Threads and gears

Gear tooth micrometerGear tooth micrometer (disc micrometer)

Also called: disc micrometer, span measurement tooth thickness micrometer

≡Standards, measuring range and accuracy

Standard

Not confirmed in public sourcesNo standalone standard was found

Measuring range

—Not confirmed in public sources

Resolution / graduation

Description Reading the scale is the same as for an outside micrometer

^[1]1 source, for reference

Accuracy (instrument error / maximum permissible error)

—Not confirmed in public sources

▦Types

An outside micrometer with disc-shaped measuring faces. Some have rotating discs and some have non-rotating discs

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

◎Part names and functions

#	Name	English	Function
1	Disc-type anvil and spindle (otherwise the same as an outside micrometer)	Disc anvil / spindle	Contacts the tooth flanks and clamps the base tangent length (span over teeth).^[1][2]

◷How to read the scale

Theoretical base tangent length W (spur gear): W = m·cosα·{π(k − 0.5) + z·invα} + 2x·m·sinα (k is the number of teeth spanned)^[4][5][1]
Example (Kyoiku table): m = 1, α = 20°, z = 24, x = 0 → k = 3, W = 7.7165 mm^[5]
Example (KHK): m = 3, α = 20°, z = 24, x = +0.4 → k = 4, W = 32.8266 mm^[4]

✋Usage steps

Measures the base tangent length of spur and helical gears (the distance between two parallel planes touching the outer flanks of k consecutive teeth)^[6][3][1]
For helical gears, check that there is enough face width for the measurement (about W·sinβb or more). If not, use the over-pin method^[4][5]

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

✓Care, storage, inspection and calibration

Care and storage

Same as for an outside micrometer^[7]

Inspection and calibration

Same as for an outside micrometer (specific procedure not confirmed)Not confirmed in public sources

The formulas and examples are shared with the gear database (span_measurement_* and examples in research/gears/formulas.json).

⇄Related instruments

Over-pin (over-ball) measurement Outside micrometer Span measurement (base tangent length) calculation

📚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.