Machinery Repairman

FEED ROD
The feed rod transmits power to the apron to drive
the longitudinal feed and cross feed mechanisms. The
feed rod is driven by the spindle through a train of gears,
and the ratio of its speed to that of the spindle can be
varied by changing gears to produce various rates of
feed The rotating feed rod drives gears in the apron.
These gears in turn drive the longitudinal feed and
cross-feed mechanisms through friction clutches, as
explained in the description of the apron.
Lathes that do not have a separate feed rod have a
spline in the lead screw to serve the same purpose. The
apron shown in figure 6-9 belongs to a lathe of this type
and shows clearly how the worm that drives the feed
mechanism is driven by the spline in the lead screw. If a
separate feed rod were used, it would drive the feed
worm in the same manner, that is, by means of a spline.
The spline permits the worm, which is keyed to it, to
slide freely along its length to conform with the
movement of the carriage apron.
LEAD SCREW
The lead screw is used for thread cutting. Along its
length are accurately cut Acme threads that engage the
Figure 6-10.—Quick-change gear box (rear view).
6-8
threads of the half-nuts in the apron when half-nuts
are clamped over it. When the lead screw turns in the
closed half-nuts, the carriage moves along the ways a
distance equal to the lead of the thread in each
revolution of the lead screw. Since the lead screw is
connected to the spindle through a gear train
(discussed later in the section on quick-change gear
mechanism), the lead screw rotates with the spindle.
Therefore, whenever the half-nuts are engaged, the
longitudinal movement of the carriage is directly
controlled by the spindle rotation. The cutting tool is
moved a definite distance along the work for each
revolution that the spindle makes.
The ratio of the threads per inch of the thread being
cut and the thread of the lead screw is the same as the
ratio of the speeds of the spindle and the lead screw. For
example: If the lead screw and spindle turn at the same
speed, the number of threads per inch being cut is the
same as the number of threads per inch of the lead
screw. If the spindle turns twice as fast as the lead screw,
the number of threads being cut is twice the number of
threads per inch of the lead screw.
You can cut any number of threads by merely
changing gears in the connecting gear train to get the
desired ratio of spindle and lead screw speeds.