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
should be used in the construction of a boiler that does not improve
with punching, and further on I will show by the experiments made by
Hoopes & Townsend, of Philadelphia, that good material is improved by
punching; that is to say, with properly made punches and dies, by the
upsetting around the punched hole, the value of the plate is increased
instead of diminished, the flow of particles from the hole into the
surrounding parts causing stiffening and strengthening.

_Drilling Rivet Holes._--In the foregoing I have not referred to the
drilling of rivet holes in place of punching. The great objection to
drilling rivet holes is the expense, from the fact that it takes more
time, and when drilled of full rivet size we are met with the
difficulty of getting the rivet holes to correspond, as they are when
punched of full rivet diameter. When two plates are drilled in place
together, the drill will produce a _burr_ between the two plates--on
account of their uneven surfaces--which prevents them being brought
together, so as to be water and steam tight, unless the plates are
afterward separated and the burr removed, which, of course, adds
greatly to the expense.

The difference in strength between boiler plates punched or drilled of
full rivet size may be either greater or l