where
D = diameter of hole in die block;
d = diameter of cutting edge of punch;
t = thickness of plate in fractions of an inch;
that is to say, the diameter of the die hole equals diameter of punch
plus two-tenths the thickness of the plate to be punched.
_Example_.--Given a plate 3/8 or 0.375 of an inch thick, the diameter
of the punch being 13/16 or 0.8125 of an inch, then the diameter of
the die hole will be as follows:
Diameter of die hole = 0.8125 + 0.375 X 0.2 = 0.8875 inch diameter,
or say 7/8 or 0.875 inch diameter.
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
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 less than the difference in
strength between unperforated plates of equal areas of fracture
section. When the metal plates are very soft and ductile, the
operation of punching does no appreciable injury. Prof. Thurston says
he has sometimes found it actually productive of increased strength;
the flow of particles from the rivet hole into the surrounding parts
causing stiffening and strengthening. With most steel and hard iron
plates the effect of punching is often to produce serious weakening
and a tendency to crack, which in some cases has resulted seriously.
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.