LATHE OPERATIONS - OD1645 - LESSON 1/TASK 1
changing gears to produce various rates of feed.
The rotating feed rod
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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