Lathes Page: 62
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38 AIR CORPS
permit and clamped as short as possible to prevent unnecessary
b. For straight boring, best results may be obtained if the cutting
point of the tool bit is set 5 above center, although in small holes
where the bar has very little room this angle must usually be reduced
in order to keep the bar from rubbing in the bore. Care should be
exercised in setting the tool bit and boring bar so that the top of the
bit will have a positive rake of approximately 10, measured tan-
gential to the radius of the bore. This position (fig, 56) gives a
WORK HELD IN CNUCiK
y B BORING TOOL
FIGURE 56.-Boring tool position,
freer cutting action than would be possible if the tool and bar were
set to a point where the tool bit topl would have a negative rake
in relation to the tangent.
(1) When boring tapered holes, the tool bit must be set at center
height with the same amount of rake as used for straight boring.
(2) By reducing the speed to approximately one-half the normal
cutting speed, a much better finish may be obtained.
(3) Due to the spring of the tool, there is a general tenldenicy to
bore bell-mouthed holes (larger at the beginning than at, the end)'.
In order to prevent this error, a light finishing cut with a sharp tool
should be taken with the tool feeding into the bore. At the cor1-
pletion of this cut, the feed should be reversed and the tool allowed
to travel out with the same setting,
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/64/: accessed May 22, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.