the dogged or driving
end, the last part turned will be eccentric to the first. Therefore, the
lathe centers should be kept true in order to produce turned surfaces
that are true or concentric with the centered ends, as it is often
necessary to change the part being turned "end for end" for finishing,
and any eccentricity between the different surfaces would, in many
cases, spoil the work.

[Illustration: Fig. 32. Grinder for Truing Lathe Centers]

Some lathes are equipped with hardened centers in both the head-and
tailstock and others have only one hardened center which is in the
tailstock. The object in having a soft or unhardened headstock center is
to permit its being trued by turning, but as a soft center is quite
easily bruised and requires truing oftener than one that is hard, it is
better to have both centers hardened. Special grinders are used for
truing these hardened centers. One type that is very simple and easily
applied to a lathe is shown in Fig. 32. This grinder is held in the
lathe toolpost and is driven by a wheel _A_ that is held in contact with
the cone-pulley. The emery wheel _B_ is moved to a position for grinding
by adjusting the carriage and cross-slide, and it is traversed across
the conical surface of the center by handle _C_. As the grinding
proceeds, the wheel is fed inward slightly by manipulating the
cross-slide.

This grinder is set to the proper angle by placing the two centered ends
_D_ and _D_{1}_ between the lathe centers, which should be aligned as
for straight turning. The grinding spindle will then be 30 degrees from
the axis of the lathe spindle. The grinder should be carefully clamped
in the toolpost so that it will remain as located by the centered ends.
After the tailstock center is withdrawn, the emery wheel is adjusted for
grinding. As the wheel spindle is 30 degrees from the axis of the lathe
spindle, the lathe center is not only ground true but to an included
angle of 60 degrees, which is the standard angle for lathe centers.
There are many other styles of center grinders on the market, some of
which are driven by a small belt from the cone-pulley and others by
electric motors which are connected with ordinary lighting circuits. The
tailstock center is ground by inserting it in the spindle in place of
the headstock center. Before a center is replaced in its spindle, the
hole should be perfectly clean as even a small particle of dirt may
affect the alignment. The center in