Tc 9-524 fundamentals of machine tools

Polishing on the Lathe
Polishing with either abrasive cloth or abrasive paper is
desirable to improve the surface finish after filing. Emery
abrasive cloth is best for ferrous metals while abrasive paper
often gives better results on nonferrous materials. The most
effective speed for polishing with ordinary abrasives is
approximately 5,000 feet per minute. Since most lathes are not
capable of a speed this great for an average size workpiece, it
is necessary to select as high a speed as conditions will permit.
In most cases the abrasive cloth or paper is held directly in
the hand and applied to the workpiece, although it may be
tacked over a piece of wood and used in the same manner as a
file. Improvised clamps may also be used to polish plain
round work.
Since polishing will slightly reduce the dimensions of the
workpiece, 0.00025 to 0.0005 inch should be allowed for this
operation. Figure 7-101 shows how to hold the abrasive strip
when polishing. Note that the ends of the strip are separated.
This prevents the strip from grabbing and winding around the
work, which could pull the operator’s hand into the work.
Move the polishing strip slowly back and forth to prevent
material building up on the strip which causes polishing rings
to form on the work. To produce a bright surface, polish the
work dry. To produce a dull satin finish, apply oil as the
polishing operation is in progress.
ECCENTRIC WORK ON THE LATHE
Eccentric work is work that is turned off center, or not on
the normal center axis. An engine crankshaft is a good
example of an eccentric workpiece. Crankshafts normally
have a main center axis, called a main journal, and offset axes,
which produce the throw and the eccentric diameters of the
mechanism. An eccentric shaft may have two or more
diameters and several different center axes. The amount of
eccentricity, or half of the throw, is the linear distance that a
set of center holes has been offset from the normal center axis
of the workpiece. Eccentric turning on the lathe is used for the
following eccentric turning situations:
When the throw is large enough to allow all centers to be
located on the workpiece at the same time.
When the throw is too small to allow all centers to fit into
the end of a workpiece at the same time. (The center drilled
holes are too large.)
When the throw is so great that all centers cannot be located
on the work, or in other words, a throw larger than the largest
diameter of the workpiece. (This type of crank is usually made
in separate pieces and connected together, since the cost of
wasted material would be too great if constructed from one
piece on the lathe).