Lathes Page: 6
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6 AIR CORPS
tenacious metals. This wedge will show the best results when its
angle is approximately 61 for the machining of soft steels. For the
harder materials this angle is increased to give the cutting edge
b. In order for the tool bit to cut effectively, the side and end are
ground so as to leave a projecting edge. This is known as side and
end relief. If the amount of relief is not sufficient, the tool bit has a
tendency to rub against the work, causing excessive heat and leaving
a surface of rough appearance. If the amount of relief is too great,
the cutting edge will be weak and will break due to insufficient
c. The top of the tool bit should slope away from the cutting edge
so that the tool bit may be forced into the metal with less resistance,
and the chip of metal being removed will not be so greatly distorted,
thus reducing the power required. This sloping of the top of the
tool bit is known as rake. Figure 2 illustrates the various angles
ground on a tool bit and gives the proper nomenclature involved.
d. A table of tool bit angles for various materials is given below:
Material Side relief Front relief Back rake Side rake
Degrees Degrees Degrees Degrees
Low carbon steel _ 12 8 16;2 18
Medium carbon steel ... . 10 8 12 14
High carbon steel_ 10 8 8 12
Cast iron ___________________ 10 8 5 12
Stainless steel ______________- _ 12 10 16% 10
Copper 14 12 16;2 20
Bronze_ 10 8 0 0
Brass_ 10 8 0 0
Aluminum ____________--_ 12 8 35 15
Monel_ ------------------------ 15 13 8 14
Silicon bronze ---------------- 10 10 10 6
e. For the simple removal of metal it is best to have a large radius at
the cutting edge of the tool bit. This shape distributes the cutting
pressures and also gives more surface through which to radiate heat.
It will wear better and last longer than a tool that is pointed or has
sharp corners, although it is sometimes necessary to use a tool that
is pointed for machining operations, such as squaring a shoulder, cut-
ting a thread, etc. When using this type of tool care must be exer-
cised due to its inherent weakness.
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/8/: accessed December 12, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.