Of copying by punching
133. This mode of copying consists in driving a steel punch
through the substance to be cut, either by a blow or by pressure.
In some cases the object is to copy the aperture, and the
substance separated from the plate is rejected; in other cases
the small pieces cut out are the objects of the workman's labour.
134. Punching iron plate for boilers. The steel punch used
for this purpose is from three-eighths to three-quarters of an
inch in diameter, and drives out a circular disk from a plate of
iron from one-quarter to five eighths of an inch thick.
135. Punching tinned iron. The ornamental patterns of open
work which decorate the tinned and japanned wares in general use,
are rarely punched by the workman who makes them. In London the
art of punching out these patterns in screw-presses is carried on
as a separate trade; and large quantities of sheet tin are thus
perforated for cullenders, wine-strainers, borders of waiters,
and other similar purposes. The perfection and the precision to
which the art has been carried are remarkable. Sheets of copper,
too, are punched with small holes about the hundredth of an inch
in diameter, in such multitudes that more of the sheet metal is
removed than remains behind; and plates of tin have been
perforated with above three thousand holes in each square inch.
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