INTRODUCTION DRAW RINGS 4g handbook vukotaboljanovicsheetmetalformingprocesses

160 Deep Drawing Dies 11.6 MULTI-OPERATION DIES Multi-operation dies are combination dies designed to perform both drawing and non-drawing operations in one press stroke. Fig. 1 1.13 shows a multi-operation die intended for use in a single-action press. First, a blank is cut from a strip of stock at contour A with a blanking punch and blanking plate 1; then, the drawing operation is performed at contour B with the drawing punch 3 and the inside of the blank- ing punch which functions as the draw ring. The pressure pad 4 puts pressure on the blank so that as it is pushed through the drawing ring, the blank is held in place. Enough force is exerted to prevent the material from rising but not enough to pre- vent the material from being able to slide under the pad and over the die ring radius. When the press slide moves up, the stripper pulls scrap strip from the blanking punch, and the knockout bar pushes the workpiece out. A strip of stock is guided by the guide pins 8 and stopped by a stop pin 9. The pressure pad gets power a mechanism located under the press bed. 1-blanking stop 2-blanking punch 10-strip 3-drawing punch 6-knockout plate 12-work piece Fig. 1.13 Multi-operation die for single-action press. 5-scrap stripper 1 1-pressure pad pin

11.7 PROGRESSIVE DIES

Progressive drawing dies are designed to do separate operations: blanking, punching if necessary, draw- ing, and redrawing shells in successive workstations. These dies are used to produce pieces of smaller dimensions in mass production. A strip of stock is usually moved automatically through the die. Moving the drawn shell from one station to the next through progressive dies is sometimes difficult, especially if the shell is drawn deeply. There are two drawing methods: in the first, the workpiece is not initially cut so Deep Drawing Dies 161 2nd reduction 1 punches die rings 3-punching punch 4-punching die ring 5-extrusion punch 6-extrusion die ring 7-blanking punch 1 1-stripper 14-spring 8-blanking ring 1 5 -spring 9-pressure pad 10-pressure pad spring 16-helical spring Fig. 1.14 Progressive die and scrap strip: a progressive die; b scrap strip with successive shells drawn in this die. that it may be carried from one station to the next until it reaches the last station, where it is cut and pushed out of the die. In the second method, the blank is partially retained in the strip called cut and carry and carried to the scrap strip from one station to the next until the workpiece reaches the last station, where it is finally cut clear and pushed out of the die. Fig. 11.14 shows a progressive die for the drawing of a bushing by the first method, and the scrap strip with successive shells drawn in this die Fig. 1 Drawing, with successive reductions of the shells diam- eter is done at the first three stations with punches and attached to the lower die block set, and drawing rings and attached to the upper die block set. At the fourth station, the bottom is punched with the punch and punching die ring 4. In the fifth station the bushing wall is extruded to the final dimensions with the punch 5 and the die ring 6. At the sixth station, the flange is blanked with the blanking punch 7 and blanking die ring 8. The final piece is dropped through the hole in the lower die block. The pressure pad 9 functions as the upper stripper when the press slide is moved up. The lower strip- per 1 1 allows all operations to be done without interference between the die components and the work- piece. The die is equipped with a mechanism for advancing the strip stock. The mechanism is equipped with a hook a cam slide a leaf spring 1 5, and a helical spring 1 6. The mechanism pulls the strip left when the press slide is moved up. The leaf spring 14 prevents the strip stock sliding while the pulling mechanism returns to its previous position as the press slide is moved up. Progressive dies for drawing by the second method - cut and carry - are of similar design to dies for drawing by the first method. The layout for drawing a similar bushing, where metal movement from the strip into the cup must be allowed for, is shown in Fig. 11.15. 162 Deep Drawing Dies Punching Extrusion Blanking flange 2nd drawing I I I Feed Fig. The scrap strip with successive shells drawn in a “cut and carry” die An I-shaped relief cutout is notched at the first station of the die in Fig. 1 1.15. The cup is successively drawn at the second and the third stations. During all the drawing stages, metal is pulled in from the cutout edges as well as from the edges of the strip, thus narrowing the strip width. At the fourth station, the bottom hole is punched. At the sixth station, the bushing is extruded to its final dimensions. At the seventh station the flange is blanked, and the workpiece is separated the strip and pulled out from the station. This type of relief cutout can be used when a series of shallow draws needs to be made without wrinkling the strip skeleton.

