Lathes Page: 12
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8 AIR CORPS
the number of feet, at the outside surface of the material, that pass
the tool bit point in 1 minute.
b. There are five factors that should be considered in regulating
the cutting speed:
(1) Hardness of metal being cut.-Hard metals require a lower
cutting speed than softer metals, because they are cut with a tool
having a blunt angle which will heat more rapidly than the sharper
edge used for softer metals. A lower speed is therefore necessary in
order that the tool bit may last for a reasonable length of time.
(2) Tool bit material.-High speed tool bits have the ability to
withstand high temperatures, and therefore higher speeds may be
used with this type of tool bit. The usual procedure, when using a.
carbon steel bit, is to reduce the speed to one-half of that used when
operating with the high speed tool.
(3) Feed and depth of cut.-Both feed and depth of cut are de-
termining factors in the selection of cutting speed, and any increase
in the depth of cut or coarseness of feed at a given speed will result
in a proportional increase in heat generated. Heat is one of the
main considerations in tool bit life.
(4) Diameter and length of work.-When the diameter of the
work is small and its length is great enough to set up vibrations due
to speed (thereby causing poor finish), it is necessary to reduce the
(5) Lubricant.-The cutting speed can be increased about 40 per-
cent if a large stream of lubricant is directed upon the chip at the
point of contact of the tool.
c. The cutting speeds in feet per minute (f. p. m.) given in the follow-
ing table are considered safe speeds when using a high speed tool bit.
It is proper to start machining operations at these speeds, observing
the effect on the tool bit and work. If the tool bit does not wear
satisfactorily the speed should be reduced.
Feet per Feet per
Kind of metal minute minute
Low carbon steel ..............._ --- ... 80 to 100 35
Medium carbon steel__ 60 to 80 ...... 25
High carbon tool steel -------------------------50 to 75 20
Bronze. ------------- 80 to 100 ... 25
Cast iron ----------------------------------- 50 to 80 25
Soft yellow brass--------------------------- 150 to 200____ 50
Aluminum ----- ---------- 200 to 300____ 50
Monel metal________________________-- ____ 100 to 120... 35
Stainless steel ---- ------------ 40 to 50 _ . 15
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/14/: accessed August 16, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.