Tolerances and fits, explained

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Reading H7/g6

The capital letter is the hole, the lower-case letter the shaft. The letter sets the fundamental deviation: where the tolerance band sits relative to the nominal size. H puts the hole band starting exactly at nominal and going up; g puts the shaft band a little below nominal. The number is the IT grade, the width of the band. Lower grades are tighter and cost more to make.

Most drawings use the hole-basis system: the hole is always H, and the fit is chosen by changing the shaft letter. Holes are made with reamers and drills of fixed sizes, while a shaft can be turned or ground to any diameter, so holding the hole fixed keeps the tooling list short.

Clearance, transition, interference

Subtract the extremes. The largest gap is the biggest hole minus the smallest shaft; the smallest gap is the smallest hole minus the biggest shaft. If even the smallest gap is positive, every pair slides together: a clearance fit. If even the largest gap is negative, every pair must be pressed or shrunk: an interference fit. If the sign depends on which parts you pick, it is a transition fit.

Common choices in the hole-basis system: H7/g6 for a precise sliding fit, H7/h6 for a locating fit that still assembles by hand, H7/k6 for a transition fit that locates accurately, and H7/p6 for a light press fit that transmits load without a key.

Where the numbers come from

ISO 286 builds every tolerance from the standard tolerance unit i, which grows with the cube root of size, because larger parts are harder to hold to the same absolute accuracy. Each grade is a fixed multiple of i, and the tables use the geometric mean of each size step for D, so a 25 mm part takes its values from the 18 to 30 mm row. Use the tables for real work; the formula is how the tables were made.

A worked number

A 25 mm H7/g6 fit. From ISO 286, H7 for 18 to 30 mm is 0 to +21 µm, and g6 is −7 to −20 µm. Maximum clearance is 21 − (−20) = 41 µm. Minimum clearance is 0 − (−7) = 7 µm. Both positive, so it is a clearance fit: it always slides, never by more than 41 µm.

The formula agrees with the table: the geometric mean of 18 and 30 is about 23.2 mm, i ≈ 1.31 µm, and IT7 = 16i ≈ 21 µm.

Common mistakes

• Reading the letter as the size of the tolerance. The letter is the position of the band; the number is its width.
• Pairing extremes the wrong way round when computing clearance, which gives the opposite range.
• Specifying a tighter grade than the function needs. IT6 instead of IT8 can double the machining cost.