Lathes Page: 77
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(4) Optical comparator.-By this method an enlarged outline of
the thread being cut is projected on a screen for visual comparison
with a perfect thread outline of the same size.
44. Pipe threads.-Pipe threads are cut in a manner similar to
ordinary 60 threads, with the exception that a taper attachment
must be used to give the thread a taper of 3/4 inch per foot (fig. 70).
TAPER AT TACHMENT
FIGURn 70.-C(utting tapered pipe thread.
Pipe threads are measured by means of a gage, aid their specifications
may be calculated by the following formulas:
Pipe thread truncation = 0.033 x P
Length of pipe thread=0.80 OD+6.8P
45. Square threads.-a. Square threads are used for feed
screws, jack screws, vise screws, etc.
b. For square threading, the compound rest is set )arallel to the
axis of the machine. A tool bit, ground from 0.002 inch to 0.008
inch undersize, is used for the roughing cut while finishing is ac-
complished with tool bits having left and right rake. The front
and back sides of the thread are finished by moving the comiipound
rest parallel to the axis. (See fig. 71.)
c. Both the depth and the width of all standard square threads are
equal to one-half the pitch (fig. 72 0). Square threads are measured
by means of a micrometer and single wire or drill of the proper
size (fig. 72 ).
46. Acme threads.-- . Acme threads are used for adjusting
screws and feed screws on various machines. These threads have
an included angle of 29 (fig. 73 ().
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/79/: accessed June 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.