That punching, or the forcible detrusion of a circular piece of metal
to form a rivet hole, has a more or less injurious effect upon the
metal plates surrounding the hole, is a fact well known and admitted
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.
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