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. Punches are generally made flat on their cutting edge, as shown in Fig. 12. There are also punches made spiral on their cutting edge, as shown in Fig. 13. This punch, instead of being flat, as in Fig. 12, is of a helical form, as shown in Fig. 13, so as to have a gradual shearing action commencing at the center and traveling round to the circumference. Its form may be explained by imagining the upper cutter of a shearing machine being rolled upon itself so as to form a cylinder of which its long edge is the axis. The die being quite flat, it follows that the shearing action proceeds from the center to the circumference, just as in a shearing machine it travels from the deeper to the shallower end of the upper cutter. The latter is not recommended for use in metal of a thickness greater than the diameter of the punch, and is best adapted for thicknesses of metal two-thirds the diameter of the punch. Fig. 14 shows positions of punch and attachments in the machine. [Illustration: FIG. 14.] It is of the greatest importance that the punch should be kept sharp and the die in good order. If the punch is allowed to become dull, it will produce a fin on the edge of the rivet hole, which, if not removed, will cut into the rivet head and destroy the fillet by cutting into the head. When the punch is in good condition it will leave a sharp edge, which, if not removed, will also destroy the fillet under the head by cutting it away.