Measurement and Workshop Practice
What is tested
Measurement and Workshop Practice tests instrument selection, least count, zero error, calibration and correct reading. Expect vernier calipers, micrometers, dial gauges, steel rules, feeler gauges and limit gauges.
Core rules
- Least count of a metric vernier caliper is the difference between one main-scale division and one vernier division, commonly 0.02 mm. A metric micrometer reads 0.01 mm per thimble division.
- Check zero before use. Record zero error with its sign and subtract it from the reading.
- Measure at the correct location and with light contact; a micrometer has a ratchet or friction thimble to prevent over-tightening.
- Avoid parallax by reading the scale squarely. Let the workpiece and instrument stabilise to room temperature for precision work.
- Calibration is comparison against a known standard; a certificate alone does not prove the instrument is currently accurate.
Worked example
A vernier caliper has a least count of 0.02 mm. The main scale shows 12 mm before the vernier zero, and the 7th vernier line coincides with a main-scale line. Reading = 12 + (7 × 0.02) = 12.14 mm. The reasoning is that each vernier division represents 0.02 mm, so the coinciding line tells you the fractional part. If the instrument had a zero error of +0.03 mm, the corrected size would be 12.14 − 0.03 = 12.11 mm.
Traps
Reading the wrong vernier line, forgetting the sign of zero error, and using the caliper jaws as a wrench or scriber. Over-tightening a micrometer gives a small false reading and damages the thread. Also, a dial gauge must be mounted rigidly; a loose stem makes every reading unreliable.