Fits and Clearances
Tolerance, allowance and the three fits
Every dimension on a drawing is a nominal size with an upper and a lower limit; the difference between the limits is the tolerance. Allowance is the intentional difference between the nominal sizes of the hole and the shaft (minimum clearance or maximum interference); clearance is the actual gap in an assembled pair. The three families are clearance fit (hole always larger than shaft), interference fit (shaft always larger, requiring force or a temperature difference) and transition fit (may be either).
The ISO limits-and-fits system uses the hole-basis system as the default: the hole is designated H with its lower deviation at zero, and the shaft letter sets the fit. H7/g6 is a close running fit, H7/h6 a sliding or location fit, H7/k6 a transition fit and H7/p6 an interference fit. The number is the international tolerance grade; larger numbers mean a wider tolerance band. Fit is not finish: a fine surface finish does not create a clearance, and an interference fit changes the bore size after pressing.
Traps: quoting allowance as clearance, forgetting that in a hole-basis system the hole is fixed and the shaft is varied, assuming a larger IT number means a tighter fit, and ignoring thermal expansion when measuring hot or cold parts.
Worked example: hole 25.000/25.021 mm, shaft 24.980/24.993 mm. Maximum clearance = 25.021 − 24.980 = 0.041 mm; minimum clearance = 25.000 − 24.993 = 0.007 mm. Both positive, so it is a clearance fit with 0.007 to 0.041 mm clearance.
Rule of thumb: always calculate worst-case maximum and minimum from the limits, never from one pair of measured parts.