the
screw feed and the regular turning feed cannot be engaged at the same
time. As previously mentioned, lead-screw _S_, Fig. 1, is rotated from
the lathe spindle, through gears _a_, _b_ and _c_, called change gears.
An assortment of these gears, of various sizes, is provided with the
lathe, for cutting screws of different pitch. The gears to use for any
pitch within the range of the lathe are given on the plate _I_.

=Example of Cylindrical Turning.=--Having now considered the principal
features of what might be called a standard lathe, the method of using
it in the production of machine parts will be explained. To begin with a
simple example of work, suppose a steel shaft is to be turned to a
diameter of 2-1/4 inches and a length of 14-1/2 inches, these being the
finished dimensions. We will assume that the rough stock is cut off to a
length of 14-5/8 inches and has a diameter of 2-5/8 inches. The first
step in this operation is to form conically shaped center-holes in each
end of the piece as indicated at _c_ in Fig. 5. As all work of this kind
is held, while being turned, between the centers _h_ and _h_{1}_, holes
corresponding in shape to these centers are necessary to keep the work
in place. There are several methods of forming these center-holes, as
explained later.

After the work is centered, a dog _A_ is clamped to one end by
tightening screw _s_; it is then placed between the centers of the
lathe. The dog has a projecting end or "tail," as it is commonly called,
which enters a slot in the faceplate _F_ and thereby drives or rotates
the work, when power is applied to the lathe spindle onto which the
faceplate is screwed. The tailstock center _h_1_, after being oiled,
should be set up just tight enough to eliminate all play, without
interfering with a free rotary movement of the work. This is done by
turning handle _n_, and when the center is properly adjusted, the
tailstock spindle containing the center is locked by tightening handle
_p_. (Ordinary machine oil is commonly used for lubricating lathe
centers, but a lubricant having more "body" should be used, especially
when turning heavy parts. The following mixtures are recommended: 1. Dry
or powdered red lead mixed with a good grade of mineral oil to the
consistency of cream. 2. White lead mixed with sperm oil with enough
graphite added to give the mixture a dark lead color.)

[Illustration: Fig. 5. Plan View showing Work Mounted between Centers of
Lathe]