(d) Before using a micrometer, wipe it off and pull a piece of paper
between the anvil and end of the spindle.
(e) The micrometer should operate freely with no play in its travel. If
a micrometer has play, or if it binds, it should be returned to the manufacturer
for reconditioning. This condition is caused by abuse or uneven wear.
(f) Check the micrometer screw periodically with a precision gage block
in at least four places other than zero to verify its accuracy. Simply measure a
selected group of blocks ranging from 0 to 1 inch.
(g) Clean micrometer mechanism whenever it becomes gummy, contains
abrasive grit, or whenever it is to be adjusted. Use an approved cleaning agent.
(h) When the faces of the spindle and anvil become worn and they are no
longer flat and parallel to each other, the error should not exceed 0.0002 inch on
a micrometer which is graduated to control measurements to a limit of 0.001 inch
and should not exceed 0.00005 inch on a micrometer which is graduated to control
measurements to a limit of 0.0001 inch. Measuring a ball at several points over
the surface of the anvils will show up any error in parallelism. Parallelism can
be tested by means of two balls mounted in an aluminum holder. If the anvils are
in error more than the allowable maximum, the micrometer should be returned to the
manufacturer for repair.
(i) In adjusting a micrometer to read correctly, the thimble is not set
to 0 when the anvil is in contact with the spindle, but is set at some dimension to
distribute the error. For example, if a micrometer screw had an accumulating error
of 0.0003 inch in the length of its travel, and it were set correctly at 0, it
would be 0.0003 at 1 inch. However, if the micrometer were set correctly in the
center of its travel, it would be 0.00015 under at 0 and 0.00015 over at 1 inch,
which is a much better condition. Because a micrometer does not return exactly to
0 when the anvil and spindle contact, it does not mean that it is not adjusted
properly. Turn the friction sleeve with a small spanner wrench to compensate for
minor wear on the anvil and spindle or on the screw.
f. Thickness (feeler) gages.
(1) General. Thickness (feeler) gages are fixed in leaf form to permit the
checking and measuring of small openings such as contact points, narrow slots and
so forth. They are widely used to check the flatness of parts in straightening and
grinding operations and in squaring objects with a try square. Thickness gages are
made in many shapes and sizes; usually 2 to 26 blades are grouped into one tool and
graduated in thousandths of an inch (fig 61). Most thickness gage blades are
straight, but some are bent at the end at 45 and 90 angles. Some thickness gages
are grouped so that there are several short and several long blades together.
Thickness gages are also available in single blades and in strip form for specific
measurements. For convenience, groups of thickness blades are equipped with a
locking screw in the case that locks the blade to be used in the extended position.
(2) Care of gages. Always coat metal parts of all gages with a light film
of oil when not in use to prevent rust. Store gages in separate containers. Do
not pile gages on each other. Always return blades of leaf-type gages to the case
after use. Keep graduations and markings on all gages clean and legible. Do not
drop any gage--minute scratches or nicks will result in inaccurate measurements.
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