Figure 2-22.—Nomenclature of an outside micrometer
caliper.
Figure 2-23.—Sleeve and thimble scales of a micrometer.
numbered 1, 2, 3, 4, and so on, representing 0.100
inch,0.200 inch, and so on. When you turn the thimble
so its edge is over the first sleeve line past the 0 on the
thimble scale, the spindle has opened 0.025 inch. If you
turn the spindle to the second mark, it has moved 0.025
inch plus 0.025 inch, or 0.050 inch.
When the beveled edge of the thimble stops
between graduated lines on the sleeve scale, you must
use the thimble scale to complete your reading. The
thimble scale is divided into 25 equal parts; each part
or mark represents 1/25th of a turn. And, 1/25th of
0.025 inch equals 0.001 inch. Note that in figure 2-23
every fifth line on the thimble scale is marked 5, 10,
15, and so on. The thimble scale permits you to take
very accurate readings to the thousandths of an inch.
Figure 2-24.—Enlarged micrometer scale.
The enlarged scale in figure 2-24 can help you
understand how to take a complete micrometer reading
to the nearest thousandth of an inch.
The thimble is turned far enough to expose the 7 on
the sleeve scale but not far enough to expose the first
mark after the 7. Therefore, the measurement must be
between 0.700 inch and 0.725 inch, Exactly how far
between 0.700 inch and 0.725 inch must be read on the
thimble scale.
As you can see, the thimble has been turned
through 12 spaces of its scale, and the 12th graduation
is lined up with the reference line on the sleeve. When
the value on the sleeve scale is added to the value on
the thimble scale that is lined up with the reference line
on the sleeve scale, the space between the anvil and
spindle must be 0.712 inch (seven hundred and twelve
thousandths of an inch).
MICROMETER-READING EXERCISE.—
Occasionally you attain a reading in which the
horizontal reference line of the sleeve scale falls
between two graduations on the thimble scale, as shown
in figure 2-25. Note the horizontal reference line is
closer to the 15 mark than to the 14 mark. To read this
measurement to THREE decimal places, simply round
off to the 15 mark, as shown in example A of figure
2-25. To read this measurement to FOUR decimal
places, estimate the number of tenths of the distance
between thimble scale graduations the horizontal
reference line has fallen. Each tenth of this distance
equals one ten-thousandth (0.0001) of an inch. Add the
ten-thousandths to the reading as shown in example B
of figure 2-25.
2-9
