Precision Measurement and Layout
Instruments and what each proves
Calipers (0.001 in / 0.02 mm resolution) suit general work; outside micrometers read to 0.0001 in; telescoping and small-hole gauges are transferred to a micrometer for bores; bore gauges read directly; dial indicators measure runout, TIR, and alignment; feeler gauges check clearances; height gauges, gauge blocks, and a surface plate support layout and inspection; a sine bar sets angles from sin θ = h/L; precision levels and laser systems establish flatness and elevation.
Resolution is not accuracy. A 0.0001 in micrometer with worn anvils is precise but wrong. Zero the instrument, check it against a known standard, and keep calibration traceable.
Error sources
Thermal expansion dominates: carbon steel grows about 6.5 µin/in/°F, so a 10 in part 100°F above the 68°F standard reads roughly 0.0065 in large. Parallax, excess measuring force, Abbe error (measurement axis offset from the scale axis), dirt, and burrs all matter at tenths.
Worked example
A shaft measures 2.4990 in with a micrometer; the matching bore, transferred from a telescoping gauge, measures 2.5005 in. Diametral clearance = 0.0015 in. If the specification is 0.0010–0.0020 in, the fit is acceptable - provided the gauge and micrometer were zeroed together and both parts are at room temperature.
Traps
Measuring a bore with a caliper; setting a dial indicator with its plunger not perpendicular to the surface; trusting a zero without checking; averaging away a burr instead of removing it.
Rule of thumb
Measure three places 120° apart and two axial positions, and record the worst case - not the average.