Chapter 4 Measurement
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tolerances so critical on some aerospace and
other applications, it is important that coun-
tersunk holes and tapers on fasteners meet
specifications. A chamfer micrometer makes it
possible to check these critical areas.
Special micrometers are available for other
applications. These micrometers are devised to
handle nonstandard measurement tasks.
4.2.2 Reading an Inch-Based Micrometer
A micrometer uses a very precisely made screw
thread that rotates in a fixed nut. The screw thread
is ground on the spindle and is attached to the
thimble. The spindle advances or recedes from the
anvil as the thimble is rotated. See Figure 4-11. The
threaded section has 40 threads per inch; therefore,
each revolution of the thimble moves the spindle
1/40″ (0.025″).
The line engraved lengthwise on the sleeve is
divided into 40 equal parts per inch (corresponding
to the number of threads per inch on the spindle).
Each vertical line equals 1/40″, or 0.025″. Every
fourth division is numbered, representing 0.100″,
0.200″, etc.
The beveled edge of the thimble is divided into
25 equal parts around its circumference. Each divi-
sion equals 1/1000″ (0.001″). On some micrometers,
every division is numbered, while every fifth divi-
sion is numbered on others.
The micrometer is read by recording the highest
number on the sleeve (1 = 0.100, 2 = 0.200, etc.). To
this number, add the number of vertical lines visible
between the number and thimble edge (1 = 0.025,
2 = 0.050, etc.). To this total, add the number of thou-
sandths indicated by the line that coincides with the
horizontal sleeve line.
Figure 4-10. This screw thread micrometer can measure
threads as wide as 7/8″. (L. S. Starrett Co.)
Figure 4-11. Basic parts of a micrometer caliper.
0.458″
Example 1
Add the readings from the sleeve and the thimble:
4 large graduations: 4 0.100 = 0.400
2 small graduations: 2 0.025 = 0.050
8 thimble graduations: 8 0.001 = 0.008
Total mike reading = 0.458″
0.289″
Example 2
Add the readings from the sleeve and the thimble:
2 large graduations: 2 0.100 = 0.200
3 small graduations: 3 0.025 = 0.075
14 thimble graduations: 14 0.001 = 0.014
Total mike reading = 0.289″
Anvil
Spindle
Lock nut
Sleeve
Thimble
Ratchet
stop
Frame