Measurement uncertainty systematic and random errors
Resolution, Abbe's principle and why the temperature in the workshop matters
Quality control with precision measuring tools is one of the competence goals that runs through the entire education. If you can't measure correctly, you don't know if the workpiece meets the tolerance — and then all your machining is guesswork. The two workhorses are the calliper for quick, versatile measurements and the micrometer for the fine work where every hundredth counts. Knowing their resolution and limitations is as important as being able to read them.
§The sliding gauge: fast and versatile
The sliding gauge measures external and internal dimensions as well as depths. A digital sliding gauge typically has a resolution of 0.01 mm, while the classic vernier sliding gauge is often read to 0.05 mm. The sliding gauge is versatile but not the most accurate tool: the jaws are long and measuring pressure is difficult to keep uniform. For dimensions where tolerance is a few hundredths, the sliding gauge is not enough — a micrometer is needed. Use the sliding gauge for estimates and for measurements with ample tolerance.
§The micrometer and Abbe's principle
The micrometer in standard design has a scale division of 0.01 mm. The spindle normally has a pitch of 0.5 mm, and the drum is divided into 50, so one turn moves the spindle half a millimetre. The micrometer is more accurate than the calliper, among other things because it follows Abbe's principle: the measuring line is in line with the scale being read. On the other hand, one micrometer only covers 25 mm — there are separate 0–25, 25–50, 50–75 mm and so on. The ratchet knob ensures uniform measuring pressure so the reading is reproducible.
| Tools | Typical resolution | Best for |
|---|---|---|
| Vernier calliper | 0.05 mm | Quick estimates, coarser measurements |
| Digital slide gauge | 0,01 mm | Comprehensive dimensions with generous tolerance |
| Micrometer | 0,01 mm | Fine external dimensions, a few hundredths |
§The 20 degrees
Metal expands with heat. Therefore, the international reference temperature for length measurement is 20 °C (established in ISO 1). If you measure a hot workpiece right after machining, or a cold workpiece from the warehouse in winter, you get an incorrect measurement — and both workpiece and measuring tool can have different temperatures. At finer tolerances, you therefore let the workpiece equalize to room temperature before measurement, and the measurement room is kept close to 20 °C. It is an invisible source of error that has cost many good workpieces unnecessary rejection.
§Calibration and traceability
A measuring tool that is not calibrated is an opinion not a measurement. Hand measuring tools are regularly checked against gauge blocks or master gauges and professional workshops have their equipment calibrated with traceability back to the national standards — in Denmark through accredited laboratories under DANAK. Traceability means there is an unbroken chain of comparisons from your micrometer to the international metre. Without it you cannot vouch for a number.
- 01Always reset caliper and micrometer before use
- 02Use the ratchet button on the micrometer for consistent measuring pressure
- 03Clean both the item and measuring surfaces – a chip easily deceives a few hundredths
- 04Let the piece stabilize to 20 °C before final measurement
- 05Check measuring tool against a standard and have it calibrated at regular intervals
“You machine to the tolerance you can measure — not to the one you hope for.”