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
plate punched, or should do so in all carefully executed work, for it
is easy to understand that the die which might give a suitable taper
in a three-fourths inch plate would give too great a taper in a
three-eighths inch plate. There is no fixed rule; practical experience
determines this in a rough and ready way--often a very rough way,
indeed, for if a machine has to punch different thicknesses of plate
for the same size of rivets, the workman will seldom take the trouble
to change the die with every variation of thickness. The maker of
punches and dies generally allows about three sixty-fourths or 0.
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