142. Tube drawing. The art of forming tubes of uniform
diameter is nearly similar in its mode of execution to wire
drawing. The sheet brass is bent round and soldered so as to form
a hollow cylinder; and if the diameter outside is that which is
required to be uniform, it is drawn through a succession of
holes, as in wire drawing: If the inside diameter is to be
uniform, a succession of steel cylinders, called triblets, are
drawn through the brass tube. In making tubes for telescopes, it
is necessary that both the inside and outside should be uniform.
A steel triblet, therefore, is first passed into the tube, which
is then drawn through a succession of holes, until the outside
diameter is reduced to the required size. The metal of which the
tube is formed is condensed between these holes and the steel
cylinder within; and when the latter is withdrawn the internal
surface appears polished. The brass tube is considerably extended
by this process, sometimes even to double its first length.
143. Leaden pipes. Leaden pipes for the conveyance of water
were formerly made by casting; but it has been found that they
can be made both cheaper and better by drawing them through holes
in the manner last described.
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