Lathes Page: 82
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47-48 AIR CORPS
(4) By counting the number of teeth in the gear on the sector arm
at the head of the lathe. After one pitch is cut, the gear is disengaged
and turned the number of teeth necessary to bring the tool point in
place for the next pitch. This method is only possible when the num-
ber of teeth on the gear is evenly divisible by the thread to be cut.
48. Left-hand threads.-A left-hand screw thread is one that is
advanced by counterclockwise rotation with respect to the head of
the screw. Left-hand threads are cut in much the same way as the
right-hand type except in the reverse order. The proper procedure
is outlined as follows:
a. Reverse direction of rotation of lead screw causing carriage to
move toward tailstock.
FIGURE 75.-Undercut for left hand threads.
b. Set compound rest in reverse direction to that used for right-
c. Relieve tool bit for left-hand threads.
d. Undercut stock to minor diameter (fig. 75). This undercut
should be of a width equal to the pitch of the thread and is provided
for easy starting of the tool bit.
e. Proceed as in cutting right-hand threads.
TYPES OF FITS
Types of fits ------------ 49
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/84/: accessed April 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.