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