|The inside micrometer is the most direct measuring tool used for
inspection (Figure 1). The total length of the inside micrometer is
itself the overall length being measured. The inside micrometer is a
much more accurate measuring tool for inside measurement than the
The inside micrometer is almost always purchased as a set. The
only way to use the inside micrometer over a range of sizes is by
using extension rods. Only one micrometer head is needed, along with
extension rods, to cover a broad range of measurements. Tubular style
extension rods are available in the intermediate to extended length,
while solid extension rods are used for ranges of up to
approximately 32 inches.
Figure 1 Inside
The smallest sizes of inside micrometers have a travel range of ¼
inch. The largest sizes of inside micrometers have a range of 1 inch.
The most popular inside micrometer sets have a ½ inch range.
The inside micrometer is very similar to the outside micrometer,
except that it has no frame (Figure 3). In some cases a micrometer
handle (shown in figure 1) is used to reach into inaccessible places or
in small areas where the inspector's fingers would get in the way.
Figures 2 and 3 An inside
micrometer is the same as a plain micrometer without the frame.
Using the Inside Micrometer
|When using the inside micrometer it is necessary to rotate the
head end of the micrometer in an axially direction as well as up and
down (Figure 4), all the while adjusting the head to the proper
feel. Rotating the head in this manner assures that the micrometer
is accurately centralized in the part. The inside micrometer, in
most cases, is removed from the part while the actual reading is
being done. For this reason the barrel lock is relatively snug to
retain the reading.
Figure 4 When possible, hold the
reference end of the inside micrometer in one hand while rotating
and adjusting the micrometer head.
Reading the Inside Micrometer
Reading an inside micrometer is comparable to reading a standard
micrometer except that extension rod and collar lengths must be added to
the head measurement (Figure 5).
||2 - to - 3 - inch rod (the shortest).
Reading is 2.350
|2 - to - 3 -inch rod with a .500 collar.
Reading is 2.850
|3 - to - 4 -inch rod.
Reading is 3.350
Figure 5 A
2-inch inside micrometer with ½-inch range
Calibration of the Inside Micrometer
The inside micrometer must be calibrated before each use and after
changing rods. The calibration must be done against a known standard,
such as an inspection ring. If an inspection ring is not available, a
properly calibrated outside micrometer may be used (Figure 6).
Checking the calibration of the inside micrometer against
a known standard