Machinery Repairman

Figure 6-47.—Examples of work mounted between centers.
When you mount work between centers for
machining, there should be no end play between the
work and the dead center. However, if the work is
held too tightly by the tail center, when the work
begins revolving it will heat the center point and
destroy both the center and the work. To prevent
overheating, lubricate the tail center with a heavy oil
or a lubricant specially made for this purpose. If you
are using a ball bearing center, no lubricant is
necessary.
Holding Work on a Mandrel
Many parts, such as bushings, gears, collars, and
pulleys, require all the finished external surfaces to
run true with the hole that extends through them. That
is, the outside diameter must be true with the inside
diameter or bore.
A mandrel is simply a round piece of steel of
convenient length that has been centered and turned
true with the centers. Commercial mandrels are made
of tool steel, hardened and ground with a slight taper
(usually 0.0005 inch per inch). On sizes up to 1 inch
the small end is usually one-half of one thousandth of
an inch under the standard size of the mandrel, while
on larger sizes this dimension is usually one
thousandth of an inch under standard. This taper
allows the standard hole in the work to vary according
to the usual shop practice, and still provides the
necessary fit to drive the work when the mandrel is
pressed into the hole. However, the taper is not great
enough to distort the hole in the work. The
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countersunk centers of the mandrel are lapped for
accuracy, while the ends are turned smaller than the
body of the mandrel and are provided with flats,
which give a driving surface for the lathe dog.
General practice is to finish the hole to a standard
size, within the limit of the accuracy desired. Thus, a
3/4-inch standard hole will have a finished dimension
of from 0.7505 to 0.7495 inch, or a tolerance of
one-half of one thousandth of an inch above or below
the true standard size of exactly 0.750 inch. First,
drill or bore 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.
Press the piece on a mandrel tightly enough so the
work will not slip while it is machined and 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
hole in the piece.
The size of the mandrel is always marked on the
large end to avoid error and for convenience in
placing work on it. The work is driven or pressed on
from the small end and removed the same way.
When the hole in the work is not standard size, or
if no standard mandrel is available, make a soft
mandrel to fit the particular piece to be machined.
Use a few drops of oil to lubricate the surface of
the mandrel before pressing it into the work, because
clean metallic surfaces gall or stick when pressed
together. If you do not use lubricant, you will not be
able to drive the mandrel out without ruining the
work.
Whenever you machine work on a mandrel, be
sure the lathe centers are true and accurately aligned;
otherwise, the finished turned surface will not be true.
Before turning accurate work, test the mandrel on
centers before placing any work on it. The best test
for runout is one made with a dial indicator. Mount
the indicator on the toolpost so the point of the
indicator just touches the mandrel. As the mandrel is
turned slowly between centers, any runout will be
registered on the indicator dial.
If runout is indicated and you cannot correct it by
adjusting the tailstock, the mandrel itself is at fault
(assuming that the lathe centers are true) and cannot
be used. The countersunk holes may have been
damaged, or the mandrel may have been bent by
careless handling. Be sure you always protect the