To machine the inside of piece A it is necessary to hold it in the
lathe by means of an independent four-jaw chuck, as shown in Fig. 21.
Work of this kind is usually chucked so that the outside surfaces will
be within ¹⁄₃₂ of an inch of running true.
[Illustration: Fig. 21]
The process of chucking the work is as follows:
=Centering Work in the Chuck.=—Place the work in the chuck and adjust
the jaws until they are all at approximately equal distances from the
circles on the face of the chuck. Then put a cutting-off tool loosely
in the tool post and move it close to the work and as near as possible
to the end of the chuck jaws. Revolve the lathe by hand to prove if
the work is centered. If it is not centered to within ¹⁄₃₂ of an inch,
readjust the jaws until it is. Now move the cutting-off tool to the end
of the work and turn the lathe by hand. If the end runs out of true,
rap it with a hammer at such points as will correct its position.
=Advantage of Proper Chucking.=—Fig. 21 shows the work held by the
middle step of the cone. One reason for holding it in this way is to
permit the rough turning of the larger step while in the chuck. If the
work were held by the small end, it would be apt to work loose when
taking the heavy roughing cuts on account of the distance that the work
projects out and the small diameter on which the chuck grips compared
with that of the large end which is to be turned.