Measuring Instruments and Workshop Practice
This section covers Measuring Instruments and Workshop Practice. The exam tests selection and reading of steel rules, vernier calipers, micrometers, try squares, feeler gauges and dial indicators, plus basic marking, datum and workshop discipline.
Core concepts
- A steel rule is a direct-reading tool; read it squarely to avoid parallax. Its smallest division is often 0.5 mm.
- Vernier caliper least count equals one main-scale division minus one vernier-scale division. A common metric caliper has 0.02 mm least count. Read main scale before the zero of the vernier, then add the coinciding vernier division multiplied by least count.
- A micrometer has least count 0.01 mm; read the sleeve and thimble. Use the ratchet stop to avoid over-tightening.
- A try square checks squareness, not length. A feeler gauge checks gaps such as valve clearances. A dial indicator shows small displacement or runout.
- Marking needs a datum, scriber, centre punch and marking blue. Punches should be struck vertically.
Worked example
A vernier caliper with 0.02 mm least count has its main-scale zero reading at 24 mm. The 13th vernier division coincides with a main-scale line. Measurement = main reading + (vernier division × least count) = 24 + (13 × 0.02) = 24.26 mm. The reasoning is to add a fraction of a millimetre; the vernier does not replace the main scale.
Traps that cost marks
Do not read the vernier division before the main scale. Check zero error before measuring. Do not use the outside jaws for an inside diameter, and do not force a micrometer thimble. A steel rule cannot reliably measure to 0.02 mm.