c 113 9.3 Values of Cs 120 9.4 Values for Handh 125 9.5 Values of the French notch width w 127 11.1 Values of coefficient kfor different materials 151 11.2 Work tolerances for draw ring and punch 152 13.1 Basic designation for SAE and AISI steels 176 13.2 Influence of alloying elements on the characteristic properties of steels 177 13.3 Machinability rating for some types of steels 178 13.4 Mechanical properties of selected carbon and alloy steels 178 13.5 Mechanical properties of quenched and tempered carbon and alloy steels 179 13.6 Typical applications and characteristics of various carbon and alloy steels 180 13.7 Basic types of tool and die steels 180 13.8 Typical alloying elements for cold work tool and die steels 181 13.9 Typical applications of common tool and die steels 181 A-1.1 Equations for calculating blank-diameters of drawn shells 185 A-2.1 Preferred fits using the hole-basis system 192 A-2.2 Metric system tolerance for holes 192 A-2.3 Metric system tolerance for shafts 194 A-2.4 ISO-symbols for geometric tolerances in engineering drawings 196 A-3.1 Group of ASTM standards for sheet, and strip 200 A-3.2 Nominal thickness of sheet and strip 201 A-3.3 SI base units 202 A-3.4. Examples of SI derived units 203 A-3.5 SI derived units with special names and symbol 203 A-3.6 Examples of SI derived units whose names and symbols include SI derived units with special names and symbols 205 A-3.7 SI prefixes 205 A-3.8 Units outside the SI that are accepted for use with the SI 206 A-3.9 Metric conversion factors ( approximate) 207 xv PREFACE A very large variety of sheet-metal forming processes is used in modern sheet-metal press-working shop practice. Many of these deformation processes, used in making aircraft, automobiles, and other products, use complex equipment that is derived from the latest discoveries in science and technology. With the ever- increasing knowledge of science and technology, future deformation processes promise to be even more complex to satisfy the demand for more productivity, lower cost, and greater precision. However, for all their advantages, the more sophisticated deformation processes of today have not replaced the need for basic sheet-metal forming processes and dies. This book draws on the author’s 30-plus years of experience as an engineer and provides a com- plete guide to modern sheet-metal forming processes and die design — still the most commonly used manufacturing methodology for the mass production of complex, high-precision parts. Much more prac- tical than theoretical, the book covers the “hows” and “whys” of product analysis, and the mechanisms of blanking, punching, bending, deep drawing, stretching, material economy, strip design, movement of metal during stamping, and tooling design. Readers will find numerous illustrations, tables, and charts to aid in die design and manufacturing processes; Formulas and calculations needed for vari- ous die operations and performance evaluation are included; and designations, characteristics, and typ- ical applications of various carbon and alloy steels for different die components are evaluated. The book concentrates on simple, practical engineering methods rather than complex numerical tech- niques to provide the practicing engineer, student, technician, and die maker with usable approaches to sheet-metal forming processes and die design. The first part of the book deals with the structures of metals and the fundamental aspects of the mechanical behavior of metals. Knowledge of structures is necessary to controlling and predicting the behavior and performance of metals in sheet-metal forming processes. The second part of the book covers all aspects of