Fits, Tolerances and Metrology
Fits, Tolerances and Metrology
This domain tests the ISO limits-and-fits system and the correct use of measuring instruments: vernier caliper, micrometer, dial indicator, slip gauges, sine bar and comparators. Expect a reading calculation, a fit interpretation, or a question on calibration and standard temperature (20 °C).
Limits and fits
- IT grades set tolerance size; IT01 to IT4 are gauge work, IT5–IT7 precision fits, IT8–IT11 general machining.
- Hole-basis system: the hole is H (lower deviation zero) and the shaft letter sets the fit. a–g gives clearance, h–js approximately location, k–n transition, p–zc interference.
- Allowance is the intentional difference between the hole minimum and shaft maximum; clearance can be positive or negative.
Measuring instruments
Vernier least count = one main-scale division divided by number of vernier divisions. Micrometer least count = pitch divided by thimble divisions, typically 0.5 mm / 50 = 0.01 mm. Slip gauges are wrung together to build a dimension; a sine bar gives angle from sin θ = height / length between rollers. Calibration compares an instrument with a traceable standard.
Worked example A - vernier reading
Main scale 0.5 mm per division, vernier 25 divisions, so LC = 0.5/25 = 0.02 mm. Main scale reads 12.5 mm and the 13th vernier division coincides: 13 × 0.02 = 0.26 mm, giving 12.76 mm. Zero error is +0.03 mm, so the true size is 12.76 − 0.03 = 12.73 mm.
Worked example B - fit
For 25 H7/g6: hole limits +0.021/0, shaft −0.007/−0.020. Maximum clearance = 0.021 + 0.020 = 0.041 mm; minimum clearance = 0 + 0.007 = 0.007 mm. It is a clearance fit.
Traps
Reversing the zero-error sign; measuring a warm workpiece; using a caliper for IT6 work; forgetting parallax and ratchet slip on a micrometer.