Setting zero and then reading one full-range gauge block shows that those two stations agree with the reference. It does not show what the screw does in between. Sparber published a mid-range error pattern on his own micrometer. MeasureProof has not repeated that series on a 0–25 mm frame.
The claim that needs a table
A common shop habit is: clean the anvils, close to zero, then close on the largest convenient block in the range. If both look right, the instrument is treated as trustworthy. Sparber’s 2017 note is written against that habit. He treated zero plus a 1-inch block as necessary and then showed that it was not sufficient for the instrument in his hand.
That is a method lesson. It is not a statement that every 0–25 mm micrometer behaves the same way. A UK shop that wants the same point in millimetres still has to record its own mid-range blocks.
What Sparber actually wrote
The following figures are Sparber’s, in inches, on his instrument. They are not MeasureProof readings and they are not converted here into a British “typical error”.
| What he compared | What he reported |
|---|---|
| Steps from 0.1000 to 1.0000 inch | Reading errors from 0 to +0.0003 inch, plus an operator contribution he wrote as about ±0.0001 inch. |
| The same instrument, 0.5000–0.6000 inch, with stacked pads | Errors of 0.0004–0.0014 inch, with local minima near whole turns on his plot. |
He used that mid-range pattern to talk about screw and nut error, not to claim that he had completed an optical-parallel inspection. His pads are his reference, not a calibration-grade UKAS artefact. The inch numbers stay in inches because that is the form of the source.
If a conversion is needed only to picture the size of those figures: 0.0001 inch is 0.00254 mm exactly as an arithmetic identity (1 inch = 25.4 mm). That identity does not transfer his table onto a 0–25 mm micrometer in a UK shop.
What a millimetre record would have to show
To make the same argument for a 0–25 mm outside micrometer, a shop record needs at least:
- zero closure, recorded;
- a block near 25 mm, recorded;
- blocks that sit away from simple 5 mm steps, recorded on the same day;
- the same contact method at every station;
- temperature, block identity and whether any stack was wrung.
Machinist Guides’ 4:1 example — a 0.0001 inch instrument paired with a 0.000025 inch block, citing MIL-STD-45662 — is a US document habit. It is not rewritten here as a UK legal requirement. NPL GPG 40 is the UK starting point for how callipers and micrometers are used; a shop still needs its own blocks and its own numbers.
MeasureProof has not published a 0 / mid-range / 25 mm contrast table. Until that record exists, this page can only point at Sparber’s method and leave the millimetre column blank.
Sources and scope
Sources checked on 23 September 2026. Numbered references point to the original publishers. Their sample measurements are not MeasureProof measurements. Manufacturer and forum material is identified separately.
- Sparber, How Accurate is Your Micrometer? ↗Personal workshop PDF, version 3.3, 31 October 2017, CC BY 4.0. The inch table is the author’s own micrometer, not a MeasureProof test.
- Machinist Guides, Ultimate guide to micrometer calibration ↗Page dates: 18 April 2021 published, 1 March 2026 updated. Cites a 4:1 ratio from MIL-STD-45662. That is a US document, not a UK workshop statute.
- NPL Good Practice Guide 40, Callipers and micrometers ↗National Physical Laboratory. UK authority for use conditions, measuring force and temperature. It is not a workshop certificate.
No instrument in this note has been tested or certified by this publication. Evidence policy · Corrections
Keep the conditions with the reading.
The CSV has column headings and one empty row. It contains no fabricated sample readings.
Download workshop-check record ↓