forming sheet metal. It presents the fundamental sheet-metal forming operations of shearing, blanking and punching, bending, stretching, and deep draw- ing. Mechanics of various drawing processes indicate ways in which the deformation, loads, and process limits can be calculated for press forming and deep drawing operations. The book includes various draw- ing processes (nosing, expanding, dimpling, spinning and flexible die forming) mostly used in the aircraft and aerospace industry. Dies are very important to the overall mass production picture, so they are discussed in the last sec- tion of the book, which presents a complete picture of the knowledge and skills needs for the effective design of dies for sheet-metal forming processes described. Special attention is given to: • Formulas and calculations needed for various die parts. • Rules of thumb and innovative approaches to the subject. • Properties and typical applications of selected tool and die materials for various die parts. xiii xiv ▼ Preface Although the book provides many examples of calculations, illustrations, and tables to aid in sheet- metal forming processes, die design, and die manufacturing, it should be evident that it is not possible to present all the data, tables, statistics, and other information needed to design complicated dies and other tools for sheet-metal forming in one text. However, the book endeavors to provide most of the information needed by a die designer in practical situations. The author owes much to many people. No book can be written in a vacuum. I am grateful to my wife, who understands my need for long periods of isolation. I also wish to express my deepest appreciation and thanks for the competent work of Em Turner Chitty, who labored countless hours at editing and proof- reading. Finally, I wish to thank my English language teacher from The University of Tennessee, Anwar F. Accawi, who encouraged me to begin writing this book. Vukota Boljanovic Knoxville, Tennessee TABLE OF CONTENTS Preface xiii List of Tables xv Introduction xvii Basic Characteristics of Sheet-Metal Forming Processes xvii Categories of Sheet-Metal Forming Processes xvii Characteristics of Sheet-Metal Parts and Their Technological Properties xviii PART I THEORETICAL FUNDAMENTALS 1 Chapter 1 The Structure of Metals 3 1.1 Introduction 3 1.2 The Crystal Structure of Metals 3 1.3 Deformation and the Strength of Crystals 6 1.3.1 Imperfections in the Crystal Structure of Metals 7 1.3.2 Grain Sizes and Boundaries 9 1.3.3 Strain Hardening 10 1.4 Recovery and Recrystalization 11 Chapter 2 Mechanical Behavior of Materials 13 2.1 Introduction 13 2.2 Stress / Strain Curves 16 2.3 Ductility 18 2.4 True Stress and True Strain 19 PART II MANUFACTURING PROCESSES 21 Chapter 3 Shearing Process 23 3.1 Mechanics of Shearing 23 3.2 Shearing Forces 23 3.2.1 Shearing with Straight Parallel Cutters 24 3.2.2 Shearing with Straight Inclined Cutters 25 3.3.3 Shearing with Rotary Cutters 27 Chapter 4 Blanking and Punching 29 4.1 Blanking and Punching Mechanism 29 4.2 Blanking and Punching Clearances 31 4.3 Punch Force 33 4.3.1 Punch and Die with Parallel Cut Edges 33 vii viii ▼ Table of Contents 4.3.2 Punch and Die with Bevel-cut Edges 34 4.4 Material Economy 35 4.4.1 Scrap Strip Layouts 35 4.4.2 Altering the Design of the Workpiece 37 4.4.3 Multi-line Layout 37 4.4.4 Utilizing Scrap from One Piece as Material for Another Piece 38 4.5 Shaving 39 4.5.1 Shaving a Punched Workpiece 39 4.5.2 Shaving a Blanked Workpiece 43 Chapter 5 Bending 45 5.1 Introduction 45 5.2 Mechanics of Bending 45 5.3 Moment of Bending 46 5.3.1 Moment of Bending in Elastic-Plastic Domain 49 5.3.2 The Moment of Bending in the Purely