Lathes Page: 72
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43 AIR CORPS
engaged, and by moving cross feed and compound rest cranks bring
point of tool bit into groove.
(8) Using a piece of white paper underneath work to provide good
visibility, position tool in such a manner that it clears front side of
groove between 0.005 inch and 0.010 inch when it bears on thread back
(tool bit and work must be thoroughly cleaned before making this
(9) Reset cross feed dial to zero without disturbing tool and re-
check to make sure that tool will return to same setting. Using
fine cuts, bring thread to its finished size.
c. A final threading operation that should be used on all precision
work consists of finishing the front and back sides of thread in the
(1) To finish the back side of the thread, the tool is advanced for
each cut by the cross feed screw. Several cuts will be necessary to
finish one side of the thread, and even though the marks of the
roughing tool have been removed, cuts must be taken until the point
of the tool reaches the truncation left by the roughing operation.
If the side of the thread does not have the proper finish at that time,
the tool will have to be reset and the operation repeated. Any addi-
tional cuts attempted after the bottom of the thread is reached will
interfere at. the front side of the V, tearing the surface.
(2) Next in order will be the finishing of the front side of the
thread. This is accomplished with the compound rest at 300, and
the tool set with the gage contacting the front side of the bit. The
tool may then be shifted to catch the thread, leaving a clearance of
0.002 inch to 0.005 inch at the back of the V. The cross feed dial is reset
to zero and the tool fed into the front side of the thread by means
of the compound rest. Since the tool is now advanced by the com-
pound, as many cuts as are necessary may be taken to bring the
thread to size without danger of tearing the metal.
(3) For best results the last few finish cuts should be very thin
(.0005 inch or less, if possible). A cut of this depth will produce a
mirror-like finish, provided all other conditions are right. The tool
must at all times take a positive chip from the work. If, due to dull-
ness or too light a cut, the tool rides over the work, the surface will be
hardened to some extent, and on the next pass the tool may take a heavy
cut and gouge the work.
(4) For very fine threads and for threads requiring only an ordinary
degree of finish, the finishing tool may be omitted and the thread
finished to size with the roughing tool.
d. Internal threading is accomplished in the same manner as the
cutting of external threads with the following exceptions:
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/74/: accessed December 14, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.