oilstone; clean the measuring surfaces again, and then
recheck the micrometer for zero setting.
The micrometer is one of the most used, and often
3. If the reading is less than zero, or if you do
one of the most abused, precision measuring
not obtain a zero reading after making the correction
instruments in the shop. Careful observation of the
described in step 2, you will need to adjust the
do's and don'ts in the following list will enable you to
spindle-thimble relationship. The method for setting
take proper care of the micrometer you use:
zero differs considerably between makes of
Always stop the work before taking a measure-
locks the thimble to the spindle; some have a special
ment. Do NOT measure moving parts because the
rotatable sleeve on the barrel that can be unlocked;
micrometer may get caught in the rotating work and
and some have an adjustable anvil.
be severely damaged.
To make adjustments to micrometers follow
Always open a micrometer by holding the
frame with one hand and turning the knurled sleeve
1. To adjust the THIMBLE-CAP TYPE, back
twirling the frame, because such practice will put
the spindle away from the anvil, release the thimble
unnecessary strain on the instrument and cause
cap with the small spanner wrench provided for that
excessive wear of the threads.
purpose, and bring the spindle into contact with the
anvil. Hold the spindle firmly with one hand and
Apply only moderate force to the knurled
rotate the thimble to zero with the other; after zero
thimble when you take a measurement. Always use
relation has been established, rotate the spindle
counterclockwise to open the micrometer, and then
instrument. Too much pressure on the knurled sleeve
tighten the thimble cap. After tightening the cap,
will not only result in an inaccurate reading, but may
check the zero setting again to be sure the
also cause the frame to spring, forcing the measuring
thimble-spindle relation was not disturbed while the
surfaces out of line.
cap was being tightened.
When a micrometer is not in actual use, place it
2. To adjust the ROTATABLE SLEEVE
where it is not likely to be dropped. Dropping a
TYPE, unlock the barrel sleeve with the small spanner
micrometer can cause the frame to spring; if dropped,
wrench provided for that purpose, bring the spindle
the instrument should be checked for accuracy before
into contact with the anvil, and rotate the sleeve into
any further readings are taken.
alignment with the zero mark on the thimble. After
completing the alignment, back the spindle away from
Before a micrometer is returned to stowage,
the anvil, and retighten the barrel sleeve locking nut.
back the spindle away from the anvil, wipe all exterior
Recheck for zero setting, to be sure you did not
surfaces with a clean, soft cloth, and coat the surfaces
disturb the thimble-sleeve relationship while
with a light oil. Do not reset the measuring surfaces
tightening the lock nut.
to close contact because the protecting film of oil in
these surfaces will be squeezed out.
3. To set zero on the ADJUSTABLE ANVIL
TYPE, bring the thimble to zero reading, lock the
A micrometer caliper should be checked for
spindle if a spindle lock is provided, and loosen the
zero setting (and adjusted when necessary) as a matter
anvil lock screw. After you have loosened the lock
of routine to ensure that reliable readings are being
screw, bring the anvil into contact with the spindle,
obtained. To do this, proceed as follows:
making sure the thimble is still set on zero. Tighten
the anvil setscrew lock nut slightly, unlock the
1. Wipe the measuring faces, making sure that
spindle, and back the spindle away from the anvil;
they are perfectly clean, and then bring the spindle
then lock the anvil setscrew firmly. After locking the
into contact with the anvil. Use the same moderate
setscrew, check the micrometer for zero setting to
force that you ordinarily use when taking a
make sure you did not move the anvil out of position
measurement. The reading should be zero; if it is not,
while you tightened the setscrew.
the micrometer needs further checking.
2. If the reading is more than zero, examine
l The zero check and methods of adjustment of
the edges of the measuring faces for burrs. Should
burrs be present, remove them with a small slip of
measure to zero; the PROCEDURE FOR LARGER