In Fig. 306 of Tool-Making, Part III, are shown a blanking punch and die for use on heavy stock such as boiler plates. In a simple tool of this character it is immaterial which part is made first. But, should this same type of tool be required for piercing a small hole in heavy stock, the tendency would be to spring the slender punch, and, therefore, in such a case, the punch should be supported and guided by the stripper. Assuming that the punch is of ¼-inch diameter and is to pierce hard rolled stock 1/8 inch thick, the first step is to find the difference in diameters between the punch and the die.
The rule for clearance is to multiply the thickness of stock in thousandths of an inch by .06; the answer being the difference in thousandths of an inch between the punch and the die.
Whether to increase the size of the die or to decrease the size of the punch depends upon the nature of the stock and whether the piece punched out must be of a certain diameter or whether the diameter of the hole must be maintained. If the blank or piece punched out must be of a certain diameter, say .250 inch, then the die is made .250 inch, whereas, if the hole pierced is to be maintained .250 inch, then the punch is made .250 inch in diameter, and the clearance or difference between the punch and the die is obtained by increasing the size of the die. This is only essential, however, where the diameter of the hole or of the blank is to be maintained in thousandths of an inch.
When punching holes in sheet metal, the actual diameter of the blank1, using the same die and punch, varies with the temper of the stock. For instance, on hard rolled stock the blank would break somewhere between the diameter of the punch and that of the die as exaggerated in Fig. 1. This is more noticeable on heavy stock as the thick stock is stiff enough to withstand the pressure of the punch. If soft stock were used in a die as in Fig. 1, the stock would bend down between the punch and the die, causing a heavy burr on both the blank and the hole. A tight-fitting punch and die also causes heavy burrs on both hole and blank, when punching thick stock.
Fig. 1. Section of Die in Which soft Stock Is used.
The main reason why a difference in the diameters of punch and die for piercing thick stock is necessary is to prevent breaking the punch. If a punch that snugly fits the die were used to pierce ¼-inch stock, the stock would be such a tight fit on the punch that it would be hard to strip the stock from the punch. Again, the severe rubbing of the punch as it passes through the stock would cause the punch to roughen, or, to use the shop term, to pick up, which causes the stock to stick to the punch, and which is one cause of the punch breaking.
Another cause of breaking, and one which the die-maker must guard against, is when the underside of the stripper, where the stock comes in contact with it in stripping, is not parallel with the die, or rather is not at right angles with the travel of the punch. It is readily seen that, if ¼-inch stock is snugly gripping a small punch and the stock comes in contact with the underside of a stripper plate which is on an angle, the stock is going to adjust itself to the surface of the stripper, which will snap off the end of the punch.
Following the rule for clearance given above, we find the difference between the punch and the die to be .0075 inch, and, assuming that the punch is of small diameter, it now becomes important which part is made first, as the punch should be guided and supported. The term guided, when applied to a punch attached to the ram of a power press that travels in a positive channel, appears at first glance to be a misnomer, but any uneveness of the stock surface, such as caused by a slight kink in the stock or even by a piece of foreign substance on the stock, causes the punch to be deflected from its line of travel, resulting in a broken punch.
The sequence of operations for one good method of making the punch and die, Fig. 2, is to cut off a piece of round tool steel for the bushing o, Fig. 2, say 2 inches longer than desired, and, gripping the steel in a lathe chuck, to rough-drill the hole to within, say, 1/32 inch of size, then to turn the outside diameter to the desired dimension, and finally to bore the hole to the desired size. Boring and turning at the same setting insures concentricity of the hole and the outside, providing, however, that one diameter is not finished, before starting to machine the other diameter. For instance, if the outside were turned to exactly the right diameter, then the hole spotted, drilled, and bored, the pressure of spotting and drilling might cause the rod to spring or to shift in the chuck, resulting in the finished hole being eccentric with the outside. The tool-maker must constantly guard against any element of chance.
After both diameters are obtained, the bushing is cut from the rod, using a cutting-off tool in the tool post of the lathe. The Clearance in the bushing can be bored by setting the slide rest at the desired angle, say ½ of a degree - the shop term for expressing 0° 30'. If a taper reamer is employed, we have no assurance that the finished taper hole will be concentric with the outside, as the reamer can be started on an angle with the hole. The bushing is now hardened and drawn to a dark straw color.
Fig. 2. Punch and Die Showing Use of Die Bushing.
The next step is to plane the bottom of the die shoe. As the top and bottom surfaces of die shoes must be parallel, it is obvious that we must hold the die shoe by clamping it to the bed of a shaper or on the faceplate of a lathe. If gripped by its rough sides in a shaper vise, difficulty would be experienced in obtaining parallel surfaces.