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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 todays 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 bath-

tubs? 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 repouss 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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CHAPTER ONE

FIGURE 1-1 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

FIGURE 1-3 Various sheet-metal products.

Metal stamping die production output can be enormous, with huge quantities of high-

quality 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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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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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

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 pow-

der 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 manu-

facturing 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 influ-

ences the parts 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 open-

ing 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 grain-

dependent 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 mater-

ial, 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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CHAPTER ONE

FIGURE 1-8 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 direc-

tions (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 pro-

duced by many other manufacturing methods. For example, die cast parts can be impres-

sive 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

FIGURE 1-9

(Continued)

Sintered metals have no grain-generated backbone at all and may fail if used in high-

stress 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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CHAPTER ONE

FIGURE 1-11 Forged and extruded parts.

quite low strength when compared to metal parts of the same shape, they suffer from crack-

ing 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, todays 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 todays 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 depen-

dence 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 prompt-

ing 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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FIGURE 1-12 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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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 products

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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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

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 volum-

nar 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 sce-

nario 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 equipments 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 exis-

tence of temporarily unrewarded financial investments into the purchase of material, work-

force compensation, taxes, utilities, and overhead. These all need to be taken into account since

they will only increase the final cost of the product, long before it can be sold to a customer.

To properly evaluate the situation, all applicable expenditures should be added up as follows:

1. Cost of the storage space (prorated rent or property taxes, cost of the building and

improvements)

2. Cost of all packaging and repackaging material, storage containers, protective barriers,

and insulation

3. Cost of stacking and restacking of parts, sorting them out, and discarding rusty or dam-

aged pieces

4. Spoilage of possible storage-sensitive material and the scrap rate

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CHAPTER ONE

5. Cost of raw material and other production-related necessities

6. Overhead, such as electricity, cost of heating or cooling, water, and fuel applicable to

the storage of parts

7. Cost of labor, including possible overtime

8. Cost of paperwork involved with storage and subsequent handling of products

9. Interest rate at which the monies allocated to the above activities could have generated

when invested otherwise

The combined expenses 1 through 9, when added up, should be equal to or less than the

combined:

1. Cost of the removal of a die from the press

2. Cost of the installation of a die in the press (for the subsequent run)

3. Cost of the machines downtime during the die removal and installation

4. Cost of the press operators standby, if applicable

5. Cost of the press adjustments and trial runs

6. Cost of the first piece inspection and the cost of further adjustments and approvals, if

applicable

7. Cost of the extra material and supplies, which must be purchased ahead of the time

even if not immediately utilized

8. Overhead, such as cost of electricity, heating, cooling, water, and fuel

9. Cost of all subsequent billing and paperwork

10. Combined interest (per going rate) the finances allocated to the above causes would

have generated when invested otherwise

The length of each run and its influence on the need for sharpening and maintenance of tool-

ing must be evaluated for the entire production run. Should a maintenance-related interruption

be necessary, a possible split of the previously planned combined run should be considered.

A definite advantage of the die production is its unrivaled consistency in the products

quality and dimensional stability. In absence of design and construction mistakes, the die,

once built, needs minimal amount of alterations, aside from regular sharpening.

Some dies, true, are more sensitive than others, which is mostly attributable to exces-

sive demands on close tolerance ranges of parts and on the variation in material thickness.

With some bending and drawing operations, the consistency in hardness of stock can be

essential as well. But a regular die, well designed and well built, can deliver a great load of

products before its punches begin to wear and a need for repair or sharpening arises.

Generally, it may be claimed that if the conditions of the die-operating process are kept

the same and if the tool was not dropped off the forklift or similarly mangled, the parts from

the die will emerge consistent with previous runs.

1-3 DESIGN CRITERIA FOR

DIE-MANUFACTURABLE PRODUCTS

Todays world places greater and greater demands on products and materials, from which

they are made. Years ago, many designers never figured out stress and strain, elasticity,

fatigue, or similar values. If it broke, then you just made it 2 inches thicker, or 3 inches, or

5 inches, whatever you preferred.

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But that is not how current manufacturing is governed. Resources are getting scarcer,

perhaps even limited in some cases, and designers are forced to economize. After all, why

should a car body be thick and heavy, when a thinner-gauge galvanized or galvannealed

steel will bring about the same, if not better, results.

