turning attachment is shown in Fig. 30. This is used
for machining the ends of gasoline engine pistons. The cross-slide has
bolted to it a bar _A_ carrying a roller which is pressed against a
forming plate _B_ by a heavy spring _C_. The forming plate _B_, which is
attached to a cross-piece fastened to the ways of the lathe bed, is
curved to correspond with the radius required on the piston end, and
when the tool is fed laterally by moving the cross-slide, it follows the
curve of plate _B_. The piston is held in a special hollow chuck which
locates it in a central position and holds it rigidly.

In connection with lathe work, special attachments and tools are often
used, especially when considerable work of one class must be turned;
however, if a certain part is required in large quantities, it is
usually more economical to use some semi-automatic or automatic turning
machine, especially designed for repetition work.

=Turning with Front and Rear Tools.=--In ordinary engine lathe practice,
one tool is used at a time, but some lathes are equipped with
tool-holders at the front and rear of the carriage so that two tools can
be used simultaneously. Fig. 31 shows a detail view of a lathe in which
front and rear tools are being used. These tools are of the inserted
cutter type and the one at the rear is inverted, as the rotary movement
of the work is, of course, upward on the rear side. This particular
lathe was designed for taking heavy roughing cuts and has considerable
driving power.

[Illustration: Fig. 31. Front and Rear Tools used for Roughing]

The part shown in this illustration is a chrome-nickel steel bar which
is being roughed out to form a milling machine spindle. It is necessary
to reduce the diameter of the bar from 5-7/16 inches to 3-3/4 inches for
a length of 27 inches, because of a collar on one end. This reduction is
made in one passage of the two tools, with a feed of 1/32 inch per
revolution and a speed of 60 revolutions per minute. The use of two
tools for such heavy roughing cuts is desirable, especially when the
parts are required in large quantities, because the thrust of the cut on
one side, which tends to deflect the work, is counteracted by the thrust
on the opposite side.

[Illustration: Fig. 32. Lo-swing Lathe for Multiple Turning]

Sometimes special tool-holders are made for the lathe, so that more than
one tool can be used for turning different surfaces or diameters at the
same time, the