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ISO fits explained

A shaft that must turn freely in a bore, a pin that must stay put once pressed in, a bearing seat that must locate without play: each is a fit between a hole and a shaft. ISO 286 gives these a shared code, so that "⌀25 H7/g6" means the same thing in every drawing office and workshop.

The two parts of ISO 286

  • ISO 286-1 sets out the system: standard tolerance grades, fundamental deviations, and how they combine into tolerance classes and fits.
  • ISO 286-2 tabulates the limit deviations of the tolerance classes for holes and shafts.

A tolerance class is a letter and a number

A tolerance class such as H7 or g6 has two parts:

  • The letter is the fundamental deviation: where the tolerance zone sits relative to the nominal size. Capital letters are for holes (internal features), lower-case letters for shafts (external features).
  • The number is the standard tolerance grade: how wide the zone is. ISO 286 defines 20 grades, from IT01 and IT0 to IT18; a higher number means a wider zone.

The width also depends on the size: the same grade allows more on a larger part. IT7 is 21 µm for nominal sizes over 24 up to 30 mm, and 35 µm over 80 up to 100 mm. All the values: ISO 286 standard tolerance grades.

Where the zone sits

  • Hole H: the lower deviation EI is zero, so the zone starts at the nominal size and goes up.
  • Shaft h: the upper deviation es is zero, so the zone ends at the nominal size and goes down.
  • Shafts a to g lie below the nominal size, shafts k to zc above it; holes A to G lie above it. The further the letter from h (or H), the further the zone from the nominal size.
  • js and JS are symmetric about the nominal size: plus and minus half the tolerance.

Capital ES and EI are the upper and lower deviations of a hole; lower-case es and ei those of a shaft.

Hole and shaft together: the fit

Put a hole class and a shaft class of the same nominal size together and you have a fit, written hole first: H7/g6. Depending on where the two zones lie, the fit is:

  • a clearance fit: there is always clearance; the smallest hole is at least as large as the largest shaft;
  • an interference fit: there is always interference; the largest hole is at most as large as the smallest shaft;
  • a transition fit: clearance or interference, depending on the actual sizes of the parts.

Most fits are chosen in the hole-basis system: every hole is H and the shaft letter selects the fit. The shaft-basis system does the opposite, with every shaft h. Hole-basis is often preferred because a hole is harder to make to many different sizes than a shaft; keeping it at H keeps the number of reamers and plug gauges down.

Worked example: ⌀25 H7/g6

At 25 mm (the size step over 24 up to 30 mm):

  • Hole 25 H7: EI = 0 µm, ES = +21 µm, so the hole is 25.000 to 25.021 mm.
  • Shaft 25 g6: es = −7 µm, ei = −20 µm, so the shaft is 24.980 to 24.993 mm.
  • The clearance is between 7 µm (smallest hole, largest shaft) and 41 µm (largest hole, smallest shaft): a clearance fit.

The same table for every size step: H7/g6. Other common fits: ISO 286 fits.

On the drawing

  • On a part drawing the class follows the size: 25 H7 on the hole, 25 g6 on the shaft. The deviations need not be written next to a class unless they are needed (see ISO 129-1 dimensioning).
  • On an assembly drawing the fit is written with the hole class first: 25 H7/g6.
  • Limits and fits refer to the standard reference temperature of 20 °C.

In MechPilot

Give a dimension a tolerance class such as H7 in the Properties panel and MechPilot writes the class and takes its deviations from its ISO 286 data (library/fits/iso-286.json, which cites ISO 286-1 and ISO 286-2 and says how each value was checked). A class that is not in that data is reported as such; it is never guessed.