The term deep drawing is applied to dies that are employed to produce long shells or deep cups, such as that in Fig. 37. These dies might properly be classed as redrawing dies. It is obvious that the shell in Fig. 37 could not be produced in a drawing die in one operation, for the diameter of the drawing punch would be so small relative to the draw that the punch would push through the blank. In other words, the pressure required to transform the large blank into a long slender tube, at one stroke, is greater than the pressure required to push the punch through the stock.
To produce the shell in Fig. 37, there is required a series of redrawing dies such as that in Fig. 38, and as can best be understood by referring to the successive drawing operations indicated in Fig. 39. When a cup has passed through several redrawing operations, the stock in the cup becomes very hard and the cups must be annealed before further drawing. The cup in Fig. 39 would require possibly two annealing operations - the deeper the cup, the greater the number of draws and annealing operations. The dies should be designed for the successive operations in practically the shapes shown by the dotted lines'in Fig. 39, and, as in Fig. 40, should be provided with a plunger actuated by a powerful spring, to insure the cup being forced from the die, and also with a close fitting stripper surrounding the punch, care being exercised that the stripper for the punch is high enough from the face of the die to allow the blank to be readily removed from the die.
When a punch passes through the strip of stock to be punched, the blank punched out is really broken from the stock, rather than being cut apart, and this breaking leaves a ragged edge on the blank and in the hole in the stock. The thicker the stock, the more pronounced this ragged edge is. Blanks punched from, say, 1/4-inch sheet stock are decidedly tapering, as in Fig, 41. When punching cams, levers, eccentrics, or small parts for typewriters, adding machines, cash registers, etc., where the action of the cams and levers must be smooth, a shaving die, shown in its simplest form in Fig. 42, is employed for finishing the blank. The blanking die is designed to punch the blank very close to the desired size or shape, leaving an allowance of only a few thousandths for shaving. In connection with such a piece as the eccentric washer, Fig. 43, the outer edge of which must be smooth, as it runs in a bearing, the designer should employ the subpress die, Fig. 21, because this type of die produces the most uniform blanks.
Fig. 37. Typical Deep Drawing Blank.
Fig. 38. Series of Redrawing Dies for Deep Drawing work.
As a rule, the shaving die is not given clearance, as is the case with blanking dies, and, as the punch must fit the die very closely, it is the general practice to make the shaving die in accordance with subpress construction, which insures alignment of punch and die. The shaving die can be fitted with a close fitting spring stripper, as in Fig. 42, or the blanks can be pushed through, as desired. By making the die, say, 1 inch deep, and without clearance, and by keeping the die and blanks well lubricated, and pushing the blanks clear through, a highly polished edge is produced on the blank. The designer should bear the shaving die in mind and use it wherever feasible, for it is a very rapid method of producing pieces of uniform size.
Fig. 39. Six Stages in Process of Forming Deep Drawn Shell, Fig. 37.
Fig. 44 shows samples of ordinary embossing - though for heavy embossing, as at a, Fig. 44, a drop press is best suited. If the embossing desired is somewhat heavy and there is danger of springing the shaft of the punch press, and the designer must of necessity use a punch press, it is better to use a die of design such as that in Fig. 45, rather than to have the blow struck directly. Extruding Die. If a cartridge shell were filled with butter, and a lead pencil forced to the bottom of the shell, the butter would ooze or flow up the pencil. This holds true with metal; it is simply a matter of pressure. Small eyelets, hollow rivets, and thin tubes are economically made with extruding dies, such as in Figs. 46 and 47. In operation, referring to Fig. 47, a punched washer of predetermined thickness is dropped into the recess of the die, and as the punch fits the recess there is only one path for the metal to flow in, and it is then simply a question of pressure. As it requires time for the metal to flow, the press must travel comparatively slow, and therefore hydraulic presses are used. If an extruding die were fitted to a punch press running at ordinary speed for blanking - about 100 revolutions per minute - it would be apt to break the shaft of the press, as there would be practically a direct blow. Another type of extruded work is shown at a, Fig. 46. This scheme is often employed where a tapped hole is desired in thin stock, as at b in the same figure.
Fig. 40. Punch and Die for Producing No. 2, Fig. 39.
Fig. 48 shows a forming die in its simplest form. Forming dies can be made part of many dies that pierce and cut off, or, if the blank has straight sides as at a, Fig. 49, the stock can be purchased in the proper width, and by adding the forming die to the end of the piercing-and-blanking die, as shown in Fig. 49, the blank can be pierced, cut off, and formed. The designer should always bear in mind that the forming points in a punch and die may be changed from the position shown in Fig. 48, and that formed pieces, such as at a, Fig. 50, can be easily made in one stroke.
Fig. 42. Shaving Die.
Fig. 43. Blank for Eccentric Washer.
Fig. 44. Typical Examples of Embossing.
Fig. 45. Embossing Press and Die.
Fig. 46. Typical Extruding Die.
Fig. 49. Piercint and Forming Die Work.
Fig. 51 shows a plain design of water forming die. The designer can elaborate upon this design and produce intricate forms, using water.
The dies are designed in halves, one-half stationary, while the other half slides, being actuated by a lever that can be locked when the halves are firmly pressed together. In operation a drawn cup such as a, Fig. 52, is filled with water, the cup is inserted in the die and the halves of the die are closed and locked. The punch, which is attached to a drop hammer, fits the inside of the cup closely, and, as the hammer falls, the punch strikes the water, forcing the metal into all parts of the die. The object in making the dies in halves is to allow removing the blanks when they are formed to a larger diameter as in 6, Fig. 52.
Fig. 50. Typical Forming Die and Blank.
Fig. 51. Design for Water Forming Die.
This same design of die can be used for thin stock when a spring rubber plug is employed. The rubber forces the thin metal walls of the cup to all parts of the die, and as rubber assumes its original shape after pressure is released, it can be easily removed and used over and over.