Demands for special alloys are continuously expanding, and they are in equal com-

petition with all the new and increasingly better alloys that are being produced. Ferrous

and nonferrous alloys, titanium and its and alloys, and alloys with traces of rare metals

added for additional qualities are all available to fill that specific gap where they are

needed.

Manufacturing methods are next on the list of economizing designers. Avoiding sec-

ondary operations whenever possible, designers apply cost-conscious strategies and plan-

ning not only in small shops, but in medium and large plants as well.

This certainly is a good approach to any given problem, since every product has its

price. If manufacturing costs become greater than the value of a product, such an item

becomes unsalable.

For these reasons, manufacturability of products is extremely important. Almost any-

thing can be manufactured somehow, if people put their minds to it. But at what cost? And

who will be willing to pay for it?

Out of this ever-present regard for price versus actual value, new methods are being

devised daily, new approaches to old problems sought for. Crowds of engineers, designers,

tool makers, model makers, and representatives of other professions are nit-picking new,

almost new, or old problems, in an attempt to come up with a simple, straightforward, and

cost-effective answer.

Sometimes, however, shortcuts are taken, where cheaper materials, thinner coatings,

less durable tools, or less experienced labor are used. These steps are just what they present

themselves as: shortcuts. They usually produce more returns, more repairs, more problems

around their drawbacks, and even more expenses. There is a time and a place for every-

thing, but these remedies are not always helpful. You pay for them later.

A good, sound design and overall manufacturability cannot be replaced by trinkets.

The old saying if it isnt good, fix it should perhaps be replaced by if it isnt good,

redesign it!

1-3-1 Manufacturability Aspects

The manufacturability of products depends on many factors. Sometimes a lack of space

may prevent a mechanic from reaching the area of concern, and long hours may be lost

before this obstacle is overcome. Or a wrong sequence of operations will cause the final

product to become distorted. Sometimes an adhesive may not hold because the part was not

degreased enough, or a screw may fall out because someone forgot to add that second nut

or a drop of Loctite.

In die work, the manufacturability of parts is dependent on much narrower range of

influences. The main areas of concern are

1. Grain direction of the material

2. Openings, their shape and location

3. Bends and other three-dimensional alterations to the flat part, their shape and location

4. Outline of the part and its size

5. Applicable tolerance ranges

6. Surface finish, flatness, straightness, and burr allowance

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CHAPTER ONE

1-3-1-1 Grain Direction of the Material. The ever-present grain of material must be

taken into consideration first. Unless absolutely necessary, it should not appear alongside

a bend, a joggle, or any other deflection and elevation in the parts surface.

Every sheet-metal material behaves differently alongside the grain line and across it.

Forming, drawing, and even simple punching may sometimes show differences in the size

and shape of the hole when evaluated for the grain influence. An extruded opening, shown

in Fig. 1-16, illustrates this claim. By cutting across the grain line, the material behaves

almost as if constantly in tension, which, when forcibly removed by the cutting process,

causes the material to back off.

If a bracket such as the one shown in Fig. 1-17a will be rotated 90 and positioned on

the strip with its bends along the grain line, these flanges may sometimes crack in forming

or even much later, in service, afterward. For that reason, wherever the problem of multi-

ple bends occurs and there is no chance of avoiding their placement alongside the grain line,

an angular positioning on the strip or sheet, shown in Fig. 1-17b, should be considered.

Such a grain-line pattern should be used quite habitually with materials of the 6061-T4

(T6) aluminum group, as they are prone to cracking. Especially if, for some reason, parts

are belt-sanded in flat prior to bending, their proneness to cracking will be enhanced. A

greater bend radius, as well as vibratory sanding, or belt-sanding under an angle, may help

to alleviate the problem to a degree.

In parts with several formed sections, the shear strength and resistance to columnar

stress of their flanges will vary with their variation from the materials grain, as shown in

Fig. 1-18. Should a force A, parallel with the grain line, be applied to the bend-up section,

the greatest shear strength will be encountered. However, we already know that bends run-

ning parallel with the bend line are prone to cracking in forming and are not recommended.

