Theory of Dies

TYPES AND CLASSIFICATIONS OF DIES

Dies can be classified according to a variety of elements and in keeping with the diversity of die designs. We will primarily talk about die classifications depending on the product quantities of stamping pieces (whether high, medium or low) and the number of stations. In choosing there, we are not trying to downplay or ignore other classifications such as the number of operations, manufacturing processes, or guide methods.

1) Die classification depending on the production quality of parts

Depending on the production quality of pieces - High, medium, or low – stamping dies can be classified as follows:

Class A. These dies are used for high production only. The best of materials are used. All easily worn items or delicate sections are carefully designed for easy replacement. A combination of long die life, constant accuracy throughout the die life, and ease in maintenance are prime considerations, regardless of tool cost.

Class B. These dies are applicable to medium production quantities and are designed to produce the designated quantity only. Die cost as related to total production becomes an important consideration. Cheaper materials may be used, provided they are capable of producing the full quantity. Less consideration is given to the problem of ease of maintenance.

Class C. These dies represent the cheapest usable tools that can be built. They are suitable for low-volume production of parts.

1.1) Die classification according to the number of stations

According to the number of stations, stamping dies may be classified as:

·         Single-station dies

·         Multiple-station dies

A) Single-Station Dies

Single-station dies may be either compound dies or combination dies.

Compound die. A die in which two or more cutting operations are accomplished to produce a part at every press stroke is called a compound die.

Combination die. A die in which both cutting and noncutting operation are accomplished to produce a part at one stroke of the press is called a combination die.

B) Multiple-Station Dies

Multiple station dies are arranged so that a series of sequential operation is accomplished with each press stroke. Two die types are used:

·         Progressive dies

·         Transfer dies

Progressive die. A progressive die is used to transform coil stock or strips into a completed part. This transformation is performed incrementally, or progressively, by a series of stations that cut, form, and coin the material into the desired shape. The components that perform operations on the material are unique for every part. These components are located and guided in precision cut openings in plates, which are in turn located and guided by pins.

The entire die is actuated by a mechanical press that moves the die up and down. The press is also responsible for feeding the material through the die, progressing it from one station to the next with each stroke.

Transfer die. In transfer die operations, individual stock blanks are mechanically moved from die station to die station within a single die set. Large workpieces are done with tandem press lines where the stock is moved from press to press at which specific operations are performed.

2) Types of dies

There are 20 types of dies, and each is distinct and different from all the other types. However, as you study the descriptions to follow, observe how the element are applied and reapplied with suitable modifications to adapt them for each particular job to be performed.

2.1) Blanking dies

A blanking die produces a blank by cutting the entire periphery in one simultaneous operation. Three advantages are realized when a part is blanked.

a)      Accuracy. The edges of blanked parts are accurate in relation to each other.

b)     Appearance. The burnished edge of each blank extends around its entire periphery on the same side.

c)      Flatness. Blanked parts are flat because of the even compression of material between punch and die cutting edges.

The inset at A shows a material strip ready to be run through a blanking die. At B is shown the top view of the die with punches removed. The section view at C shows the die in open position with the upper punch raised to allow advance of the strip against the automatic stop. At D, the die is shown closed with a blank pushed out of the strip. Blanking dies may produce plain blanks as shown in inset E, but more frequently holes are pierced at one station and the part is then blanked out at the second station. Such dies are called “pierce and blank” dies.

2.2) Cut-off dies

The basic operation of a cut-off die consists of severing strips into short lengths to produce blanks. The line of cut may be straight or curved, and holes, notches, or both may be applied in previous operations. Cut-off dies are used for producing blanks having straight, parallel sides because they are less expensive to build than blanking dies. In operation, the material strip A is registered against stop block B. Descent of the upper die causes the cut-off punch C to separate the blank from the strip. Stop block B also guides the punch while cutting occurs to prevent deflection and excessive wear on guideposts and bushings. A conventional solid stripper is employed.

2.3) Piercing Dies

Piercing dies pierce holes in stamping. There are two principal reasons for piercing holes in a separate operation instead of combining piercing with other operations:

1.       When a subsequent bending, forming, or drawing operation would distort the previously pierced hole or holes.

2.       When the edge of the pierced hole is too close to the edge of the blank for adequate strength in the die section. This occurs in compound and combination dies in which piercing and blanking are done simultaneously.

The inset at A shows a flanged shell requiring four holes to be pierced in the flange. If the holes were pierced before the drawing operation, they would become distorted because of the blank holder pressure applied to the flange in the drawing process.

