rriage are worn unevenly, the tool
as it moves along does not remain in the same plane and this causes a
variation in the diameter of the part being turned.

=Application of Drivers or Dogs.=--Work that is turned between centers
is sometimes driven by a dog which is so short for the faceplate that
the bent driving end bears against the bottom _a_ of the faceplate slot,
as shown at _A_, Fig. 13. If the dog is nearly the right length, it may
allow the headstock center to enter the center in the work part way,
with the result that the turned surface is not true with the centers.
When a driving dog of this type is used, care should be taken to see
that it moves freely in the faceplate slot and does not bind against the
bottom. By using a straight dog (_B_), which is driven by a pin _b_
bolted to the faceplate, all danger from this source is eliminated. The
straight dog, however, is used more particularly to do away with the
leverage _l_ of a bent dog, as this leverage tends to spring a flexible
part when a cut is being taken.

[Illustration: Fig. 13. (A) Dog that is too Short for Faceplate. (B)
Straight Driving Dog]

Straight dogs are also made with two driving ends which engage pins on
opposite sides of the faceplate. This type is preferable because it
applies the power required for turning, evenly to the work, which still
further reduces the tendency to spring it out of shape. The principal
objection to the double-ended type lies in the difficulty of adjusting
the driving pins so that each bears with equal pressure against the dog.
The double-ended driver is often used for large work especially if deep
roughing cuts are necessary.

=Lathe Arbors or Mandrels.=--When it is necessary to turn the outside of
a part having a hole through it, centers cannot, of course, be drilled
in the ends and other means must be resorted to. We shall assume that
the bushing _B_, Fig. 14, has a finished hole through the center, and
it is desired to turn the outside cylindrical and concentric with the
hole. This could be done by forcing a tightly-fitted arbor _M_, having
accurately-centered ends, into the bushing and inserting the mandrel and
work between the lathe centers _h_ and _h_{1}_ as shown. Evidently, if
the arbor runs true on its centers, the hole in the bushing will also
run true and the outside can be turned the same as though the arbor and
bushing were a solid piece. From this it will be seen that an arbor
simply forms a tempora