11.8 DRAWING DIES FOR PIECES OF SPHERICAL AND PARABOLIC SHAPE

General problems in drawing these kinds of pieces include a tendency to wrinkles appearing at the top flange of the cup, and thinning of the material at the lower part of the drawn pieces. To avoid these prob- lems, the following are necessary: The blank diameter needs to be a little larger than calculated; The excess the material must be cut off after each drawing operation; The order of drawing must be divided into two stages. In the first stage, the workpiece is drawn with a spherical bottom, with or without a flange. In the second stage of the drawing operations, two methods are used: Reverse drawing, Drawing in the die with a circular draw bead. Fig. 1 1.16 shows a design for a die for drawing spherical pieces. In the first operation a piece with a con- vex bottom is drawn. Then a reverse drawing method is used for the final drawing of the spherical piece. This method is used for drawing different shapes of pieces that must have a very fine surface without wrin- kle marks. With this process, wrinkles do not form because the forces of compression encountered during the drawing operation are changed to forces of tension. The technical components of the reverse die are as follows: the die drawing ring with a die ring radius which should be not less than 4 times the material thickness; a pressure pad 3; a reverse drawing punch 1; and a workpiece ejector 4. If this method is used for drawing a parabolic piece, a reverse draw- ing to the next radius reduction, finally resulting in the final dimension, sometimes needs to be done by a spinning process. Deep Drawing Dies 163 Fig. 1 1.16 Die for drawing spherical piece. ring Fig. 1 1.17 shows a design for a die with a circular draw bead ring. The components of the die are the die ring with circular draw bead the punch the pressure pad and the ejector 4. The die is used on a double-action press. The first blank forms a spherical shell in a separate die, which is then drawn in the die as shown in Fig. 11.17. The punch is attached to the inner slide and the pressure pad to the outer slide. The pressure pad puts pressure on the spherical workpiece, so that the punch forms it into a para- bolic shape. The workpiece is held with a force great enough to prevent the material from rising and light enough that the material is able to slide out from under the pad without wrinkles being formed. The ejector 4 is designed with a parabolic inside contour matching the shape of the final piece so that it performs a coining die function before the workpiece is ejected from the die. This method is much more reliable than the reverse drawing method. The next radius reduction, which results in the final dimen- sions, is done by a spinning process. 1 drawing 2-die drawing ring 3-pressure pad 4-ejector 5-punch holder 6-die shoe Fig. 1.17 Die with a circular draw bead ring. 164 Deep Drawing Dies

11.9 IRONING DIES

Frequently, the final operation in a series of draws consists in ironing the shell walls to reduce the thick- ness of the material and ensure a smooth uniform surface throughout. This work is done by making the clearance between the punch and the die ring slightly less than the thickness of the workpiece wall, so that the material is both thinned and elongated. Fig. 1 1.18 shows an ironing die. The ironing die consists of the punch holder 9 and the die shoe without a guide system. The punch is attached to the punch holder by the ring whose inner side has a conic profile and whose out- side is threaded. The punch is made of two parts: the technical part of the punch 1 is made of tool steel, and the extension part is made of carbon steel. The upper set of the die is fixed to the ram 1 2, by a clamp 15. In the die shoe 10 are fixed the die ring the workpiece centering ring and the seg- mental stripper with the helical spring 6. The lower set of the die is attached to the bed of the press by the clamp 1 6. The segmental stripper consists of four segments connected by the ring of the helical spring located in slots in the segments. When the ram moves down, the segments are moved apart by the radial pressure on the workpiece, and when the ram is moved up, the sharp edges of the stripper strip the workpiece from the punch. The most efficient drawing process occurs when the first drawing achieves a reduction by diameter; reduction of the wall thickness of the workpiece occurs after that. Deformation by diameter done separately from deformation by wall-thickness reduction is advisable not only because of the favorable drawing ratio but because for wall-thickness reduction operations, ironing dies are simple and may be used on single-action \ I J 1 -punch 2-die ring 7-punch extension 8-ring 13-bed of press 3-workpiece 4-workpiece centering ring 14-holder plate 5-clamp 10-die shoe 5-segmental stripper 11-backing plate 6-helical spring 16-clamp Fig. 1.18 An ironing die