Handbook of Die Design 2nd Edition PDF

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Cataloging-in-Publication Data is on file with the Library of Congress Copyright © 2006, 1998 by Ivana Suchy. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a data base or retrieval system, without the prior written permission of the publisher. 1 2 3 4 5 6 7 8 9 0 DOC/DOC 0 1 0 9 8 7 6 5 ISBN 0-07-146271-6 The sponsoring editor for this book was Larry S. Hager and the production supervisor was Richard C. Ruzycka. It was set in Times Roman by International Typesetting and Composition. The art director for the cover was Handel Low. Printed and bound by RR Donnelley.

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Copyright © 2006, 1998 by Ivana Suchy., McGraw-Hill

11/1/2007 8:46 AM

Source: HANDBOOK OF DIE DESIGN

CHAPTER 1

BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

1-1 SHEET-METAL STAMPING IN COMPARISON WITH OTHER METAL FABRICATING PROCESSES In today’s practical and cost-conscious world, sheet-metal parts have already replaced many expensive cast, forged, and machined products. The reason is obviously the relative cheapness of stamped, or otherwise mass-produced parts, as well as greater control of their technical and aesthetic parameters. That the world slowly turned away from heavy, ornate, and complicated shapes, and replaced them with functional, simple, and logical forms only enhanced this tendency. Remember old bathtubs? They used to be cast and had ornamental legs. Today they are mostly made of coated sheet metal, if not plastics. Manufacturing methods for picture frames, chandeliers, door and wall hardware, kitchen sinks, pots and pans, window frames, and doors were gradually replaced by more practical and less costly techniques. But, sheet-metal stampings can also be used to imitate handmade ornamental designs of previous centuries. Such three-dimensional decorations can be stamped in a fraction of time the repoussé artist of yesterday needed. Metal extrusions, stampings, and forgings, frequently quite complex and elaborate, are used to replace handmade architectural elements. Metal tubing, metal spun products, formings, and drawn parts are often but cheaper substitutes of other, more expensive merchandise. Metal stampings, probably the most versatile products of modern technology, are used to replace parts previously welded together from several components. A well-designed sheet-metal stamping can sometimes eliminate the need for riveting or other fastening processes (Fig. 1-1). Stampings can be used to improve existing designs that often are costly and labor-intensive. Even products already improved upon, with their production expenses cut to the bone, can often be further improved, further innovated, further decreased in cost. The metal stamping die (Fig. 1-2) is an ideal tool that can produce large quantities of parts that are consistent in appearance, quality, and dimensional accuracy. It is a press tool capable of cutting the metal, bending it, drawing its shape into considerable depths, embossing, coining, finishing the edges, curling, and otherwise altering the shape and the outline of the metal part to suit the wildest imaginable design concepts. Figure 1-3 shows samples of these products.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS 2

FIGURE 1-1

CHAPTER ONE

Threaded part, replaced by other, less expensive means.

The word “die” in itself means the complete press tool in its entirety, with all the punches, die buttons, ejectors, strippers, pads, and blocks, simply with all its components assembled together. When commenting on these little technical ingenuities, it is important to stress the role of designers of such products, both artistic and technical. Their thorough knowledge of the manufacturing field will definitely enhance not only the appearance, but the functionality, overall manufacturability, and cost of these parts.

FIGURE 1-2

Metal stamping dies.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

FIGURE 1-3

3

Various sheet-metal products.

Metal stamping die production output can be enormous, with huge quantities of highquality merchandise, as shown in Figs. 1-3 and 1-4; pouring forth from the press. For that reason technical ignorance is not readily excusable, as the equal quantities of rejects can be generated just the same way. 1-1-1 Grain of Material Often, parts produced by various manufacturing methods can be redesigned to suit the sheet-metal mass production (Fig. 1-5).

FIGURE 1-4

Metal-stamped replacements.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS 4

CHAPTER ONE

FIGURE 1-5

Additional sheet-metal replacements.

When designing such replacements, there are several aspects to be evaluated. The first and probably the most important is the grain of material (Fig. 1-6). Sheet metal of every form, be it a strip or a sheet, displays a definite grain line. It is the direction along which the material was produced in the mill-rolling process. In coils, the grain direction always runs lengthwise, parallel with the longer edge. The grain direction

FIGURE 1-6

Grain of materal in sheet-metal strip.

