This type of die is familiarly known as a drawing die. The most common example of the forming die is that used for drawing a flat, circular blank, shown at A, Fig. 358, into a cup-shaped piece, shown at B. This operation can be done in an ordinary punching press by means of a forming die of the shape known as a push-through die, so called from the fact that the piece is formed to shape and pushed through the die at one operation. This form of die is shown in Fig. 359. The face of the die is cut to receive the blank; this depression is known as the "set edge". The opening in the die is given a "draw" of from 1/4 to 1/2 of a degree, making it larger at the top; the upper edge is rounded over and left very smooth, and the bottom edge is made vary sharp, in order that the piece may not be carried back with the punch as it returns.
Fig. 354. Bending Die for Light Work.
Fig. 355. Spring Bending Die and Finished Piece.
Fig. 356. Compound Bending Die.
Fig. 357. Punch at Bottom.
Fig. 358. Flat Blank Drawn into Cup Shape.
This form of die is left as hard as possible, and the walls of the opening are made as smooth as they can be polished. It is sometimes advisable to finish the walls with a lateral rather than a circular finish.
Drawing and redrawing dies having holes which pass entirely through them, as shown in Fig. 360, give considerable trouble when hardened unless proper methods of treatment are used. The principal difficulties experienced are alteration in the size of the hole, and soft spots in the walls of the holes.
As there is no need for the exterior of the die being hard, the whole attention of the hardener should be given to getting the walls of the hole as hard as possible, as this portion is subjected to considerable strain and to excessive abrasive action, and soft portions render the die useless. This is especially true of dies used for such work as redrawing cartridge shells and similar pieces.
In order to harden the walls of the hole, and yet leave the circumference of the die soft, it is necessary to make a fixture to cover the portions desired soft. Such a fixture is shown in Fig. .360. It may be made from a piece of cast iron, the portion A being a little, say 1 inch larger than the diameter of the die. The opening to receive the die should be 1/16 inch larger than the die. The balance of the hole should be somewhat larger than the hole in the die, say ) inch. A cover may be made of the same material, and it should be a loose fit on the holder. The hole in the cover should be 1/4 inch or more larger than the hole in the die and beveled as shown.
When the die is heated to a uniform red, it is placed in the fixture, the cover put on, and the whole held under a water pipe, or faucet, Fig. 362, while the water is allowed to flow through the hole as shown. A mistake sometimes made consists in placing the fixture in a bath and then attempting to force the water through the hole; unsatisfactory results always follow if this is done, for the water cannot flow through the hole, pockets of steam form which prevent hardening, and soft spots result. The fixture should not be immersed in water, but should be held so that the water can pass unretarded through the hole and carry the steam with it. The water supply should be sufficient to fill the hole and should pass through under a fair head, but not too swiftly. This method, when properly executed, gives excellent results. As a rule, dies of this kind are left dead hard, the temper not being drawn at all.
Fig. 359. Push Through Die for Cup-Shaped Work.
Fig. 360. Fixture for Covering Soft Portion of Die.