Intermediate shear strength will be encountered in the direction of the C force line in

Fig. 1-18a, whereas the B force line will display the least shear strength, as the flange may

tend to bend under it. Whenever a bent-up flange is acted upon by a secondary bending

force, it has a tendency to follow that forces direction only if consistent with the initial

movement of the flange in forming. A force applied against the direction of bending will

not flatten the material, but will break it.

FIGURE 1-16 Elongation of openings (exaggerated) caused by the direction of grain.

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FIGURE 1-17 Grain direction in formed sheet-metal parts.

FIGURE 1-18 Stresses and their relationship to the direc-

tion of grain. (From Frank W. Wilson, Die Design

Handbook, New York, 1965. Reprinted with permission from

The McGraw-Hill Companies.)

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CHAPTER ONE

Bending style shown in Fig. 1-18b, with flanges at 45 off the grain line, is considered

a fair practice.

The value of the bend radius is another factor influencing the parts behavior in

formingthe smaller the radius, the greater the materials proneness to cracking. There is

a certain minimal bend radius for various materials and thicknesses, which is discussed in

Chap. 8.

1-3-1-2 Openings, Their Shape and Location. Openings in the part should not be

located too close to each other and certainly not too close to the edge of the sheet or strip

(Fig. 1-19). At this point, it should suffice to compare the sheet-metal cutting operation to

that of slicing a block of Swiss cheese. The closer to each other the cuts are placed, the more

distorted they will be.

The shape of openings other than rounded, has a considerable effect on the parts

behavior in further manufacturing as well as in service (Fig. 1-20). Sharp edges in cutouts

become the points of accumulated stresses and may turn into points of failure. Sharp edges

are also difficult to protect from rust and corrosion, which may seep into the part through

these areas. For that reason, rounded edges are preferable whenever possible.

Some minimal dimensions for punched parts are shown in Fig. 1-21. Should an open-

ing be located too close to a bend, the recommended practice would be to first produce the

bend and only subsequently to pierce the opening. By following this procedure, a greater

dimensional stability can be achieved. Because if such an opening is pierced first and the

bend produced afterward, distortion of the opening will occur (Fig. 1-22).

1-3-1-3 Bends and Other Three-Dimensional Alterations to the Flat Part, Their Shape

and Location. The location of formed portions and their dependence on the direction of

grain was already addressed in Sec. 1-3-1-1. In some situations, however, bending along all

four edges of a square or rectangular opening cannot be avoided. This is a condition in

which the results of bending along the grain and bending perpendicularly to it differ. There

are charts and guidelines ready to provide us with the data on the size of the bending radius

and bending allowance in either situation. But often, a simple trial run and a careful exam-

ination of the bend may serve the purpose.

A slightly different problem is the formation of flanges (i.e., sides) in a four-sided enclo-

sure. Here a question of the most suitable joining technique of side flanges is often brought

up. Often, the sides of such a unit can be left with a small gap for welding (Fig. 1-23a), or

be provided with an additional bent-up flange and spotwelded together (Fig. 1-23b).

FIGURE 1-19 Distances between pierced openings.

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FIGURE 1-20 Openings other than round.

Where the enclosure has not only four sides but the frontal, or face flanges as well

(Fig. 1-23c, d, e), this dilemma is still greater. Basically, there are but three solutions to this

problem. For face plates, the gaps between the joining flanges can be weld-filled and

sanded smooth. For unexposed areas, or where another plate is to be used as a cover, rough-

sanding to flatten the surface may be good enough.

Gaps between the flanges may be large, small, or almost nonexistent. Their size and

quality depends on the bend calculation, condition of tooling, and experience of the opera-

tor (in manual bending situations).

All bent-up portions should be provided with proper bend relieves (Figs. 1-24 and 1-25).

These not only ease the bending process but also prevent the material from being pulled in

the wrong direction, wrinkled, or torn.

FIGURE 1-21 Minimal practical punching and blanking dimensions (t = material thickness).

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CHAPTER ONE

FIGURE 1-22

Influence of bending and piercing sequence of operations.

FIGURE 1-23 Different methods of joining side and face flanges.

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BASIC DIE DESIGN AND DIE-WORK INFLUENCING FACTORS

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Handbook of Die Design: Basic Die Design and Die-Work Influencing Factors

Handbook of Die Design: Basic Die Design and Die-Work Influencing Factors