ny parts can be held by simply
gripping them with one hand. From the foregoing it will be seen that the
small centers made by punch _C_, Fig. 25, serve as a starting point for
the drill and also as a support for the outer end of the work while the
first hole is being drilled.

[Illustration: Fig. 26. Drilling Centers in the Lathe]

The form of center-hole produced by a combination drill and reamer is
shown by the lower left-hand view in Fig. 27. A small straight hole a in
the bottom prevents the point of the lathe center from coming in contact
with the work and insures a good bearing on the conical surface _c_. The
standard angle for lathe centers is sixty degrees, as the illustration
shows, and the tapering part of all center-holes should be made to this
angle.

[Illustration: Fig. 27. Centers of Incorrect and Correct Form]

[Illustration: Fig. 28. Special Machine for Centering Parts to be
Turned]

=Centering Machine.=--Many shops have a special machine for forming
centers which enables the operation to be performed quickly. One type
of centering machine is shown in Fig. 28. The work is gripped in a chuck
_C_ that automatically locates it in a central position so that it is
not necessary to lay out the end before drilling. There are two spindles
_s_, one of which holds the drill and the other the countersink, and
these are rotated by a belt passing over pulley _P_. Each of these
spindles is advanced by lever _L_ and either of them can be moved to a
position central with the work, as they are mounted in a swiveling
frame. In operating this machine, a small straight hole is first made by
a twist drill held in one of the spindles; the other spindle is then
moved over to the center and the hole is reamed tapering. The
arrangement is such that neither spindle can be advanced by the feeding
lever except when in a central position. The amount that each spindle
can be advanced is limited by a fixed collar inside the head, and there
is also a swinging adjustable stop against which the end of the work
should be placed before tightening the chuck. These two features make it
possible to ream center holes of the same size or depth in any number of
pieces.

[Illustration: Fig. 29. The Imperfect Center Bearing is the Result of
Centering before Straightening]

=Different Forms of Centers.=--In some poorly equipped shops it is
necessary to form centers by the use of a center-punch only, as there is
no better tool. If the end o