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5

in sheets may vary, and designers must always make themselves familiar with it prior to planning a production run of any kind. In contrast, cast or forged parts display a different grain direction, and in sintered powder metal parts the grain is completely gone. For this reason, each of these manufacturing methods can be used to produce items for different applications. For example, a part, shown in Fig. 1-7, will display a different reaction to various forces and stresses when made by the forging method than when obtained through other manufacturing processes. Where the forging would possess a great resistance to tensile and compressive forces along the A-A line, the same part, when made from sintered powder metal, may break or collapse under the same force. With this shape being cast, the location of the gate is of extreme importance, as it influences the part’s sturdiness in various directions. In the casting gated at the longer end (as pictured in Fig. 1-7b), the opposite end will be more susceptible to breakage, as the molten metal will reach that portion later, when already cooling down. The existence of an opening in that area will divide the flow of material and thus create a so-called knit line, along which a separation, resulting in defects and possible breakage, may occur. The same casting, when gated in the middle (Fig. 1-7c), will have an equal breakage proneness at both ends. However, these ends will be somewhat sturdier, as the molten metal will reach them sooner than in the case of Fig. 1-7b. Of course, the existence of openings may have the same detrimental effect described earlier. A similar product, made of sheet metal, as pictured in Fig. 1-8, will also display a graindependent behavior; the part with the lengthwise grain will be considerably sturdier along the A-A line of force than the same shape positioned across the grain line. Where used sensibly, the grain in sheet-metal material can serve as a backbone of future products. In formed parts where bends are oriented perpendicularly to the grain of material, such bends are rarely seen cracking or becoming distorted, and the whole structural

FIGURE 1-7

Forces applied to a casting.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS 6

FIGURE 1-8

CHAPTER ONE

Grain variation in sheet-metal strip.

consistency of the part is greater. Where such bends “across the grain” cannot be achieved, bends under an angle should be attempted (see Fig. 1-9). In parts with bends in both directions (Fig. 1-9b), a 45° deviation from the grain line can be extremely helpful. Aside from other advantages, sheet-metal parts are stronger and sturdier than parts produced by many other manufacturing methods. For example, die cast parts can be impressive with their intricate shapes, nonconcentric rounds, and full-bodied mass. But they have no distinct grain direction, and where strength is required their increased thickness often serves as a substitute for sturdiness (see Fig. 1-10).

FIGURE 1-9

Grain of material in bending.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

FIGURE 1-9

7

(Continued)

Sintered metals have no grain-generated backbone at all and may fail if used in highstress applications. Forged materials do have their strength and sturdiness, but this is, again, outweighed by their bulkiness, as shown in Fig. 1-11a. Same with extruded materials (Fig. 1-11b): the grain is there, the strength is there, the columnar strength is impressive, but the increased bulkiness cannot be overlooked. Additionally, the span of applications for these products is limited and highly specific. Plastic parts, similarly to cast products, have but the material flow to depend on and that provides them with more defects than support. And since plastic materials are generally of

FIGURE 1-10

Sample of a cast part.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS 8

CHAPTER ONE

FIGURE 1-11

Forged and extruded parts.

quite low strength when compared to metal parts of the same shape, they suffer from cracking when stressed or flexed, often brittle, pestered with serious aging problems, and greatly affected by weathering effect. They are almost useless in many applications where sheet metal can substitute for them with ease. Yet, for some reason, today’s manufacturers often go into extremes of supporting a fragile plastic insert with a sturdy wire mesh or producing a complicated sheet-metal structure covered by a plastic wrapper, just to be able to use plastics. Where fillers are used in plastics moldings, the proneness of such parts to cracking can be greatly enhanced, with dependence on the percentage of filler material utilized. And considering the pressure today’s plastic parts’ production places on the petroleum industry, we actually may have no plastic parts to speak of 50 years down the road, especially when taking into account the enormity of our mass production and mass consumption.

