, let
us assume that the hole through the cast-iron disk, Fig. 39, has been
cored considerably out of center, as shown. If the work is set by the
outside surface _S_, as it would be ordinarily, the hole is so much out
of center that it will not be true when bored to the finished size, as
indicated by the dotted lines. On the other hand, if the rough hole is
set true, the outside cannot be finished all over, without making the
diameter too small, when it is finally turned. In such a case, the
casting should be shifted, as shown by the arrow, to divide the error
between the two surfaces, both of which can then be turned as shown by
the dotted lines in the view to the right. This principle of dividing
the error when setting work can often be applied in connection with
turning and boring. After a casting or other part has been set true by
the most important surface, all other surfaces which require machining
should be tested to make sure that they all can be finished to the
proper size.

=Inaccuracy from Pressure of Chuck Jaws.=--Work that is held in a chuck
is sometimes sprung out of shape by the pressure of the chuck jaws so
that when the part is bored or turned, the finished surfaces are untrue
after the jaws are released and the work has resumed its normal shape.
This applies more particularly to frail parts, such as rings, thin
cylindrical parts, etc. Occasionally the distortion can be prevented by
so locating the work with relation to the chuck jaws that the latter
bear against a rigid part. When the work cannot be held tightly enough
for the roughing cuts without springing it, the jaws should be released
somewhat before taking the finishing cut, to permit the part to spring
back to its natural shape.

[Illustration: Fig. 39. Diagram Illustrating Importance of Setting Work
with Reference to Surfaces to be Turned]

[Illustration: Fig. 40. Drilling in the Lathe]

=Drilling and Reaming.=--When a hole is to be bored from the solid, it
is necessary to drill a hole before a boring tool can be used. One
method of drilling in the lathe is to insert an ordinary twist drill in
a holder or socket _S_, Fig. 40, which is inserted in the tailstock
spindle in place of the center. The drill is then fed through the work
by turning the handle _n_ and feeding the spindle outward as shown by
the arrow. Before beginning to drill, it is well to turn a conical spot
or center for the drill point so the latter will start true. This is
of