TM 1-1500-204-23-9
Figure 3-42. Reading a Measurement on a
Micrometer
thimble (3). Each line represents 0.025 inch (5), and
each fourth line is numbered, representing tenths of an
inch (2).
(b) Thimble lines. The beveled edge of the
thimble
is
graduated
into
25
parts,
each
line
representing 0.001 inch (4). One complete and exact
revolution of the thimble will indicate 0.025 inch. Every
fifth line is numbered to assist in reading these marks.
(3) Reading a measurement. A measurement
can be read as follows:
Read the highest whole number visible on the barrel (2)
and record................................................ 2 = 0.200 inch
Count the number of lines visible between the 2 and the
thimble edge (5) and record .................... 1 = 0.025 inch
Locate the line on the thimble (3) that coincides with or
has passed the horizontal line in the barrel (4) and
record .................................................... 16 = 0.016 inch
Total all preceding measurements.................. 0.241 inch
The measurement, therefore, is 0.241 inch.
e. Reading a Vernier Micrometer. Reading the
vernier micrometer is the same as reading the standard
micrometer. A vernier micrometer is shown in figure 3-
43. An additional step must be taken, to add the vernier
reading to the dimensions. This allows for precise
measurements which are accurate to 1/10,000 (0.0001)
of an inch. This scale furnishes the fine readings
between the lines on the thimble rather than making an
estimate as you would on a standard micrometer.
(1) Vernier scale. The ten spaces on the
vernier scale (2) are equivalent to 9 spaces on the
thimble (3). Therefore, each unit on the vernier scale is
equal to 0.0009 inch and the difference between the
sizes of the units on each scale is 0.0001 inch.
(2) Reading the measurement. A measurement
can be read as follows:
Read the highest whole number visible on the barrel (6)
and record................................................ 2 = 0.200 inch
Count the number of lines visible between the 2 on the
barrel and the thimble edge (5) and
record ...................................................... 3 = 0.075 inch
Locate the line on the thimble (3) that coincides with or
has passed the horizontal line in the barrel (4) and
record ................................................... 11 = 0.011 inch
Locate the line on the vernier scale (2) that coincides
with a division line on the thimble (3) and
record .................................................... 2 = 0.0002 inch
Total all preceding measurements................ 0.2862 inch
The measurement, therefore, is 0.2862 inch
f. Reading a Metric Micrometer. The same principle
is applied in reading the metric graduated micrometer,
but some changes in graduations are used. These
graduations are shown in figure 3-44.
(1) Spindle movement. The pitch of the
micrometer screw under the thimble is 0.05 millimeters
(mm). One revolution of the thimble advances or
withdraws the spindle a distance equal to 0.5 mm.
(2) Barrel graduations. The barrel (1) is
graduated in millimeters from 0 to 25. It takes two
revolutions of the thimble to move the barrel 1 mm.
3-17
