Measuring Instruments and Metrology
What this domain tests
Measuring Instruments and Metrology tests selection, reading, calibration and care of workshop instruments: vernier caliper, micrometer, dial indicator, height gauge, slip gauges, sine bar, and comparators. It also checks your understanding of least count, zero error, accuracy versus precision, and inspection of limits and fits.
Core rules
- Least count (LC) = smallest value an instrument can read. For a standard vernier caliper with 50 divisions on the vernier matching 49 main-scale divisions: LC = 1 MSD − 1 VSD = 1 mm − 0.98 mm = 0.02 mm.
- Micrometer: pitch 0.5 mm, thimble 50 divisions, LC = 0.5/50 = 0.01 mm. Read the sleeve main scale, then the thimble division, then add or subtract zero error.
- Slip gauges are wrung together to build a dimension; select the minimum number of blocks, largest first, and never use them as a hammer or leave them wrung.
- A sine bar gives angle by sin θ = h/L; it needs slip gauges and a surface plate.
- Calibrate against a known standard and record temperature; steel expands about 11–12 µm/m per °C, so a 5 °C error matters on 500 mm.
Worked example
A vernier caliper has LC 0.02 mm. Main scale reads 24 mm; the 21st vernier division coincides. Reading = 24 + 21 × 0.02 = 24.42 mm. If the jaws are closed and the instrument reads +0.06 mm, subtract 0.06 from every reading: true size = 24.36 mm.
Traps
Using the wrong LC; ignoring zero error; reading the thimble in the wrong direction; measuring a rotating part; using a caliper where a micrometer is needed; forgetting that a dial indicator measures variation, not absolute size.