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 problems, 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. 11.16 shows a design for a die for drawing spherical pieces. In the first operation a piece with a convex 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 wrinkle 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 (2), 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 drawing to the next radius reduction, finally resulting in the final dimension, sometimes needs to be done by a spinning process.

Fig. 11.16 Die for drawing spherical piece.

Fig. 11.17 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 (2), the punch (1), the pressure pad (3), 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 parabolic 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 dimensions, is done by a spinning process.

Fig. 11.17 Die with a circular draw bead ring.

11.9 IRONING DIES

Frequently, the final operation in a series of draws consists in ironing the shell walls to reduce the thickness 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. 11.18 shows an ironing die.

The ironing die consists of the punch holder (9) and the die shoe (10), without a guide system. The punch is attached to the punch holder by the ring (8), whose inner side has a conic profile and whose outside 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 (7), is made of carbon steel. The upper set of the die is fixed to the ram (12), by a clamp (15). In the die shoe (10) are fixed the die ring (2), the workpiece centering ring (4), and the segmental stripper (5), with the helical spring (6). The lower set of the die is attached to the bed of the press by the clamp (16). The segmental stripper consists of four segments connected by the ring of the helical spring (6), 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

Fig. 11.18 An ironing die

presses. However, reduction by both diameter and wall thickness of the workpiece may be combined in one multi-stage drawing die. Fig. 11.19 shows one design for a multi-stage ironing die.

In the multi-stage ironing die, a precut blank is inserted into a nest and held there by the pneumatic pressure pad (10). The blank is pushed through the die ring (2) for reduction by diameter, and then it is drawn. I Ironing operations are then done by the punch (1) and three reduction draw rings (3, 4, and 5). The distance rings (6, 6a, and 6b) assure the correct distances between the draw rings. If the workpiece, in process of drawing, leaves one draw ring before beginning to enter into the next draw ring, the machinery may perform another stroke that will often cause a crack or cracks in the workpiece.

Fig. 11.19 Multi-stage ironing die

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