Lathes Page: 73
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(1) An undercut is necessary. The major diameter of the thread
desired should be placed where the thread runs out for easy backing-off
of the tool.
(2) The tool must be set in a reverse position.
FIGURE: 65.-Setting internal threading tool.
(3) The tool is advanced by moving the compound toward the
(4) Lighter cuts are taken than for external threading due to the
spring of the internal tool.
(5) More end relief is required to prevent the heel of the tool from
rubbing than when cutting external threads.
(6) Sufficient clearance must be allowed between the tool and the
inside diameter of the hole to permit the backing out of the tool when
the end of the cut has been reached.
(7) It is necessary to calculate the minor diameter of the thread by
means of the following formula:
Minor diameter= major diameter- . 1.026
number of threads
e. It is always best to check the pitch of an external or internal
thread after the first light cut has been taken over the work. This
may be done by using a screw pitch gage or by counting the number of
threads per inch with a scale or rule (fig. 66).
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/75/: accessed October 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.