left. (This decarbonization is
probably due to the action of the oxygen of the air on the bar during
the process of manufacture.) If stock for a reamer is so centered that
the tool removes the decarbonized surface only on one side, as
illustrated to the right, evidently when the reamer is finished and
hardened the teeth on the side _A_ will be harder than those on the
opposite side, which would not have been the case if the rough bar had
been centered true. To avoid any trouble of this kind, stock that is to
be used for hardened tools should be enough larger than the finished
diameter and so centered that this decarbonized surface will be entirely
removed in turning.

[Illustration: Fig. 31. Three Methods of Facing the Ends Square]

=Facing the Ends of Centered Stock.=--As a piece of work is not properly
centered until the ends are faced square, we will consider this
operation in connection with centering. Some machinists prefer lathe
centers that are cut away as shown at _A_, Fig. 31, so that the point of
the side tool can be fed in far enough to face the end right up to the
center hole. Others, instead of using a special center, simply loosen
the regular one slightly and then, with the tool in a position as at
_B_, face the projecting teat by feeding both tool and center inward as
shown by the arrow. Whenever this method is employed, care should be
taken to remove any chips from the center hole which may have entered. A
method which makes it unnecessary to loosen the regular center, or to
use a special one, is to provide clearance for the tool-point by
grinding it to an angle of approximately forty-five degrees, as shown at
_C_. If the tool is not set too high, it can then be fed right up to the
lathe center and the end squared without difficulty. As for the special
center _A_, the use of special tools and appliances should always be
avoided unless they effect a saving in time or their use makes it
possible to accomplish the same end with less work.

=Truing Lathe Centers.=--The lathe centers should receive careful
attention especially when accurate work must be turned. If the headstock
center does not run true as it revolves with the work, a round surface
may be turned, but if the position of the driving dog with reference to
the faceplate is changed, the turned surface will not run true because
the turned surface is not true with the work centers. Furthermore, if
it is necessary to reverse the work for finishing