Hand tools, measuring instruments and metrology
Hand tools, measuring instruments and metrology questions test correct tool selection, safe use, and the reading of vernier, micrometer and dial instruments, together with calibration and traceability.
Hand tools
Use the exact spanner size; an adjustable or worn open-end spanner rounds hexagon heads. Ring and socket spanners carry higher torque safely. A torque wrench is set to the manufacturer's figure and returned to zero afterwards, never extended with a pipe. A puller grips the ring being removed - the inner ring when removing a bearing from a shaft - and never pulls through the rolling elements; soft jaws or a brass drift protect machined faces. Tap drill size is approximately D − pitch, so M10×1.5 uses 8.5 mm. Circlip pliers match the eye size, and the clip's flat side takes the load.
Metrology
Vernier caliper 0.02 mm, micrometer 0.01 mm, dial test indicator 0.01 mm, bore gauge, feeler and thread gauges, gauge blocks and sine bar. Measurements are valid at 20 °C, with the instrument cleaned, zeroed, and a micrometer's ratchet used to control measuring force. Calibration certificates provide traceability; an out-of-calibration instrument invalidates the inspection result.
Reading the scales
On a 0.02 mm caliper, if the main scale reads 32 mm and the seventh vernier line coincides, the reading is 32 + 7 × 0.02 = 32.14 mm. On a micrometer, barrel 7.5 mm plus thimble 28 × 0.01 = 0.28 mm gives 7.78 mm; always check whether the half-millimetre line on the barrel is visible before reading the thimble.
Worked example
A shaft gives micrometer readings 24.98, 24.99 and 24.99 mm against a drawing value of Ø25 h6, whose limits are 24.987–25.000 mm. Since 24.98 mm is below 24.987 mm the shaft is undersize, even though it looks like 25 mm. The reasoning: compare with the calculated limits, not the nominal size, and repeat readings to reveal a trend rather than a single slip.
Traps
Using a caliper for tolerance-grade work, parallax when reading a vernier, measuring a rotating part, checking zero with dirty jaws, and forcing the thimble instead of using the ratchet.