Lathes Page: 75
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information as to the use of the thread-chasing dial for various threads,
and should be carefully observed before starting the threading (opera-
tion. Without the use of this dial, trouble will be experienced in
getting the tool point to retrace the path already cut. Some types of
lathes are not equipped with thread-chasing dials but have a reversing
lever to take its place. When cutting threads with this type of lathe,
it is only necessary to withdraw the tool at the end of the cut and
reverse the direction of motion of the lead screw.
h. Accurate measurement is necessary during all thread cutting
operations. Several methods used for measuring 60 threads are
outlined as follows:
(1) Thread calipers.-The calipers have the ends of their legs
dressed to fit into the thread form. They are set to a screw or bolt of
the size required and this size transferred to the work being done.
(2) Thread micrometers.-The micrometers are made with the spin-
dle pointed and the anvil notched so as to bear on the side walls of the
thread being measured. The point of the spindle is blunt to prevent
interference with the bottom of the thread groove, and the amount of
this bluntness of truncation limits the micrometer to the measurement
of a definite range of thread sizes. Thread micrometers are therefore
manufactured in different size ranges, such as 1 inch: 8 to 13 threads,
14 to 20 threads, 22 to 30 threads; 2 inches: 14 to 20 threads, 22 to 30
threads, etc. As the thread micrometer measures the pitch diameter
of the thread, the micrometer reading for a screw is obtained by sub-
A EQUALS THE MAJOR DIAMETER.
OFTHE STANDARD ScLRW MINUS
THE SINGLE DEPTH OF THREAD
EXAMPLE AND SOLUTION
TO FIND THE SINGLE DEPTH OFA 8 PITCH U.S.S. THREAD
CONSTANT .6495 .-- 0811
a To s TO FIND THE MICROMETER READING FORA 1"-8 U.SS.THREAD
V uS.S. 1'-.0811 "= .9189 = THREAD MICROMETER READING
FIGURE 68.-Measuring threads using thread micrometer.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/77/: accessed February 19, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.