With first class steel or iron plates, punching is perfectly
allowable, and the cost is twenty-five per cent. less than drilling;
in fact, none but first class metal plates should be used in the
construction of steam boilers.

In the original punching machines the die was made much larger than
the punch, and the result was a conical taper hole to receive the
rivet. With the advanced state of the arts the punch and die are
accurately fitted; that is to say, the ordinary clearance for a rivet
of (say) three-fourths of an inch diameter, the dies have about three
sixty-fourths of an inch, the punch being made of full rivet size, and
the clearance allowed in the diameter of the die.

Take, for example, cold punched nuts. Those made by Messrs. Hoopes &
Townsend, Philadelphia, when taken as specimens of "commercial," as
distinguished from merely experimental punching, are of considerable
interest in this connection, owing to the entire absence of the
conical holes above mentioned.

When the holes are punched by machines properly built, with the punch
accurately fitted to the die, the effect is that the metal is made to
flow around the punch, and thus is made more dense and stronger. That
some such action takes place seems probable, from the appearance of
the holes in the Hoopes & Townsend nuts, which are straight and almost
as smooth as though they were drilled.

Therefore I repeat that iron or steel that is not improved by proper
punching machinery is not of fit quality to enter into the
construction of steam boilers.