Measuring length

Inside micrometerInside micrometer

Also called: caliper-type inside micrometer, rod-type inside micrometer, single-unit inside micrometer

≡Standards, measuring range and accuracy

Standard

Unconfirmed whether the scope of JIS B 7502 (micrometers) includes inside and depth micrometers
ASME B89.1.13 covers outside, inside, and depth micrometers

^[1]1 source, for referenceA standalone JIS number for the rod-type inside micrometer could not be confirmed

Measuring range

byMaker
- Mitutoyo IM series (rod type) 50–75 mm to 200–225 mm in 25 mm steps (133-143 to 133-149)
General Extendable to large diameters. In Mitutoyo's US materials, up to 80 inches with rods and pipes

^[6][7]1 source, for reference

Resolution / graduation

Graduation 0.01 mm (Mitutoyo IM series)

^[6]1 source, for reference

Accuracy (instrument error / maximum permissible error)

Approach Like an outside micrometer, it is controlled by indication error. A rod type is also affected by misalignment (tilt), self-weight sag, and temperature
byMaker

^[3][5]1 source, for reference

▦Types

Caliper-type inside micrometer

Because it measures while opening the jaws, the sleeve scale numbers get smaller toward the right, and the thimble scale also runs in the opposite direction to an outside micrometer. Used for small bores and side grooves^[2][3][4]

Rod-type (single-unit / extension) inside micrometer

Rods or pipes are joined to measure large bores^[5][3][6][7]1 source, for reference

◎Part names and functions

#	Name	English	Function
1	Jaws (measuring faces)	Jaws	Caliper type: two jaws pressed against the inside surface of a hole.^[2]
2	Thimble, sleeve, ratchet stop (or friction stop)	Thimble, sleeve, ratchet/friction stop	The reading section is the same as on an outside micrometer.^[2][3]
3	Heat shield cover (rod type)	Heat insulator	Wear gloves and hold here to prevent expansion from body heat.^[5][3]

◷How to read the scale

Reading is the same as for an outside micrometer. Note, however, that the scale direction is reversed on the caliper type^[2]

✋Usage steps

Apply the caliper type in the same way as a caliper's inside measurement, so that the two jaws capture the true diameter of the hole^[2]
For a rod type, rock it in the hole and use the fact that tilting axially reads long (positive side) and tilting radially (side to side) reads short (negative side) to find the position of the true diameterMitutoyo's US materials explain finding the minimum position by turning the micrometer head while rocking the tool inside the hole. The final value is obtained by adding to or subtracting the reading from the value set on a reference (setting ring, etc.)^[5][3]
Do not hold it bare-handed; wear gloves and hold the heat shield cover^[5][3]
When laying it horizontally to set the zero point, support it at two points at the Airy points (where the two end faces are most parallel) or the Bessel points (where the overall length error is smallest)^[5][3]

Printed from Kezuriba (kezuriba.net/en/gauges/inside-micrometer/)

!Common measurement mistakes and countermeasures

Misalignment (tilt)

WhyIf it is tilted axially by X, the measured length becomes √(ℓ²+X²) and reads long

CountermeasureRock it and read at the position of the true diameter^[5][3]

Expansion from body heat

WhyA long rod expands from the heat of the hand

CountermeasureGloves and a heat shield cover^[5][3]

Self-weight sag

WhySupported horizontally at two points, it bends under its own weight

CountermeasureSupport it at the Airy points (a ≈ 0.577ℓ) or the Bessel points (a ≈ 0.559ℓ)^[5][3]

✓Care, storage, inspection and calibration

Care and storage

Follow the outside micrometer (the sources have little that is specific to inside micrometers)^[3]

Inspection and calibration

Set or check the zero point with a setting ring or an outside micrometer (no primary source for the specific procedure was found)^[7]

A diagram of the scale direction on the caliper type would help prevent misreading.

⇄Related instruments

Three-point inside micrometer (bore micrometer)Dial bore gauge Outside micrometer

📚Sources

Content without a mark is what agreed across two or more sources from different publishers. Procedures and cautions are summarized in Kezuriba's own words based on the sources, and the diagrams were drawn by Kezuriba. Accuracy values are manufacturers' published figures, not values from JIS tables.