The shell is located in an accurately ground hole in the die block. Piercing punches are retained in a punch plate fastened to the punch holder, and a knockout affects stripping after the holes have been pierced.

2.4) Compound Dies

In a compound die, holes are pierced at the same station where the part is blanked, instead of at a previous station, as is done in a pierced blank die. The result is greater accuracy in the blank. Whatever accuracy is built in will be duplicated in every blank produced by the die.

Compound dies are inverted dies. The blanking punch A is located on the die holder of the die set instead of being fastened to the punch holder as in conventional dies, and it is provided with tapered holes for disposal of slugs.

The die block B is fastened to the punch holder and it is backed up by a spacer C, which retains piercing punches. A positive knockout removes the blank from within the die cavity near the top of the press stroke. A spring stripper removes the material strip from around the blanking punch.

Although most compound dies are designed for producing accurate, flat blanks, they are occasionally used for producing blanks that are too large for production in more than one station. Because all operations are performed at the same station, compound dies are very compact and a smaller die set can be applied.

2.5) Bending Dies

A bending die deforms portions of flat blanks to some angular position. The line of bend is straight along its entire length, as differentiated from a forming die, which produces workpieces having a curved line of bend. In the illustration, a flat blank is to be given a double bend to form a U shape. The blank is inserted in gages A fastened on bending blocks B. The bending blocks, in turn, are fastened to the die holder. Upon descent of the upper die, the bending punch C grips the blank between its lower face and pressure pad D. Pins E extend to the pressure attachment of the press. Shedder F strips the workpiece from the punch.

2.6) Forming Dies

The operation of forming is similar to bending except that the line of bend is curved instead of straight and plastic deformation in the material is more severe. In the image, the flat blank at A is to be formed into a part having a curved contour. The blank is positioned in nest B composed of two plates mounted on pressure pad C. When the ram descends, the blank is gripped between the bottoms of forming blocks D and the surface of pressure pad C. Further descent causes the sides of the blank to be formed to the curved shape of forming blocks D and forming punch E. At the bottom of the stroke, knockout against a hardened spacer fastened to the punch holder, thus, setting the form. When the die ascends, the part is carried up within form blocks D. Near the top of the stroke it is ejected by knockout F.

2.7) Drawing Dies

The drawing of metal, or deep-drawing manufacturing technology, is defined as the stretching of sheet metal stock, commonly referred to as a blank, around a punch. The edges of the metal blank are restrained by rings and the punch is deep drawn into a top die cavity to achieve the end shape that is desired. There are many shapes that can be made through deep drawing and stamping, such as cups, pans, cylinders, domes, and hemispheres, as well as irregularly shaped products.

In the image, at A, a flat disk is to be drawn into a cup. The blank is placed on pressure pad B of the drawing die and is located by four spring-loaded pins C. Descent of the upper die causes the blank to be gripped securely between the surface of pressure pad B and the lower surface of draw ring D. Further descent of the ram causes the blank to be drawn over punch E until it has assumed the cup shape shown in the closed view at the right. Pressure pins F extend to the pressure attachment of the press.

The amount of pressure must be adjusted carefully. Excessive pressure would cause the bottom of the cup to be punched out. Insufficient pressure would allow wrinkles to form, with the proper amount of pressure; a smooth, wrinkle-free cup is produced. Drawing dies are extensively used for producing stampings ranging from tiny cups and ferrules to large shells for pressure vessels, ships, cars, aircraft, and missiles.

2.8) Trimming Dies

Trimming dies cut away portions of formed or drawn workpieces that have become wavy and irregular. This condition occurs because of uneven flow of metal during forming operations. Trimming removes this unwanted portion to produce square edges and accurate contours.

The image at A shows a flanged shell after the drawing operation. A trimming die is requires to trim the irregular edge of the flange. The shell is placed over a locating plug B. Descent of the upper die then causes the scrap ring to be cut from around the flange. After trimming, the shell is carried up in the upper die and a positive knockout ejects it near the top of the stroke. The scrap rings are forced down around the lower trimming punch until they are split in two by scrap cutters C applied at the front and back of the die. The scrap pieces fall to the sides, away  from the operation of the press.

2.9) Shaving Dies

Shaving is the operation of removing a small amount of metal from around the edges of a blank or hole in order to improve the surface.  A properly shaved blank has a straight, smooth edge and it is held to a very accurate size. Many instruments, business machines, and other parts are shaved to provide better functioning and longer wear.

