Lathes Page: 44
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25-27 AIR CORPS
1. Polish centers with fine emery cloth after they are ground, using
a high spindle speed. The dead or tailstock center should be ground
first and the live or headstock center last. This precaution will allow
the live center to be left in place after grinding, giving it more
26. Alining centers.--a. In order to turn a shaft straight and
true between centers, it is necessary that the centers be in a plane
parallel to the ways of the lathe. The tailstock may be moved later-
ally to accomplish this alinement by means of two adjusting screws
after it has been released from the ways. At the rear of the tailstock
on the cricket are two zero lines, and the centers are approximately
alined when these lines coincide as shown in figure 40 Q. This
approximate alinement may be checked by moving the tailstock up
until the centers almost touch, and observing their relative positions
as shown in figure 40 Q.
b. Positive alinement of centers may be obtained by placing a test
bar between centers and bringing both ends of the bar to a zero read-
ing by the use of an indicator clamped in the tool post as shown in
figure 40 (. It is necessary to have the tailstock clamped to the
ways and the test bar properly adjusted between centers when taking
the indicator readings.
c. Another method commonly employed by the mechanic for posi-
tive alinement is to take a light cut over the work held between
centers. The work is then measured at each end with a micrometer
and if the readings are found to differ the tailstock is adjusted
accordingly. The procedure is then repeated until alinement is
27. Straight turning.-a. Straight turning may be performed
upon work held in the chuck; however, the majority of work turned
in the lathe is held between the lathe centers.
b. Centers must be in good condition and allowance should be
made for expansion in the work so that it will not bind. When work
is mounted and revolved between centers, there will be friction where
the tailstock center bears in the work center hole. This bearing point
must be lubricated at intervals, as the pressure of the tool bit against
the work sets up considerable friction at this point. The ordinary
machine oils do not have enough body to resist this pressure success-
fully, allowing metal-to-metal contact which will soon destroy the
accuracy of the center and center hole. A good center lubricant can
be made of white lead and machine oil mixed to about the consistency
of thick cream.
c. For straight turning, the cutting edge of the tool bit should be
slightly above center (fig. 41), as this position gives the tool bit a better
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/46/: accessed November 18, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.