the headstock is usually referred to
as the "live center" because it turns around when the lathe is in use,
and the center in the tailstock as the "dead center," because it remains
stationary.

=Universal, Independent and Combination Chucks.=--Many parts that are
turned in the lathe are so shaped that they cannot be held between the
lathe centers like shafts and other similar pieces and it is often
necessary to hold them in a chuck _A_, Fig. 33, which is screwed onto
the lathe spindle instead of the faceplate. The work is gripped by the
jaws _J_ which can be moved in or out to accommodate various diameters.
There are three classes of chucks ordinarily used on the lathe, known as
the independent, universal and combination types. The independent chuck
is so named because each jaw can be adjusted in or out independently of
the others by turning the jaw screws S with a wrench. The jaws of the
universal chuck all move together and keep the same distance from the
center, and they can be adjusted by turning any one of the screws _S_,
whereas with the independent type the chuck wrench must be applied to
each jaw screw. The combination chuck, as the name implies, may be
changed to operate either as an independent or universal type. The
advantage of the universal chuck is that round and other parts of a
uniform shape are located in a central position for turning without any
adjustment. The independent type is, however, preferable in some
respects as it is usually stronger and adapted for holding odd-shaped
pieces because each jaw can be set to any required position.

[Illustration: Fig. 33. (A) Lathe Chuck. (B) Faceplate Jaw]

=Application of Chucks.=--As an example of chuck work, we shall assume
that the sides of disk _D_, Fig. 34, are to be turned flat and parallel
with each other and that an independent chuck is to be used. First the
chuck is screwed onto the lathe spindle after removing the faceplate.
The chuck jaws are then moved out or in, as the case may be, far enough
to receive the disk and each jaw is set about the same distance from the
center by the aid of concentric circles on the face of the chuck. The
jaws are then tightened while the disk is held back against them to
bring the rough inner surface in a vertical plane. If the work is quite
heavy, it can be held against the chuck, before the jaws are tightened,
by inserting a piece of wood between it and the tailstock center; the
latter is then run out far enough