Lathes Page: 76
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43 AIR CORPS
tracting the single depth of the thread from its outside diameter.
(See fig. 68.)
(3) Saybolt or three wire system.-This method allows the use of
an ordinary micrometer, and measurement is accomplished by placing
three wires (all of the same size) into the thread groove (fig. 69). The
FIGURE G9.-Measurement of threads by three-wire system.
size of the wires should be such that they will bear on the side walls of
the thread at the pitch diameter when the thread is finished. Know-
ing the outside diameter of the screw, the wire sizes, and pitch of the
thread, the measurement over the wires can then be computed by
means of the following formula:
D= OD-1.515 P+3 W
Where: D= Measurement over the wires.
OD= Outside diameter of screws being measured.
W= Wire diameter, "
W= Wire eter Number of threads
P= Pitch of thread being cut.
Example: Three wire calculation for 3/4 inch by 10 threads.
D = 0.750- (1.515 x .10) + (3 x .057)
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/78/: accessed April 24, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.