Lathes Page: 79
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0 Form. Measurement.
FIGURE 72.-Square threads.
d. Both Acme and square threads are usually cut with a relatively
coarse pitch. Due to the large helix angle produced by this wide
thread spacing, careful attention must be given to the relief of the
tool bit. In order to check accurately the amount of relief necessary
to prevent binding of the bit in the groove, a gage may be made in
the following manner:
(1) Using a small rectangular piece of sheet metal, scribe a line
at 90 to one of its sides and on it lay off a distance equal to the
circumference of the thread to be cut at its minor diameter.
(2) At one end of this line and at right angles to it, lay off a
distance equal to the pitch of the thread to be cut.
(3) Connect the ends of these two lines, and the angle of this
third side is the angle of the required thread at its minor diameter.
(4) Cut off the sheet metal along this last line and gage the tool
bit (fig. 74). The tool bit is considered correctly relieved when its
side clears the gage from 30 to 6.
47. Multiple threads.-a. It is sometimes necessary to cut a
screw that will travel a considerable distance in one revolution. If
a single pitch coarse thread were cut on the screw, the minor diam-
eter of the thread would be so deep that the screw would be seri-
ously weakened. In a case of this kind, the multiple-pitch thread
may be used to good advantage. As an example in multiple thread
cutting let it be assumed that an eight-pitch double thread is desired.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/81/: accessed March 30, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.