Trouble is experienced at times with blanking and piercing punches because the metal clings to the punch and pulls the end off in the operation of stripping. This is especially the case when a clinging metal is being worked. The trouble can be avoided by making the punch of the design shown in Fig. 347, where the portion marked a is straight and the desired fit in the die. The portion 6 is tapered and smaller at its junction with a. When the die is set up in the press, a enters the die nearly its entire length; the tapered portion b, entering the stock, spreads it, thus enlarging the opening, and so preventing it from binding the punch during the process of stripping. It is of course necessary, when setting a punch of this design in the press, to make sure that the tapered portion does not enter the opening in the die. Multiple Die. If the shape of the pieces to be punched allows it, it is sometimes advisable to make several openings in the die of the same outline so arranged that as many pieces may be punched at a time as there are openings in the die block. This will effect a great saving where work is punched in large quantities. In the manufacture of perforated sheet-metal work, it is customary to make dies having as many as five hundred punches working into one die block at a time; but as this is an unusual application of this principle, it will not be considered.
If it is necessary to punch ten holes in the piece shown in Fig. 348, a die can be made having this number of openings. Then, by making a punch holder having an equal number of punches properly located, all the holes can be punched at one stroke of the press.
While in the case just cited the piece of stock which had the holes punched in it is the product, the punchings being scrap, the same principle can be applied to punching blanks from a sheet of stock. The design shown in Fig. 349 is the product in a shop where many thousands of this piece are used monthly. The die produces a dozen or more blanks at each stroke of the press, but for convenience in illustrating the die and punches, but four openings in the die, with a corresponding number of punches are shown, Fig. 350.
If a die were made with the openings near enough together to punch the stock, Fig. 351, there would be so little stock between the openings that the die would not stand up when used; for this reason the openings are located in such a manner that every other opening is omitted. When the punch descends, four blanks are punched, and, the stock is moved until the first opening strikes the gage pin, Fig. 350. This leaves the stock in position to punch between the openings already made. The next time the stock is moved until the gage pin strikes the wall of the last opening to the right.