Measuring length

Three-point inside micrometer (bore micrometer)Three-point internal micrometer (bore micrometer)

Also called: Holtest (Mitutoyo product name), bore micrometer, three-point internal micrometer, Borematic (digital; Mitutoyo product name)

≡Standards, measuring range and accuracy

Standard

Not confirmed in public sourcesNo standalone JIS/ISO standard could be confirmed

Measuring range

byMaker
- Mitutoyo HT-R (368-161–167) 6–8 mm to 25–30 mm (each model has a narrow range)

^[6]1 source, for reference

Resolution / graduation

byMaker
- Mitutoyo HT-R Graduation 0.001 mm (up to 12 mm), 0.005 mm (from 12 mm)

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

Accuracy (instrument error / maximum permissible error)

Approach Used comparatively, setting the zero point with a reference setting ring (master). Because contact is at three points, it gives an average value and is less able to detect shapes such as ovality, unlike the two-point type
byMaker

^[4][2]1 source, for reference

▦Types

Analog (ratchet type)

^[1][2][3]1 source, for reference

Digital (Digimatic)

^[2][4]1 source, for reference

Two-point / three-point (Mitutoyo Holtest series)

^[2]1 source, for reference

Pistol type (snap-action quick type)

^[4]1 source, for reference

◎Part names and functions

#	Name	English	Function
1	Contacts (anvils)	Contact points / anvils	The three contacts at the tip extend radially and touch the inside surface of the hole.^[1][2][3]
2	Cone	Cone	Converts the spindle's movement into radial movement of the contacts.^[3]
3	Spindle	Spindle	^[3]
4	Thimble and sleeve	Thimble, outer sleeve	The part where the scale is read.^[1][2][3]
5	Ratchet	Ratchet	Keeps the measuring force constant.^[1][2][3][5]
6	Measuring head	Measuring head	Replacing it voids the manufacturer's accuracy guarantee.^[2]

◷How to read the scale

Sleeve reading + thimble reading (graduation 0.005 mm)Example: barrel 35 mm + thimble 0.015 mm = 35.015 mm^[1][3]

✋Usage steps

Remove dirt from the contacts, turn the thimble through its full range, and check operation and the extension and retraction of the contacts^[2]
Set the zero point with a calibrated master (setting ring). On digital types, preset the reference value (the ring's calibrated value)^[2][1]
If you measure with only the tips of the contacts, also set the zero point with the tips (the value differs from measuring with the full faces)^[1][2]
Hold the measuring faces lightly in contact, turn the ratchet 5–6 times (2–3 revolutions) to apply constant pressure, and read^[2]
Three-point contact makes the tool tend to center itself in the hole, so it is more accurate than the caliper type and can measure even deep inside a hole^[5]

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

!Common measurement mistakes and countermeasures

Measuring position differs from the conditions under which the zero point was set

WhyThe value changes depending on whether the full faces of the contacts or only their tips are used

CountermeasureSet the zero point under the same conditions^[1][2]

Use outside the measuring range / pulling the spindle out too far

WhyA digital type can be damaged if the spindle is pulled back beyond its limit

CountermeasureUse it within its range, and do not turn it further when you feel resistance^[2]

✓Care, storage, inspection and calibration

Care and storage

After use, check for damage and clean, and store away from high temperature and humidity, dust, and oil mist. For long-term storage, apply micrometer oil to the contacts, and remove the battery from digital types^[2]
Do not shock the contact surfaces of the contacts^[2]

Inspection and calibration

Set and check the zero point with a setting ring (reference ring)^[2][1][4]

The pros and cons of two-point and three-point types (the two-point type can detect ovality, barrel shape, and taper but is slow; the three-point type is fast and needs no experience but gives an average value) lend themselves to a comparison diagram (Mitutoyo US materials).

⇄Related instruments

Dial bore gauge Inside 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.