by every engineer, and it has often been said that the rivet holes
ought all to be drilled. But, unfortunately, at present writing, no
drilling appliances have yet been placed on the market that can at all
compare with punching apparatus in rapidity and cheapness of working.
A first-class punching machine will make from forty to fifty holes per
minute in a thick steel plate. Where is the drilling machine that will
approach that with a single drill?

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