An internal micrometer measures inside diameters — bores, housings and rings — and comes in three families that suit different jobs and budgets. If you look after a small workshop's measuring kit and an ISO 9001 or customer audit is coming, the tool you choose also has to be traceable and in date. This guide compares the types and the record-keeping that keeps a measurement defensible.
The three types of internal micrometer
- Three-point (bore) internal micrometer: anvils at 120 degrees self-centre and read true diameter directly. Most accurate for round bores; each head covers a set range. See the practical method in our bore micrometer guide.
- Two-point tubular internal micrometer: uses extension rods to cover a wide range from one set at lower cost; you find the minimum by feel, and it reveals ovality better than a three-point tool.
- Caliper-type internal micrometer: jaws for small internal dimensions and grooves where the other types will not fit.
Choosing by range and job
Match the tool to the work: bearing housings and hydraulic bores favour a three-point head for repeatable size; wide or varied internal ranges favour tubular sets with rods; small IDs and slots need the caliper type. Whatever you pick, it needs a setting ring or standard so you can zero it before use.
Traceability when an audit is coming
For a quality-conscious shop the measurement is only as good as its paperwork. The line no quality lead wants to hear from an auditor is that the measurement system cannot be trusted, and a worn or out-of-calibration tool is exactly the finding that triggers it. Two routes keep you covered:
- A calibration certificate from a lab, ideally UKAS-accredited, recorded on the tool's asset register with the due date;
- Or a documented in-house check against a certified setting ring, logged so the record is in date and repeatable.
Do not ignore the environment either — coolant, oil and temperature swings on the shop floor all affect readings, so protect the tool and let parts stabilise before a critical check.
Where the outside set fits
External features still need an outside micrometer. The PROSTER 6-piece outside set (£126.13, free UK delivery, 12-month guarantee) covers shafts and pins for the external side of the same jobs. Be clear on scope: PROSTER does not supply internal micrometers or UKAS certificates, so plan those separately. See the outside micrometer guide.
PROSTER Outside Micrometer — 6-Piece Metric Set
Six metric frames covering 0–150 mm in 25 mm steps, carbide measuring faces and a ratchet stop, with free UK delivery, 30-day returns and a 12-month guarantee.
Honest limits
No affordable set replaces accredited calibration when a regulated audit or a customer contract demands traceability. When that is the requirement, send the tools to a UKAS lab and keep the certificates; treat in-house checks as day-to-day confidence, not a substitute for the certificate.
Frequently asked questions
What is the difference between an internal micrometer and a bore micrometer?
A bore micrometer is a three-point internal micrometer for cylinders; internal micrometer also covers two-point tubular and caliper-type tools for other inside dimensions.
Do I need a calibration certificate for an ISO 9001 audit?
You need evidence that your measuring equipment is controlled and traceable, either a calibration certificate or a documented in-house check against a certified standard, logged and in date.
Does PROSTER sell an internal micrometer?
No, PROSTER's set is a 6-piece outside micrometer set; for internal work you will need a dedicated internal or bore tool alongside it.
How often should a micrometer be calibrated?
A common interval is at least annually, and after any drop or heavy use; the exact schedule should be set by your quality system and how hard the tool is worked.