1-1-2 Edge Formation Another important aspect to be considered when designing sheet-metal replacements for parts manufactured by other methods is the formation of the edge. A cast part (Fig. 1-12a) will always exhibit a parting line to some degree. The visibility of this line is dependent on tool quality; with well-manufactured and well-maintained tooling, the line can be almost invisible, but with worn-out dies, rough machining, and crude assembly and fit, that area may bulge out and perhaps even show a burr at some places. The existence of draft angle in cast parts is another necessity the designer has to take into account. If the same part were forged, it will have the edge characteristics similar to those of its cast counterpart. Sheet-metal products’ edges will be completely different. With dependence on the thickness of material and clearance between the punch and die, the sheet-metal parts’ cut or pierced edges will show a reasonably straight portion, with a slight distortion toward the surface opposite from the punch, as shown in Fig. 1-13. The mechanism prompting such distortion to emerge at all, along with the factors contributing to its width and volumnar growth, are explained in greater detail in Chap. 2. Considering the terminology, here the word “die” describes the insert, which during the operation of the press receives the punch and retains the pierced slug or blanked part. Sometimes the term “die button” may be used interchangeably. The burr on metal-stamped products is a great aid in evaluating the sequence of the manufacturing process, as it clearly indicates the direction of punching (or blanking) of each opening and of each cut.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

FIGURE 1-12

9

Side view of the cast product.

Drawn parts’ edges are similar in that they display the characteristics of the cut metal, where produced from previously blanked material (see Fig. 1-14a). This is due to the action of blankholder, which retains the outer rim of the blank, while the middle of it is being drawn into depth.

FIGURE 1-13

Edge formation in stamped parts.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS 10

CHAPTER ONE

FIGURE 1-14

Edge formation in drawn parts.

Where no blankholder is employed, the drawn part is usually expelled through the die right after drawing, in a single, continuous motion of the press. The edges of such a part are wavy and uneven, as shown in Fig. 1-14b. A drawn cup produced from a blankholder-restrained blank and trimmed afterwards, retains a portion of the outer radius of the previously formed flange, which gives the edge of a shell a knife-resembling sharpness (see Fig. 1-14c). The formation of the cross section of the drawn portion further influences the product’s characteristics. There is often some thinning of the wall due to the drawing process, and the deeper the draw, the thinner the wall may become (Fig. 1-15).

FIGURE 1-15

Volumnar changes in drawing operation.

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11

The reason for this is obvious: The material needed for the expanded length of the drawn portion has to be taken from somewhere, and practically (and mathematically) the volumnar content of that section must be equal to that portion of the flat piece from which it was produced.

1-2 WHAT CONSTITUTES SUITABILITY FOR DIE PRODUCTION? When evaluating a part for die production, the most restrictive aspect to be considered is the cost of the tooling. To build a metal stamping die is a costly process, involving many people, many machines, and several technologies. For that reason, the demand for tooling must first be economically justified. The quantitative demands per given time span should be evaluated first, because a scenario of 50,000 washers to be delivered each month requires a different treatment from 50,000 washers to be delivered each week. A correct evaluation of the problem must be performed on the basis of: • • • •

Availability of the appropriate press The equipment’s running speed The length of production shifts Scheduling for the needed time interval

For a small run with few repetitions, a single line of tooling may be chosen. However, if the quantities are large and the time constraint exists, a multiple-part-producing tool must be built. Such a die, generating at least two or more complete parts with each stroke of a press, will speed up production admirably. But increasing the size of the tool necessitates the use of a larger and more powerful press and may even require a nonstandard width of a strip, which will certainly cost more and will have longer lead (i.e., delivery) times. With parts other than simple washers, the shut height of the press versus the height of the part (and subsequently the height of the die) is another production-influencing factor. The width of the opening in the press plus the width of the proposed die must definitely be in congruence. The possibility of reorders should be considered at this point, as they may result in an extended production run, greater material demands, and longer occupancy of the press. Such longer runs are usually beneficial from the economical standpoint, as they save on die-mounting procedures and press adjustments, while also decreasing the demand for quality control personnel involvement. On the other hand, a problem of storage of these extra parts may arise along with the existence of temporarily unrewarded financial investments into the purchase of material, workforce compensation, taxes, utilities, and overhead. These all need to be taken into account since they will only incr...