Assuming that the die is ready to harden and having the screw holes plugged - with a soft machine screw if desired - the die is heated slowly and evenly in a muffled fire preferably. A blast such as a black smut forge would cause uneven heating of the die, which means uneven expansion. If either an open forge or a muffle furnace is used, the position of the die should be constantly changed to insure even heating, and the face of the die should be up. When an even temperature of the desired degree is obtained - varying with different makes of steel - the die is gripped by tongs, plunged into the bath, and moved slightly up and down, keeping it fully submerged at all times.
The die should not be allowed to remain in the bath, however, until it becomes cold, because some parts of the die will contract faster than others. When the violent vibration on the tongs ceases, the die should be removed and plunged into an oil bath as quickly as possible. This is done to allow the heat from the heavier portions to flow into the parts that are cooler, causing a more even contraction. The die should be removed from the oil bath before the die is cold, it should be drawn to the desired temper immediately and, to allow it to cool slowly, should be set on some material which is of low heat-conductivity. If a hardened die, while hot, were set on a cold mass of steel, the chances are that cracks in the die would result.
After the die is thoroughly cool, the oil and scale are removed, and the face that is most level is placed against the grinder bed and the other face is ground. The bottom of the die need only be ground until a true surface is obtained, but the top or cutting surface should have several cuts taken across to remove any burned metal that may have been caused in hardening and also to insure the cutting edge being keen its entire length.
The next step is to make the punch. Assuming that the blanking punch has been machined, the bottom or cutting surface is blued in, the same as the die, the punch is clamped to the face of the die as at Fig. 12, and the outline of the die is transferred to the face of the punch. A very slender and sharp-pointed scriber must be used, and after the entire outline is scribed, the line must be inspected carefully before the clamp is removed. It is easy to make an error in transferring the outline, as the die is quite thick and the scriber must of necessity be tapering, and the largest diameter of the scriber can rest against the die instead of the point of the scriber being in contact with the cutting edge of the die.
If the die has narrow places where it is not possible to scribe the line, then the surface of the punch is coated with solder and machined level, and the outline of die is transferred by forcing the solder into the die.
The punch is now gripped by the shank in the chuck of a milling machine - the shank having been turned on the punch for two reasons: to facilitate milling the punch to shape; and to act as a heavy pilot to stiffen the punch on the punch plate. After milling to within, say, 1/64 inch of the line, the punch is removed and the entire cutting edge of the punch is beveled slightly, and, placing the punch in the die opening, the punch is forced in far enough to obtain the exact outline of die. This operation is called shearing the punch. The punch can then be replaced in the chuck of the milling machine, and by skillful workmanship all surplus metal can be milled away, leaving only a small amount of hand work necessary to complete the punch. If the punch is milled after it has been sheared in the die, a narrow cutter must be used to remove the small and surplus stock. A safer way for the beginner - in fact, many experienced die-makers pursue this method - is to chip the stock away, shear the punch again, and the stock that the cutting of the die causes to curl up is again chipped and scraped away, then repeating the operation until the punch is fitted the desired depth. One point that is essential when shearing a punch in the die is to make sure that the punch enters at right angles with the face of the die and also that the punch cannot tilt when being withdrawn. Any tilting when withdrawing will surely break off the weak corners of the die. Therefore, it is best to secure the punch in the ram of a press and to fasten the die securely to the bed when shearing.
Fig. 12. Laying Out Punch.
Punches and dies having no weak corners or points can be sheared by forcing the punch in the vise but the edge of the punch will be rounded off when driving out the punch, if great care is not exercised, as one end will invariably drive out ahead of the other.
In Fig. 13 at a, b, and c the punch is shown as it appears at the first, second, and third shears. The punch should not be forced in more than 1/8 inch at a time, as the die does not actually cut the metal away, but crowds it out, and, after a certain amount of stock is banked up on the punch by the crowding or pushing action, the stock tears away from the punch and deep spots will be torn in the punch that are below the size of the die. For chipping away the surplus stock, the chisel should be ground so that it does not have a tendency to dig in, and the chisel should be struck as shown at c, Fig. 13.
After each shearing operation and chiseling away of stock the surface is smoothed by scraping, d, Fig. 13, and by filing. Only the point or end of the file must be used, or else the cutting edge of the punch will be filed tapering or too small. The entire surface of the sides of punch must be reduced to less than the size of the die, governed by the thickness of metal to be punched, and the surfaces should be made smooth.
Fig. 13. Chipping Away Surplus Stock on Punch.
If the punch is to be secured to the punch plate by screws, the holes are drilled and tapped in the punch by transferring the boles from the punch plate.
The punch is now gripped by its shank in a lathe chuck and the beveled edge is faced off, leaving a sharp corner or cutting edge, after which the punch is hardened. Punches are not made as hard as dies, and a deep dark straw color or even purple proves satisfactory for stock that is not tempered by heating and dipping. For punching thin soft metals - aluminum, copper, or brass - or paper, the punch is generally left soft, for there must be a close fit between it and the die in punching thin stock, and when the punch becomes dull, which is caused by its rubbing through the material being punched, it can be upset or riveted slightly around the edge and sheared into the die without taking the punch from the press. This insures a perfect fit between the punch and the die which is essential on very thin stock-After hardening, the punch is attached to the punch plate, and the cutting face of the punch is ground by resting the back of the punch plate against the bed of the grinder. This insures the face of the punch and the back face of the punch plate being parallel.
Fig. 14. Die Fitted Tight in Die Shoe.
The stripper is now fastened to the die without the guide strip D, Fig. 307, Tool-Making, Part III, so that the stripper comes in contact with the face of the die. The outline of the die is now transferred to the stripper, and the stripper then is removed and the opening drilled and filled, much the same as for the die, except that the opening in the stripper is generally made somewhat larger on large blanking punches.
The adjustable die shoes shown in Figs. 308, 309, 310, and 311, Tool-Making, Part III, are designed more for a jobbing shop where quick changes are made, that is, where only a few blanks of each kind are made at a time; but for continued daily production it is better to fit the die tight in a recess in the die shoe, as in Fig. 14, and to have a separate die shoe for each die. The first cost of making the die shoe for each die is soon wiped out by the saving of the pressman's time in changing the dies from one shoe to another.