Measurement, Metrology and Quality Basics
This section covers Measurement, Metrology and Quality Basics: units, least count, vernier and micrometer instruments, tolerance and fits, calibration, sampling and quality tools. The exam tests correct reading and interpretation of inspection data.
Core rules
- Least count of a vernier caliper = 1 main-scale division − 1 vernier division. For a 0.02 mm caliper, 50 vernier divisions equal 49 main divisions.
- Micrometer least count = pitch/number of thimble divisions; a 0.5 mm pitch with 50 divisions gives 0.01 mm.
- Tolerance is the permitted variation; allowance is the intentional difference between mating parts. Clearance fit has positive clearance; interference fit has negative clearance.
- Calibration compares an instrument with a traceable standard. Zero error must be applied with the correct sign.
- Quality tools: Pareto prioritises few major causes; cause-and-effect (fishbone) explores causes; control charts separate common and special variation; 5 Why finds root cause.
Worked example
A vernier caliper has least count 0.02 mm. Main scale reads 12 mm and the 7th vernier line coincides. Reading = 12 + 7 × 0.02 = 12.14 mm. If the caliper has +0.03 mm zero error, subtract it: 12.14 − 0.03 = 12.11 mm.
Traps
Do not read the main scale at the vernier zero incorrectly. Do not add zero error when it should be subtracted. Do not confuse tolerance with allowance. Do not treat every control-chart point as a fault; look for trends and control limits.