Lathes Page: 39
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(3) By having the center drill in the lathe headstock and holding
the work by hand.
b. When holding the work to be drilled in the lathe chuck, its
center should be spotted with a small hole by using a sharp pointed
c. The proper size of center drill should be selected in order to
make the center hole large enough to carry the load of machining,
but not so large as to be unsightly or interfere in any manner with
operations that follow. A table of center drill sizes for various
diameters is given below:
Diameter of combination
Diameter of drill and countersink
K 0.043 N
1 . 09375 Ao
1% . 128 %o
2 .157 M%
d. When finished the center hole should be smooth and have a pol-
ished appearance. If it is rough or improperly drilled it will wear,
causing the work to become loose and scoring the center. Figure 36
O Proper drilling. Drilling too deep. Drilling at incorrect angle.
FIGURE 36.-Correct and incorrect drilling of center holes.
shows properly and improperly drilled center holes.
e. When drilling the center hole in the drill press or lathe, the center
must be located by one of the methods shown in figure 37 and lightly
f. The four principal conditions which may cause the drill to break
when drilling center holes are as follows:
(1) Dull center drill.-The drill must be sharp and in good condi-
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United States. Army. Air Corps. Lathes, book, November 29, 1940; Washington, D.C.. (digital.library.unt.edu/ark:/67531/metadc96655/m1/41/: accessed April 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.