Tools and Measuring Instruments
Tool and instrument questions test least count, reading technique, zero error and instrument selection. Match the instrument to the tolerance: steel rule for rough work, vernier for 0.02 mm, micrometer for 0.01 mm, dial gauge for deflection or runout, sine bar and slip gauges for angles.
Least count and reading
- Vernier: LC = smallest main-scale division ÷ number of vernier divisions. Common value is 0.02 mm with 50 divisions spanning 49 mm.
- Micrometer: LC = pitch ÷ thimble divisions = 0.5 mm ÷ 50 = 0.01 mm.
- Reading = main scale + (coinciding division × LC) − zero error, keeping the sign.
Selection and care
- Calipers measure outside, inside, depth and steps; micrometers are for precision outside dimensions; bore gauges for holes; go/no-go gauges check limits quickly.
- Clean the jaws, use the ratchet stop, avoid parallax, do not use calipers on hot or rough castings, and store instruments separately.
Worked example
A vernier has LC 0.02 mm. The main scale before zero reads 24 mm and the 13th vernier line coincides. Reading = 24 + 13 × 0.02 = 24.26 mm. If the closed-jaw zero error is +0.04 mm, true size = 24.26 − 0.04 = 24.22 mm. If the zero error were −0.04 mm, true size = 24.26 + 0.04 = 24.30 mm.
Traps
- Adding zero error instead of subtracting it.
- Counting the vernier zero instead of the coinciding line.
- Using a micrometer on a rotating workpiece.
- Reading a steel rule from an angle, causing parallax.
- Assuming a caliper is accurate enough for a 0.01 mm tolerance.