Figure 9-22.—Examples of work mounted between centers.
MOUNTING THE WORK.—Figure 9-22 shows
correct and incorrect ways to mount work between
centers. In the correct example, the driving dog is
attached to the work and held rigidly by the setscrew.
The tail of the dog rests in the slot of the faceplate,
without touching the bottom of the slot. The tail extends
beyond the base of the slot so that the work rests firmly
on both the headstock center and the tailstock center.
In the incorrect example, note that the tail of the dog
rests on the bottom of the slot on the faceplate at A and
pulls the work away from the center’s point, as shown
at B and C. This causes the work to revolve
eccentrically.
In mounting work between centers for machining,
be sure there is no end play between the work and the
dead center. However, do not have the work held too
tightly by the tailstock
center. If you do, as the work
revolves, it will heat the center’s point, destroying both
itself and the center. To help prevent overheating,
lubricate the tailstock center with grease or oil.
Holding Work on a Mandrel
Many parts, such as bushings, gears, collars, and
pulleys, require all the finished external surfaces to run
true with their center hole, or bore.
General practice is to finish the bore to a standard
size within the limit of the accuracy desired. Thus a
3/4-inch standard bore would have a finished diameter
of from 0.7495 to 0.7505 inch This variation is due to
a tolerance of 0.0005 inch below and above the true
standard of exactly 0.750 inch. First drill the hole to
within a few thousandths of an inch of the finished size;
then remove the remainder of the material with a
machine reamer, following with a hand reamer if the
limits are extremely close.
Then press the piece on a mandrel tightly enough so
the work will not slip while being machined Clamp a
dog on the mandrel, which is mounted between centers.
Since the mandrel surface runs true with respect to the
lathe axis, the turned surfaces of the work on the mandrel
will be true with respect to the bore of the piece.
A mandrel is simply a round piece of steel of
convenient length which has been center drilled and
ground true with the center holes. Commercial mandrels
are made of tool steel, hardened and ground with a slight
taper (usually 0.0005 inch per inch). This taper allows
the standard hole in the work to vary according to the
usual shop practice and still provides a drive to the work
when the mandrel is pressed into the hole. The taper is
not great enough to distort the hole in the work The
center-drilled centers of the mandrel are lapped for
accuracy. The ends are turned smaller than the body of
the mandrel and provided with flats, which give a
driving surface for the lathe dog.
Holding Work in Chucks
The independent chuck and universal chuck are
used more often than other work-holding devices in
lathe operations. The universal chuck is used for holding
relatively true cylindrical work when the time required
to do the job is more important than the concentricity of
the machined surface and the holding power of the
chuck When the work is irregular in shape, must be
accurately centered, or must be held securely for heavy
feeds and depth of cuts, an independent chuck is used.
FOUR- JAW INDEPENDENT CHUCK.-Figure
9-23 shows a rough cylindrical casting mounted in a
four-jaw independent lathe chuck on the spindle of the
lathe. Before truing the work, determine which part you
wish to have turned true. To mount this casting in the
chuck, proceed as follows:
1. Adjust the chuck jaws to receive the casting. The
same point on each jaw should touch the same
ring on the face of the chuck If there are no
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