Lathes Page: 54
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33 AIR CORPS
will cause chatter marks on the work. The pressure exerted on the
file with the hands should be less than when filing at the bench.
Since there are less teeth in contact with the work, the file must be
cleaned frequently to avoid scratching. Since filing should be used
for little more than to remove lathe toolmarks from work, only
0.002 inch to 0.005 inch should be left for the filing operation. Figure
50 illustrates the correct method of filing on the lathe.
FIGURE 50.-Filing on lathe.
e. Polishing with either emery cloth or sandpaper is desirable to
improve the surface after filing. Emery cloth is best for the ferrous
metals, whereas sandpaper gives better results on nonferrous mate-
rials. The most effective speed for polishing with ordinary abrasives
is approximately 5,000 f. p. m. Most lathes are not capable of speeds
this high, particularly on small work, and it is therefore necessary to
select as high a speed as conditions will permit. In most cases the
cloth or paper is held directly in the hand and applied to the work,
although it may be held or tacked over a stick of wood and used in
the same manner as a file. Polishing clamps made of wood may also
be used on plain round work such as shafting, etc.
f. Since polishing will slightly reduce the dimensions of the work,
0.00025 inch to 0.0005 inch should be allowed for this operation. The
use of oil with the abrasive will give the work a dull satin finish,
while dry polishing leaves a bright surface. Crocus cloth may be
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/56/: accessed July 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.