The most important matter in punching plates is the diameter of the opening in the bolster or die relatively to that of the punch.  This difference exercises an important influence in respect not only of easy punching but also in its effect upon the plate punched.  If we attempt to punch a perfectly cylindrical hole, the opening in the die block must be of the same diameter as the point of the punch, or, at least, a very close fit.  The point of the punch ought to be slightly larger in diameter than the neck, or upper part, as shown in Figs. 12 and 13, so as to clear itself easily.  When the hole in the bolster or die block is of a larger diameter than the punch, the piece of metal thrust out is of larger diameter on the bottom side, and it comes out with an ease proportionate to the difference between the lower and upper diameters; or, in other words, it produces a taper hole in the plate, but allows the punching to be done with less consumption of power and, it is said, with less strain on the plate.

[Illustration:  Fig. 12.]

[Illustration:  Fig. 13.]

As to the difference which should exist between the diameter of the punch and the die hole, this varies a little with the thickness of the plate punched, or should do so in all carefully executed work, for it is easy to understand that the die which might give a suitable taper in a three-fourths inch plate would give too great a taper in a three-eighths inch plate.  There is no fixed rule; practical experience determines this in a rough and ready way—­often a very rough way, indeed, for if a machine has to punch different thicknesses of plate for the same size of rivets, the workman will seldom take the trouble to change the die with every variation of thickness.  The maker of punches and dies generally allows about three sixty-fourths or 0.0468 of an inch clearance.

The following formula is also used by punch and die makers: 

    Clearance = D = d + 0.2t

where
    D = diameter of hole in die block;
    d = diameter of cutting edge of punch;
    t = thickness of plate in fractions of an inch;

that is to say, the diameter of the die hole equals diameter of punch plus two-tenths the thickness of the plate to be punched.

Example.—­Given a plate 3/8 or 0.375 of an inch thick, the diameter of the punch being 13/16 or 0.8125 of an inch, then the diameter of the die hole will be as follows: 

  Diameter of die hole = 0.8125 + 0.375 X 0.2 = 0.8875 inch diameter,
    or say 7/8 or 0.875 inch diameter.