In the image, a blank A is to be shaved, both along outside edges and in the walls of the two holes. The shaving die for this work-piece consists of an inverted shaving punch B fastened to the die holder, and a shaving die block C fastened to the punch holder. A spacer D back up the die block and it retains the shaving punches for the holes.

The blank is located in a nest E, beveled to provide clearance for the curled chip. The nest is mounted on a spring stripper plate guided on two guide pins F. The shaved blank is carried up, held in the die block with considerable pressure, and ejected near the top of the stroke by a positive knockout. Shaving die sets for better alignment. This is necessary because no clearance is applied between punches and die block.

2.10) Broaching Dies

Broaching may be considered to be a series of shaving operations performed one after the other by the same tool. A broach is provided with a number of teeth, each of which cuts a chip as the broach traverses the surface to be finished. Internal broaches finish holes; surface or slab broaches finish outside surfaces. Two conditions make broaching necessary:

1)     Blanks are too thick for shaving: If considerable metal must be removed from the edges of thick blanks, a series of shaving dies would be required to produce a smooth finish. It would then be more economical to use a broaching die.

2)     When considerable metal must be removed: this occurs when ridges or other shapes are required in the edges of the blank such shapes directly because the cutting edges would be weak and subject to breakage.

In the image, a blank at A must have small pointed serrations machined in the side. The die is provided with two broaches B supported during the cutting process by hardened backing block C. The blank is located in a nest D composed of two opposed plates machined to fit the contour, Pressure pad E, backed up by heavy springs, clamps the blank securely before cutting begins. The first three or four teeth of the broach are made undersize; ordinarily they do not cutting unless an oversize blank is introduced into the die. The last three or four teeth are sizing teeth. Intermediate teeth are called working teeth and they take the successive chips to machine the serrations.

2.11) Horn Dies

A horn die is provided with a projecting post called a horn. Bent, formed, or drawn work-pieces are applied over the horn for performing secondary operations.

In the illustration at A, a blank has been reverse bent in a previous operation and the ends are to be hooked together and seamed in a horn die. The horn B is retained in a holder C fastened to the die holder. When the ram descends, seaming punch D strikes the work-piece to form the seam.

Many other operations, such piercing and staking, are also performed in horn dies.

2.12)  Side Cam Dies

Side cams transform vertical motion from the press ram into horizontal or angular motion and they make possible many ingenious operations. In the image, at A, a flanged shell requires two holes pierced in its side. The shell is placed over die block B of the die. Descent of the upper die causes pressure pad C to seat the shell firmly over the block. Further descent causes side cams D to move the punch-carrying slide E for piercing the holes. Spring stripper F strip the shell from around the piercing punches as they are withdrawn.

2.13) Cutting Dies

A curling die forms the material at the edge of a workpiece into a circular shape or hollow ring. Flat blanks may be curled; a common application is a hinge formed of two plates each of which is curled at one side for engagement of the hinge pin. More often, curling is applied to edges of the open ends of cups and shells to provide stiffness and smooth, rounded edges. Most pans used for cooking and baking foods are curled.

 In the illustration, a drawn shell shown at A is to be curled. The shell is placed in the curling die where it rests on knockout pad B. Descent of the upper die causes the knockout pad to be pushed down until it bottoms on the die holder. Further descent causes curling punch C to curl the edge of the shell. Near the bottom of the stroke, the lip of the material contacts an angular surface machined in curling ring D to complete the curl. When the punch goes up, the knockout raises the shell for easy removal.

2.14) Bulging Dies

A bulging die expands a portion of a drawn shell causing it to bulge. There are two types: fluid dies and rubber dies. Fluid dies use water or oil as the expanding medium and a ram applies pressure to the fluid. In rubber dies, a pad or block of rubber under pressure moves the walls of the workpiece to the desired position. This is possible because rubber is virtually incompressible. Although it can be made to change its shape, the volume remains the same.

In the illustration at A, a drawn shell is to be bulged at its closed end. The shell is placed over punch B of the bulging die and its lower end is confined in lower die C. The upper end of punch B is a rubber ring within which is applied a spreader rod D. This rod is conical at its upper end and it helps the rubber to flow outward to the desired shape. When the press ram descends, the upper die applies a force to the shell bottom, and since the rubber cannot compress, it is forced outward bulging the walls of the shell. When the ram goes up, the rubber returns to its original shape and the bulged shell can be removed from the die. After bulging, a shell is shorter than it was previously.

