arrangement, the carriage is gradually moved along as the
tailstock spindle is fed outward. Some reamers are provided with
stop-collars which come against the finished side of the casting when
the hole has been reamed to size.

After the reaming operation, the casting is removed from the chuck and a
taper mandrel is driven into the hole for turning the outside of the
piston. This mandrel should run true on its centers, as otherwise the
outside surface of the piston will not be true with the bored hole. The
driving dog, especially for large work of this kind, should be heavy and
stiff, because light flexible clamps or dogs vibrate and frequently
cause chattering. For such heavy work it is also preferable to drive at
two points on opposite sides of the faceplate, but the driving pins
should be carefully adjusted to secure a uniform bearing on both sides.

The foregoing method of machining a piston is one that would ordinarily
be followed when using a standard engine lathe, and it would, perhaps,
be as economical as any if only one piston were being made; but where
such work is done in large quantities, time could be saved by proceeding
in a different way. For example, the boring and reaming operation could
be performed much faster in a turret lathe, which is a type designed for
just such work, but a turret lathe cannot be used for as great a variety
of turning operations as a lathe of the regular type. There are also
many other classes of work that can be turned more quickly in special
types of machines, but as more or less time is required for arranging
these special machines and often special tools have to be made, the
ordinary lathe is frequently indispensable when only a few parts are
needed; in addition, it is better adapted to some turning operations
than any other machine.

Fig. 12 illustrates how a taper attachment would be used for turning the
taper fitting for the crosshead end of an engine piston-rod. Even though
this taper corresponds to the taper of the hole in the piston, slide _S_
would have to be reset to the corresponding division on the opposite
side of the central zero mark, because the taper of the hole decreased
in size during the boring operation, whereas the rod is smallest at the
beginning of the cut, so that the tool must move outward rather than
inward as it advances. The taper part is turned practically the same as
a cylindrical part; that is, the power feed is used and, as the carriage
moves