Before hardening, the stripper and guide screw holes should be drilled and tapped, and the hole drilled for the gage pin or stop. If the name of the part to be punched, or the shelf number of the die is to be stamped, it should be done now. After all screw holes, stop-pin holes, etc., are filled with fire clay mixed with water to the consistency of dough, the die is ready for hardening. Extreme care should be exercised in the heating; the heat must be no greater than is absolutely necessary, and it should be uniform throughout-the corners of the die must be no hotter than the middle of the piece, and the outside surface must be of the same temperature as the interior of the steel. The water in the bath should be slightly warmed to prevent any tendency to crack. The die should be lowered into the bath and swung back and forth gently so that the bath may pass through the opening and harden the walls. As soon as the singing ceases, the die should be removed and plunged into a tank of oil and allowed to remain until cold, when it is brightened and the temper drawn. If more than a few minutes are to intervene between the time the die becomes cold and the time for commencing to draw the temper, the die should be held over a fire or placed where it can be heated, to remove the internal strains which have a tendency to crack the piece.
Fig. 328. Grinding Round Opening in Die.
When there is a heavy body of metal around the openings in a die, and a light partition between the openings, there is danger of cracking during the hardening. In such cases a little oil applied to the light portion when red hot and just before quenching, especially at the point where it connects with the heavier portion, thus prevents too rapid cooling of the parts, and so does away with the danger of cracking. The oil may be applied by means of a piece of cloth, which may be attached to a wire; in this way the oil reaches the desired spot and no other. The oil having been applied, the die may be cooled in the bath in the usual manner.
When a die of such a shape that it is likely to give trouble, is to be hardened, much more satisfactory results will follow if the pack-hardening process is used. Run the dies from one to five hours in the fire after they are red hot; then dip them in raw linseed oil and swing them back and forth to force the oil through the opening. Dies having openings that are perfect circles may be left a trifle small until after hardening, when they are ground to exact size as shown in Fig. 328. Here the die is held in a chuck and the grinder is motor driven.
A very common method of drawing the temper of dies and similar pieces, is to heat a piece of iron to a red heat and place the hardened piece on it, leaving the face of the piece uppermost. Experience shows, however, that this method of treatment is too harsh for hardened steel, especially if the job is in the hands of one not thoroughly experienced, for it subjects one side of the piece to an intense heat while the opposite side is exposed to the cooling effects of the air. If an open fire is used, a plate may be set on the fire, and the die placed on the plate before it is hot; now the temperature of the plate may be raised gradually, the die being turned occasionally. In this manner, the temper can be drawn to the desired degree with safety. When such a fire is not available, two plates may be used, one heating while the other is in use. The first one should not be very hot, the next somewhat hotter, and so on until the die is drawn to the desired color.
The punch is used to force the metal through the die, thus producing pieces of the desired shape.
In the case of small plain dies, the punch is generally made of the form shown in Fig. 329. The end A is of the same outline as the opening in the die; the shoulder B which bears against the shoulder of the punch holder, takes the thrust when the punch is working; the shank C fits the hole in the punch holder or in the ram of the press. It is customary in most shops in this country to make the die to a drawing or a templet, and then to harden it, after which the punch is fitted to it. Laying Out. The templet may be used in laying out the punch for a plain die. If the shape of the opening in the die is the same on each side, Fig. 330, and the die does not change shape in hardening, either side of the templet may be used next to the face of punch; but if the outline is of the form shown in Fig. 331, it will be necessary to exercise care to see that the proper side is used, because the side of the templet placed against the face of the punch when laying it out will be opposite to the one placed against the face of the die when laying that out.
Fig. 329. Plain Punch.
Fig. 330. Typical Die of Some Shape at Both Ends.
Fig. 331. Die of Odd Form.
In order to obviate this trouble, many tool-makers lay out the face of the punch from the opening in the die before beveling the face for shear. In order to hold the punch and the die together so that there will be no danger of the punch slipping while the shape is being transferred, a die clamp of the form shown in Fig. 332 should be used.
The punch blank should be faced on both ends, and the shank turned to size. .The end which is to fit in the opening in the die should be finished with a smooth, flat surface, and colored with blue vitriol. After coloring, it may be clamped to the face of the die by means of the die clamp, and the outline of the punch marked on the face by scribing through the opening in the die. This outline should be accurately marked with a sharp-pointed prickpunch, as the scribed line is likely to become obliterated by the various operations of machining the punch to shape.
After the outline has been carefully prickpunched, the punch is ready to be milled or planed to shape, leaving enough stock at all points to shear into the die. If the punch is milled to shape, the irregular surfaces may be produced by means of a fly cutter, Fig. 333. If it is planed, it may be held in a pair of centers, as shown in Fig. 334, in a shaper. If the die has been left soft to permit laying off the punch, it should now be beveled for shear, and hardened.
The punch should be machined close to the lines, and then placed over the hardened die and forced into it a little, about 1/16 inch. This is termed shearing-in, and is a customary process in this country.
After the punch has been sheared-in for a short distance, it may be removed and worked to size by means of chisel, file, and scraper to the witness mark, as the portion sheared-in is termed. The operation of shearing-in may be repeated until the punch enters the entire length.
If the material to be punched is thin or soft, it is necessary to make the punch a closer fit in the die than if the stock is heavy or very stiff. Thin stock requires a punch nicely fitted to the die in order to avoid ragged edges on the punched blank. When punching brass stock 1/8 inch in thickness, the punch should be .0075 inch smaller than the die, the usual difference being 6 per cent of the thickness of the stock for brass, and 7 per cent for steel. If the stock is very stiff, a greater difference should be allowed, the exact amount depending on the nature of the material to be used and the character of the tool.
Fig. 332. Die Clamp.
After the punch has been fitted to the die, the cutting end should be faced off to insure a good working surface and sharp edges. Any distinguishing names or marks necessary, should be stamped on it, after which it is ready for hardening.
Punches are hardened by heating them in an oven furnace or in a clear charcoal fire, to a low red, and cooling in water or brine, preferably the latter. Punches whose form insures strength, need be hardened only on the end; the hardening should not extend quite back to the shoulder or shank. Small, slender punches are sometimes hardened the entire length, especially if they are to punch stock nearly as thick as the diameter of the tool itself, for otherwise they would become upset when used.
Fig. 333. Shaping the Punch with Fly Cutter.
It is generally considered good practice to have the punch softer than the die; on this account it is usually drawn to a color that insures this result. If a die is drawn to a straw color, the punch is drawn until it assumes a distinct purple, or even a blue color.
The punch is sometimes left soft-not hardened at all; when this is done, it can be upset, and refitted when worn. As this would not work satisfactorily in many cases, it can be recommended only when a soft punch is advisable.
Fig. 334. Shaping Punch in a Shaper.