. 11. The file
is passed across the work and advanced sidewise for each forward stroke,
until the entire surface is finished. The file should be kept in contact
with the work continually, but on the return stroke the pressure should
be relieved. The movement of the file during the forward or cutting
stroke should be much slower than when filing in a vise. By moving the
file slowly, the work can make a number of revolutions for each stroke,
which tends to keep it round, as practically the same amount of metal is
removed from the entire circumference. On the other hand, short rapid
strokes tend to produce flat spots, or at least an irregular surface,
especially if the work can only make part of a revolution for each
cutting stroke. The pressure on the file during the forward stroke
should also be kept as nearly uniform as possible.

[Illustration: Fig. 11. Filing Work after Finishing Cut is taken]

It is very difficult to file a part smooth and at the same time to keep
it round and cylindrical, and the more filing that has to be done, the
greater the chance of error. For this reason, the amount left for filing
should be very small; in fact, the metal removed by filing should be
just enough to take out the tool marks and give a smooth finish. Very
often a satisfactory finish can be obtained with a turning tool, and
filing is not necessary at all. The file generally used for lathe work
is a "single-cut bastard" of "mill" section, having a length of from 12
to 14 inches.

Sometimes particles of metal collect between the teeth of a file and
make deep scratches as the file is passed across the work. When this
occurs, the teeth should be cleaned by using a wire brush or a file
card, which is drawn across the file in the direction of the teeth. This
forming of tiny particles between the teeth is known as "pinning" and it
can sometimes be avoided by rubbing chalk on the file. Filing is not
only done to obtain a smooth finish, but also to reduce the work to an
exact diameter, as a very slight reduction can be made in this way.

[Illustration: Fig. 12. Two Methods of Aligning Centers for Cylindrical
Turning]

If a polish is desired, this can be obtained by holding a piece of emery
cloth tightly around the work as it revolves. The coarseness of emery
cloth is indicated by letters and numbers corresponding to the grain
number of loose emery. The letters and numbers for grits ranging from
fine to coarse are as follows: _FF_, _F_,