Measurement, Instrumentation and Quality Control
This domain tests instrument reading, limits and fits, gauge use, calibration and the statistical control of a process. Expect at least one direct reading question and one tolerance calculation.
Instruments
- Vernier caliper least count = 1 main-scale division − 1 vernier division; 50 vernier divisions on a 1 mm main scale give 0.02 mm.
- Micrometer: 0.5 mm spindle pitch with a 50-division thimble gives 0.01 mm; reading = barrel + thimble × 0.01.
- Dial gauge and dial test indicator typically resolve 0.01 mm and are used for runout and alignment.
Limits, fits and gauges
Tolerance = upper limit − lower limit. H7/h6 is a clearance fit on the hole basis, H7/p6 an interference fit, H7/k6 a transition. A GO gauge checks the maximum material limit and must enter; the NO-GO gauge checks the least material limit and must not enter.
Quality control
Calibration compares an instrument with a traceable standard and records the deviation. Control charts plot the mean and range; points outside the 3σ limits, or seven in a trend, signal an assignable cause. Cp = (USL − LSL)/6σ measures potential capability; Cpk accounts for centring.
Worked example. Main scale 23 mm, vernier line 12 coincident, least count 0.02 mm: reading = 23 + (12 × 0.02) = 23.24 mm. For a shaft marked Ø30 +0.021/0: tolerance = 0.021 mm = 21 µm.
Traps. Reading the vernier scale from the wrong zero; ignoring zero error; using a caliper with worn jaws for an inside measurement; swapping GO and NO-GO functions; mixing micrometres and millimetres in a tolerance answer.