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.

Punching possesses so many advantages over drilling as to render it
extremely important that the operation should be reduced to a system
so as to be as harmless as possible to the plate. In fact, no plate