(8) Lead Screw.

drives the gears in the apron. These gears in turn drive the longitudinal

feed and crossfeed mechanisms through friction clutches, as previously

explained in (6)(c) on page 14.

(b) Lathes which do not have a separate feed rod have a spline in the

lead screw to serve the same purpose. The apron belongs to a lathe of this

type and is driven by the spline in the lead screw. If a separate feed rod

was used, it would drive the feed worm in the same manner.

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.

(8) Lead Screw.

(a) The lead screw is used for thread cutting.

Along its length are

accurately cut Acme threads which engage the threads of the half-nuts in the

apron when the half-nuts are clamped over it.

When the lead screw turns

inside 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 (to be

discussed in paragraph (9)(a) below), the lead screw rotates with the

spindle. 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 of the

spindle.

(b) 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.

(c) Any number of threads can be cut by merely changing the gears in the

connecting gear train to obtain the desired ratio of the spindle and the

lead screw speeds.

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