It is very important to keep your micrometers clean as dirt between the anvil and spindle will cause the
micrometer to read incorrectly. If you want to test to see if the micrometer is accurate, clean and bring
the anvil and spindle together carefully. If the zero line on the thimble and the axial (longitudinal) line on
the sleeve fail to coincide, wear has taken place either in the screw or contact surfaces.
Micrometers are made in a wide range of sizes and in matched sets. The ratchet on the micrometer is
used to rotate the spindle when taking a measurement and insures consistent, accurate gauging by
limiting the spindle pressure on the work piece.
There is a locknut that makes it possible to lock the micrometer spindle at any desired setting. A slight
turn of the knurled locknut ring contracts a split bushing around the spindle and makes the micrometer a
fixed gage.
If you want very accurate measurements are required, a micrometer that has an extra scale added to the
sleeve is used, enabling the micrometer to be read in ten-thousandths of an inch. This scale consists of
a series of lines on the sleeve parallel to its axis.
Ten divisions on the sleeve mark the same spaces as nine divisions on the beveled edge of the thimble.
The difference between the width of one of the ten spaces on the sleeve and one of the nine spaces on
the thimble is one tenth of a division on the thimble. Since the thimble is graduated to read in
thousandths, 1/10 of a division would be .0001 or one ten-thousandth.