The screws used by carpenters, smiths, and others, for fastening wood
or metal-work together, are generally made by machinery. A piece of
wire of the required thickness and length is first cut off, which is
then placed in a hole or mould, with its upper orifice “countersunk”
as in the annexed cut, and the head of the screw is formed by beating
the upper part of the piece of wire into the counter-sinking of the
mould. It is then called a “blank,” and a number of these blanks
are dropped into holes bored round the periphery of a wheel which
revolves slowly against a small circular saw driven with great
rapidity, and cutting a “thread” or slit in the head of each blank
in turn as it comes into contact with it; the wheel continuing
to revolve, each screw when it gets to the bottom drops out, so
that the holes when they again arrive upwards are ready to receive
fresh blanks. The worm, if cut by hand, is made by means of a die
of hard steel, having a hollow screw cut in it, which is screwed
slowly by a backward and forward movement on to the blank, cutting
it into a “worm;” when this is done by machinery, the die is held
fast while the screw is worked by a rotatory movement into it. The
best screws for wood are slightly tapered, which enables them to be
driven much more easily, while they hold equally fast. Large screws
for mechanical purposes are cut by a “lathe,” the bar revolving
whilst a cutter is held to it, and moved onwards by a pattern screw.
Small screws for various purposes can be easily cut by means of a
“screw-plate,” having holes of graduated sizes cut inside into hollow
screws, which is placed on the end of the wire, and gently turned
round till it is sufficiently cut.