Measurement and Workshop Practice
What the exam tests
Measurement and workshop practice covers vernier calipers, micrometers, dial gauges, bore gauges, least count, zero error, limits, fits, tolerances and surface finish. You must read instruments correctly and interpret workshop drawings.
Core rules and concepts
- Least count is the smallest value an instrument can read. For a vernier, LC = 1 main-scale division - 1 vernier-scale division.
- Zero error must be added or subtracted with its sign.
- A micrometer uses a screw thread; LC is usually 0.01 mm.
- A dial gauge magnifies small linear movement; a bore gauge transfers bore diameter to a dial gauge.
- Tolerance is the permitted variation; clearance, interference and transition fits describe the relationship between mating parts.
- Surface roughness affects friction, wear and sealing.
Worked example
A vernier caliper has a main-scale reading of 12 mm and the 8th vernier division coincides. LC is 0.02 mm. Reading = 12 + (8 × 0.02) = 12.16 mm. If the caliper has a zero error of +0.03 mm, actual size = 12.16 - 0.03 = 12.13 mm. Always record the zero error before measuring and apply its sign correctly.
Traps that cost marks
- Reading the vernier before the main scale.
- Applying zero error with the wrong sign.
- Parallax error from viewing the scale at an angle.
- Confusing tolerance with allowance.