Before cutting a thread between centers, it is ad-
visable to make sure that the centers themselves are
right. The point of a center should be exactly on the
axis of the center. The levels of the two centers must
be exactly alike. To test this, the screw cutting tool may
be set in the tool post and the carriage run to one center
and then to the other for the purpose of setting the tool
for height at one and of testing the other center for
agreement with this level. It may not be out of place
here to say a few words about centers. The work turns
about the tailstock center. It is advisable, then, to pre-
pare the hole in the work at the tail-stock and so that the
point of the center and the metal of the work will not
be in actual contact. This may be done by first prepar-
ing a conical hole to fit the center and then counter-
' boring it at the bottom with a small drill. This drill
hole, if deep enough, tends to prevent damage to the
center point either by wear or by friction. It is well to
counterbore the other end of the work also. Fig. 11 will
make this very clear.
