Before this piece can be finished on the outside, it must be forced on
a mandrel, or arbor, and placed in the lathe, as shown in Fig. 30. Most
commercial shops are provided with hardened steel mandrels for this
purpose, but if one is not available it can be made from soft steel in
the following manner:
[Illustration: Fig. 30]
=Making Mandrel.=—Cut off a piece of steel of suitable length, say 6
inches, and rough turn it to within ¹⁄₃₂ of an inch of the diameter of
the hole. Then turn it at the end for a distance of about ⅛ of an inch
to the size that will just fit the hole. The rest of the distance is
now turned .002, or .003 of an inch larger and filed for about 3 inches
until it will just fit the hole. The next 2 inches are filed with a
slight taper so that when the mandrel is pressed into the hole it will
fit tight enough to hold the casting while it is being turned. This
kind of a fit is called a forced, or driving fit.
When making such a mandrel, it is not necessary to turn that portion to
which the dog is fastened.
=Mounting Work on Mandrel.=—Before pressing the mandrel in, it should
be oiled to prevent it from being marred, or scored. Mandrels are
usually forced in with a mandrel press, but if one is not available,
they may be driven in with a hammer. When this method is used, a piece
of lead, or some other soft material, must be held on the end of the
mandrel to keep the hammer from marring the center.