[Footnote 1: A paper read at a meeting of the Franklin Institute.
From the journal of the Institute.]
By WILLIAM BARNET LE VAN.
[Illustration: FIG. 11.]
Fig. 11 represents the spacing of rivets composed of steel plates
three-eighths inch thick, averaging 58,000 pounds tensile strength on
boiler fifty-four inches diameter, secured by iron rivets
seven-eighths inch diameter. Joints of these dimensions have been in
constant use for the last fourteen years, carrying 100 pounds per
square inch.
_Punching Rivet Holes._--Of all tools that take part in the
construction of boilers none are more important, or have more to do,
than the machine for punching rivet holes.
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. 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 0.0468
of an inch clearance.
The following formula is also used by punch and die makers: