Lathes Page: 52
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30-32 AIR CORPS
b. When making a blind tapered hole, such as may be required in
drill sockets, etc., it is best to drill the hole carefully to correct depth
with a drill of the same size as specified for the small end of the
hole. This gives the advantage of boring to the right size without
the removal of metal at the extreme bottom of the bore, which is
rather difficult, particularly in small, deep holes.
31. Knurling.-a. Knurling, as previously mentioned, is an oper-
ation of rolling depressions into the surface of the work by means of
small wheels or rollers with helical teeth milled in their faces. Most
knurling tools produce a. diamond pattern (fig. 48), and are provided
with interchangeable rollers for coarse, medium, and fine work.
b. For knurling, the lathe should be run at approximately one-half
the normal cutting speed with the knurling tool pressed hard against
the work. The longitudinal feed of the carriage may then be en-
gaged and the knurling tool fed across the surface. A coarse power
BACK RELIEF [ND RELIEF I5E RELIEF
FIGURE 40.-Parting tool positions.
feed is used, with plenty of oil when knurling any kind of material,
and three to five passes over the work should be made to complete the
32. Parting.-a. Parting is accomplished in the lathe with a
narrow blade tool bit held in a suitable holder. In this operation
particular care must be exercised in the grinding and setting of the
tool as well as in the adjustment of the machine.
b. The tool must. be ground so that the cutting end has about 5 of
relief, and better results will be obtained if the top of the cutter is
tapered slightly toward the bottom of the blade. The cutting edge
must be sharp, and the sides of the blade should be relieved about
10 to 20. A slight concavity carried back from the cutting edge on
the top of the tool will help the cutting action. This back rake may
be produced by holding the tool upright on the grinding wheel and
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/54/: accessed December 15, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.