Lathes Page: 8
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6-7 AIR CORPS
be cooled frequently by dipping it in water. It is also necessary to see
that the wheel is dressed properly, the grinding pressure is not too
heavy, and the tool bit is not held against the wheel too long without
being cooled. The relief and rake surfaces of the finished tool should
be clean and smooth and should not have a number of facets. Con-
siderable practice is necessary before this can be accomplished by the
average person. The cutting edge should be sharp and keen and can
be greatly improved by hand stoning the tool after grinding, with a
hard, medium grain stone. This is not a remedy for a tool that has
become dulled in use (in which case it should be reground), but is for
the purpose of improving its condition after it has been ground on the
h. The majority of tool bits used at this time are of high speed steel
and are considerably more efficient than the carbon steel tools. They
are capable of retaining their hardness at higher temperatures and
therefore can be used at higher speeds. In view of the numerous steels
used in the manufacture of cutting and turning tools, it is difficult to
give a definite rule for their selection. Steels for this purpose may be
generally classed as carbon steel and high speed steel and are marketed
under various trade names, such as Rex A, Rex AA, Rex AAA, Super
Dreadnaught, Dreadnaught, etc., in the high speed steel class, and
Ketos, Black Diamond, Crescent, Sanderson, etc., in the carbon steel
class. Generally, high speed steels are an alloy of tungsten with vari-
ous proportions of chromium and vanadium. When selecting steel for
the manufacture of tools, the cutting speed must be considered. For
low speed turning and for hand tools, such as taps, readers, chisels,
and punches, carbon steel gives satisfactory results and is more eco-
nomical. For high speeds the high speed steels should be used.
7. Tool holders.-There has been a gradual decline in the use of
hand forged tools for lathe work. These have been replaced by tool
bit holders of various types. Many varieties of holders are on the
market and care should be used in their selection, so that proper tool
efficiency may be obtained. Tool bits are available in different sizes to
fit the various holders and are furnished hardened and cut to standard
lengths. Tool holders for the average line of lathe work are described
a. The straight, right, or left hand turning tool holders (fig. 4) are
used for the majority of external turning and machine ol)erations.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/10/: accessed December 13, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.