Inside micrometers
Selecting, setting, and interpreting two-point, three-point, and extension-type internal micrometers.
Tags: inspection, dimensional-metrology, micrometers
On this page
What the instrument measures
State the contact arrangement, axial station and orientation with the internal dimension.
Illustrative reference-value example: assume a ring's applicable calibrated diameter is 25.003 mm and a correctly configured comparison records the workpiece as 0.012 mm larger. The estimated diameter is 25.015 mm. Substituting the nominal 25.000 mm would instead give 25.012 mm, introducing a −0.003 mm difference. These invented values illustrate arithmetic only; they do not establish uncertainty or an acceptance decision. Confirm that the instrument's preset and output mode actually implement the intended comparison.
Two-point instruments contact opposite sides; three-point heads distribute contacts around the circumference. Mitutoyo's Holtest range includes both arrangements, so the family name alone does not establish the contact geometry. The manufacturer's Series 368 description identifies automatic centering as a feature of the three-point arrangement. This helps positioning, but does not establish that every possible bore-form error will be detected. Mitutoyo: Holtest product description
Choose the contact arrangement and reach
| Arrangement | Selection question | Measurement-plan consequence |
|---|---|---|
| Two-point internal micrometer | Can the operator establish a diametral contact in the required plane? | Record angular orientation and depth; repeat positioning to assess operator influence. |
| Three-point head | Do the contact shape, measuring range, and accessible depth suit the bore? | Use the specified setting method; self-centering does not remove the need to sample along the bore. |
| Rod or tubular extension instrument | Does the assembled configuration reach the internal feature with appropriate support? | Identify every extension and the resulting range; verify the assembly used for the job. |
Mitutoyo's inside-measurement guidance discusses support positions for long internal measuring bars, including Bessel and Airy points. Support is therefore part of the measurement configuration rather than merely a convenience. Its Holtest guidance also distinguishes contact over the full anvil from contact only at the tip: those conditions can produce different results. Set the starting point under the same contact condition used on the workpiece. Mitutoyo: inside-measurement knowledge
Reach and blind-hole access require a dimensional check of the exact head. An extension adds reach; it does not necessarily allow measurement at the bottom of a bore, across an interruption, or inside a counterbore. Establish where the actual contact surfaces will touch before selecting the instrument.
Setting and alignment
A setting operation establishes the instrument's reference. It does not replace calibration over the measuring range. Use the designated setting artifact and the instrument's prescribed adjustment or preset procedure. Record the artifact identity, applicable calibrated size, instrument configuration, and units. A nominal size printed on a ring is insufficient information for a measurement requiring correction to its calibrated value.
For two-point measurement, distinguish two alignment errors. An off-center chord across the bore can read smaller than the diameter, while inclination along the bore axis can read larger. Mitutoyo illustrates these as radial and axial misalignment with opposite error signs. Consequently, an instruction such as “rock until the maximum reading” is incomplete unless the rocking direction and instrument geometry are defined. Establish the correct plane and diametral position using the applicable instrument instructions. Mitutoyo: Quick Guide, inside-micrometer section
Clean contact surfaces and the reference before setting. Recheck the reference after changing a head or extension, after a suspected impact, or when readings become unstable. Treat unexpected drift as a reason to investigate the setup rather than repeatedly adjusting the preset to make the result appear stable.
A bounded manufacturer example
Mitutoyo's June 2020 Holtest/Borematic bulletin 2230 lists Digimatic Holtest 468-164 with a 12–16 mm range, an “accuracy” entry of ±3 μm, and maximum difference of 2 μm. It associates that size with optional 150 mm extension rod 952621. The bulletin describes a ratchet arrangement for consistent measuring force. These are specifications for that documented product configuration, not interchangeable specifications for every internal micrometer or extension assembly. Mitutoyo: bulletin 2230, printed pages 3 and 5; June 2020 footer
Preserve the original specification terminology when recording historical data. Mitutoyo announced on 16 December 2020, updated 28 June 2021, that relevant micrometer specifications changed from “accuracy” to maximum permissible error terminology; some model tolerances changed too. Therefore check the current model documentation and supplied inspection certificate rather than treating the older brochure entry as the current acceptance limit. Mitutoyo: accuracy-indication announcement
Temperature, force, and uncertainty
Displayed increments, permissible error, repeatability, and measurement uncertainty answer different questions. Resolution concerns distinguishable changes; a fine display alone does not demonstrate equivalent accuracy under working conditions. Assess the effective resolution through repeated observations in the actual setup. NIST/SEMATECH: resolution
Temperature differences between the workpiece, setting artifact, and instrument can change the comparison. NIST identifies 20 °C as the conventional reference temperature for dimensional measurements unless another temperature is specified. This is a reference condition, not proof that a recently handled instrument or warm machined bore has reached equilibrium. Record relevant temperatures and assess expansion effects for the actual materials and tolerance. NIST: Engineering Metrology FAQ
Mitutoyo's guide specifically warns about hand heating and recommends minimizing it through appropriate handling. Use the force-control device as instructed; additional tightening is not a measurement improvement. Thin walls, soft surfaces, roughness, and elastic contact behavior also warrant local evaluation. Repeated readings with removal and reinsertion help reveal positioning effects that repeated readings without moving the instrument can conceal. Mitutoyo: Quick Guide
Bore-form limits and reporting
Measure at defined depths and orientations when the inspection plan requires evidence about size variation. Separate those sampled results from a roundness or cylindricity claim. ISO 12181-1:2011, published April 2011, explicitly covers complete roundness profiles. A few internal-micrometer contacts do not constitute such a profile; this is a limitation of the sampling arrangement, even when individual size readings repeat closely. ISO: roundness standard scope
A useful record includes instrument and head identifiers, extensions, reference artifact and value, contact geometry, measurement locations, results, environmental observations, and the uncertainty and decision rule required by the job. Define additional form measurement separately when the drawing requires it.