Plastic Domain 50 5.4 Bending Forces 51 5.4.1 Force for a U-die 51 5.4.2 Forces for a Wiping Die 52 5.4.3 Forces for a V-die 53 5.4.4 Curling 55 5.4.5 Three-Roll Forming 56 5.5 Bend Radius 57 5.5.1 Minimum Bend Radius 57 5.5.2 Maximum Bend Radius 59 5.6 Bend Allowance 60 5.7 Springback 64 5.8 Clearance 67 Chapter 6 Deep Drawing 69 6.1 Introduction 69 6.2 Mechanics of Deep Drawing 70 6.2.1 Deep Drawability 72 6.3 Forces 74 6.3.1 First Drawing Operation 74 6.3.2 Subsequent Drawing Operations 75 6.4 Blank Calculations for Symmetrical Shells 76 6.5 Determining Shape of Blank for Nonsymmetrical Shells 78 6.5.1 Rectangular Shells 78 6.5.2 Square Shells 81 6.6 Drawing Practice 82 Table of Contents ▼ix 6.6.1 Defects During Deep Drawing 82 6.6.2 Lubrication in Drawing 82 Chapter 7 Various Forming Processes 85 7.1 Stretch Forming 85 7.2 Nosing 87 7.3 Expanding 92 7.4 Dimpling 94 7.5 Spinning 96 7.6 Flexible Die Forming 99 7.6.1 Force in Die Forming 99 PART III DIE DESIGN 101 Chapter 8 Basic Die Classifications and Components 103 8.1 Die Classifications 103 8.2 Basic Die Components 105 Chapter 9 Blanking and Punching Dies 109 9.1 Introduction 109 9.2 Die Blocks 110 9.2.1 Die Opening Profile 110 9.2.2 Fastening to the Die Shoe 111 9.2.3 Sectioned Die 112 9.2.4 Calculation of Die Block Dimensions 113 9.3 Punches 116 9.3.1 Punch Face Geometry 116 9.3.2 Methods for Assembling Punches 116 9.3.3 Punch Calculations 118 9.4 Stripper Plates 120 9.4.1 Stripper Force 120 9.4.2 Stripper Design 121 9.5 Die Components for Guiding and Stopping 124 9.5.1 Stock Guides and Guide Rails 124 9.5.2 Die Stops and French Notch Punch 126 9.5.3 Positioning the Individual Blank 127 9.5.4 Pilots 128 9.6 Center of Die Pressure 129 9.7 Examples of Cutting Die Designs 131 Chapter 10 Bending Dies 135 10.1 Introduction 135 10.2 Simple Die Designs 135 10.2.1 U-Profile Bend Dies 135 x ▼ Table of Contents 10.2.2 V-profile Bend Dies 137 10.2.3 Universal Bending Dies 138 10.3 Dies of Complex Design 139 10.3.1 Closing Profile Dies 139 10.3.2 Special Bending Dies 139 10.3.3 Curling and Hinge Dies 141 10.3.4 Tube-forming Dies 142 10.3.5 Multiple-bend Dies 144 10.3.6 Combination Dies 144 10.3.7 Progressive Dies 145 Chapter 11 Deep Drawing Dies 147 11.1 Introduction 147 11.2 Draw Rings 147 11.2.1 Draw Rings with Corner Radius 147 11.2.2 Draw Rings with Conical Profile 149 11.3 Clearances and Tolerances 151 11.3.1 Calculation of the Dimensions of the Punch and Die 151 11.4 Blank Holders 154 11.4.1 Blank Holders 154 11.4.2 Blank Holder Pressure 155 11.4.3 Blank Holder Force 155 11.4.4 Draw Beads 156 11.5 Single-operation Dies 158 11.6 Multi-operation Dies 160 11.7 Progressive Dies 160 11.8 Drawing Dies for Pieces of Spherical and Parabolic Shape 162 11.9 Ironing Dies 164 Chapter 12 Various Forming Dies 167 12.1 Nosing Dies 167 12.2 Expanding and Bulging Dies 170 12.2.1 Expanding Dies 170 12.2.2 Bulging Dies 171 12.3 Flanging Dies 172 Chapter 13 Tool and Die Materials 175 13.1 Introduction 175 13.2 Carbon and Alloy Steels 175 13.2.1 Designations for Carbon and Alloy Steels 176 13.2.2 Effects of Various Alloy Elements in Steels 176 13.2.3 Carbon Steels 176 Table of Contents ▼xi 13.2.4 Alloy Steels 177 13.2.5 Machinability of Steels 177 13.2.6 Mechanical Properties of Steels 178 13.2.7 Applications of Carbon and Alloy Steels 179 13.3 Tool and Die Steels 179 13.3.1 Designation and Classification of Tool and Die Steels 179 13.3.2 Cold Work Tool and Die Steels 181 13.4 Nonferrous Metals 182 13.5 Nonmetallic Materials 183 Appendix 1 Blank Diameter of Drawn Shells 185 Appendix 2 Metric System Tolerances on Linear Dimensions 189 Appendix 3 Miscellaneous Information 199 Glossary 209 Bibliography 215 Index 217