Hand Tools, Measuring Instruments and Metrology
What this domain tests
Least count, correct reading of vernier and micrometer instruments, calibration, limits and fits, and selection of the right gauge.
Core rules
- Vernier least count = value of one main-scale division ÷ number of vernier divisions. A 49 mm main scale divided into 50 vernier divisions gives 0.02 mm.
- Micrometer least count = pitch ÷ thimble divisions = 0.5 mm ÷ 50 = 0.01 mm. Read barrel (including the half-millimetre line) then thimble.
- Standard reference temperature is 20 °C; steel expands about 11 × 10⁻⁶ per °C, so a 100 mm part at 30 °C is roughly 0.011 mm longer.
- Limit system: hole-basis is normal practice. H is the basic hole, h the basic shaft; H7/g6 is a clearance fit, H7/p6 an interference fit.
- GO gauges check the maximum material condition and must enter the full length; NO-GO gauges check the minimum material condition and should not enter.
- Slip gauges are wrung together; sine bar uses sin θ = h/L.
Worked example
A vernier caliper has least count 0.02 mm. The zero of the vernier is past 12 mm and the 23rd vernier division coincides with a main-scale line. Reading = 12 + (23 × 0.02) = 12.46 mm.
Micrometer check: barrel shows 7.5 mm and the thimble is on 32. Reading = 7.5 + (32 × 0.01) = 7.82 mm.
Traps that cost marks
- Missing the half-millimetre line exposed on the barrel.
- Ignoring zero error and its sign – subtract a positive zero error, add a negative one.
- Too much measuring force, or parallax when the eye is not square to the scale.
- Using a caliper to check a tolerance tighter than its least count.