2.15) Swaging Dies

The operation of swaging, sometimes called necking, is exactly the opposite of bulging. When a workpiece is swaged a portion is reduced in size. This process causes the part to become longer than it was before swaging. In the image, at A, a shell is to be swaged at its open end. It is inserted in the swaging die where it rests on knockout pad B and its lower end is surrounded by the walls of block C. When the ram descends, swaging die D reduces portion of the diameter of the shell and this portion becomes longer.

2.16) Extruding Dies

The function of all the dies discussed so far is to perform work on sheet material – to cut sheet material into blanks, to perform further operations upon the blanks, or to perform operations on worpieces bent, formed or drawn from the blanks. We come now to some interesting classes of dies that perform secondary operations on small thick blanks called slugs. In these dies, the slugs are severely deformed to make parts having no resemblance to the slugs from which they were made.

The first class of dies are extruding die. In this type of die, each slug is partly confined in a cavity. Then extremely high pressure is applied by a punch to cause the material in the slug to extrude or squirt out, much like toothpaste is extruded when the tube squeezed. In the illustration the slug A is to be extruded into a thin-walled shell having a comical closed end. The slug is placed in die block B, backed up by a hardened plate C. The bottom of the cavity in the die block is formed by the end of knockout rod D. When the press ram descends, extruding punch E first squeezes the slug until it assumes the shape of the die cavity and of the working end of the extruding punch. Continued descent causes the material to extrude upwards between the wall of the punch and the wall of the die cavity. The amount of clearance between the two determines the thickness of the wall of the extruded shell. The extruding punch is retained in punch plate F and, because of the high pressure involved; it is backed up by hacking plate G.

                                                                                 

2.17) Cold-Coining Dies

Cold-coining dies produce workpieces by applying pressure to blanks, squeezing and displacing the material until it assumes the shape of the punch and die. In the illustration at A, a slug is to be formed into a flanged part in a cold coining die. It is placed on punch B located within spring-loaded V gages C, Descent of the upper die causes the material under the upper die block to be displaced outward to form the flange. As the flange increases in diameter, the gages are pushed back as shown. When the die goes up, the part is carried upward within it and ejected near the top of the stroke by knockout plunger D actuated by knockout rod E.

The cylindrical part is the slug (blank); another illustration is the flanged part. This is only one simple example of cold coining dies. A basic postulate of plastic deformation of material states that “the shape” (of course, dimensions, too) of blanks can be changed, but volume is constant.

2.18) Progressive Dies

All of the operations described previously may be performed in progressive dies. For example, a single die of this type may do piercing at the first station, trimming at the second station, bending at the third, forming at the fourth, etc. A progressive die may thus be considered a series of different dies placed side by side with the strip passing through each successively. This analogy has some merit, although it does not give a true picture of the extremely close interrelationship between the various stations.

In the illustration, at A, a pierced, trimmed, and bent part is to be produced complete in a simple progressive die. At the first station, the strip is notched and pierced and at the second station, the blank is cut off and bent. You should easily recognize all of the elements in this die-the die block, piercing punch, trimming punch, knockout, and stop block, along with all the others.

2.19) Sub-Press Dies

Sub-press dies blank and form very small watch, clock and instrument cam shown at A. The die components are retained in a sub-press, which is, as its name implies, actually a small press operated in a larger one. The sub-press is composed of base C, barrel B, and plunger D. A long, tapered Babbitt bearing E provided with longitudinal key slots guides the plunger and prevents rotation. Tightening spanner nut F against bearing E causes it to close around plunger D to remove all looseness. The top portion of plunger D is engaged by actuator G threaded into a central tapped hole. The slot of the actuator is engaged loosely by a yoke fastened to the press ram. Thus, the press ram does not guide the sub-press in any way. It simply applies the up-and-down motion. Sub-press dies are usually of the compound type because of the considerable accuracy required.

2.20) Assembly Dies

Assembly dies assemble two or more parts together by press fitting, riveting, staking, or other means. Components are assembled very quickly and relationships between parts can be maintained closely. In the illustration shows a link and two studs that are to be riveted together in an assembly die. The studs are positioned in die block A and they sit on plungers B. The link is then positioned over the studs, the turned-down ends of the studs engaging in holes in the link. Descent of the press ram causes riveting punches C to deform the ends of the studs into the shape of rivet heads. A hardened plate D backs up the punches to prevent the heads from sinking into the relatively soft material of the die set. Another hardened plate E backs up the plungers.