Measurement, Inspection, Quality Control and Standards
Domain 5 tests measurement, inspection and quality decisions: choosing the right instrument, reading it correctly, comparing with wear limits, and recording results against standards.
Core rules
- Vernier caliper least count = 1 main-scale division − 1 vernier division; common metric LC is 0.02 mm. Micrometer LC is usually 0.01 mm; dial gauge 0.01 mm.
- Zero the instrument, check for parallax, measure at the same temperature as the specification, and take several readings.
- Wear limits are not the same as manufacturing tolerances; a part may be within new tolerance but beyond the service limit.
- Standards such as IS, ISO and OEM service limits define acceptance; calibration records prove an instrument is trustworthy.
- Record actual readings, not OK; inspection is evidence.
Worked example
A vernier caliper has a main scale in mm and 50 vernier divisions. LC = 1 mm ÷ 50 = 0.02 mm. The main scale reads 12 mm before zero, and the 7th vernier line coincides. Reading = 12 + (7 × 0.02) = 12.14 mm. Reasoning: the vernier line tells you the fraction of a main-scale division; you add it to the last main-scale mark passed by the zero.
Traps
Reading the wrong vernier line; ignoring zero error; using a caliper for a precision bore instead of a bore gauge; measuring a hot component; recording within limit without the number.