Die Design Fundamentals. Vukota Boljanovic
stripping after the holes have been pierced.
Figure 2.2 Cut-off die.
2.2.4 Compound Dies
In a compound die (Figure 2.4), holes are pierced at the same station where the part is blanked, instead of at a previous station, as is done in a pierce and 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.
Figure 2.3 Piercing die.
Figure 2.4 Compound die.
Figure 2.5 Bending 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.
2.2.5 Bending Dies
A bending die (Figure 2.5) 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 work-pieces 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.
Figure 2.6 Forming die.
Figure 2.7 Drawing die.
Figure 2.8 Trimming die.
2.2.6 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 Figure 2.6, 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 block F applies the final form. It bottoms 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.
2.2.7 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 Figure 2.7 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.
2.2.8 Trimming Dies
Trimming dies (Figure 2.8) 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 illustration at A shows a flanged shell after the drawing operation. A trimming die is required 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.
2.2.9 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 Figure 2.9, a blank A is to be shaved, both along outside edges and in the walls of the two holes. The shaving die for this workpiece 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 backs 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 dies are ordinarily held in floating adapter die sets for better alignment. This is necessary because no clearance is applied between punches and die block.