Lathes Page: 70
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43 AIR CORPS
two tools, one for roughing out the thread and the other for finishing.
The roughing tool should be made with top rake, while the finishing
tool should be flat at the top so that this surface is horizontal in both
directions when set to the work (fig. 62) Particular attention must
be given to the end and side relief, especi illy that on the leading side
of the tool bit. Since the thread is cut around the shaft at an angle,
the leading edge of the tool bit must have more relief than the trail-
ing edge to prevent interference.
(2) The 60 center gage (fig. 63) is used for gaging the angle of the
tool bit for cutting the U. S. Standard, National coarse and fine,
metric, sharp V and all other threads of this angle. Since a tool
will tend to cut a groove slightly larger than its own angle, it will
be found that if the tool angle is made slightly less than the thread
angle desired the finished groove will be more accurate than if the
tool were made to correspond exactly to the gage. It is therefore
necessary to make the roughing tool angle 1/ to 1/40 less than the
HARD W o NANO
ANGLE OT r-EPAD 1 -3 AN0- EQF hEAb
FIGURE 63.-Grinding 00 threading tool.
gage angle, while the finishing tool bit angle should be 3 to 40 less
than the gage angle.
(3) The walls of all 600 threads are straight lines, and since the
tool bit edges are used to make these thread walls they must be straight
also. Should a tool bit be used with irregular sides, this irregularity
will be transmitted to the thread, seriously reducing its efficiency.
(4) In order that the truncation at the bottom of the finished
thread will not interfere with its mating member, the point of the tool
bit should be truncated 1/ to 2/3 the amount required by the thread.
(5) Since the compound rest is set at an angle of 30 from parallel
to the ways of the lathe and the feed for each cut is made with the
compound, the tool will cut only on one side of the thread groove as
it is advanced for each successive cut. This enables the tool to have
side rake, allowing faster cutting and producing better finishes.
Furthermore, if a tool is fed so that both sides cut at the same time,
there may be a gouging or tearing action which must be avoided as
much as possible in threading. It will also be best to decrease the
amount of feed as the tool goes deeper into the work to reduce further
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/72/: accessed February 26, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.