A fit describes the way two parts go together, typically a shaft in a bore. Depending on the difference between their actual dimensions, you get a clearance (the shaft rotates or slides), an interference (the shaft is fitted by force, by press or by thermal expansion) or a transition fit, which gives a small clearance or a small interference depending on the parts. As each part varies within its tolerance, the fit is defined by a minimum and maximum clearance or interference, never by a single value.
The ISO system (ISO 286) codes the fit by the position and extent of each part’s tolerance. A capital letter designates the bore (H…), a lower-case letter the shaft (g, h, k, p…), and the number that follows sets the extent of the tolerance. The most common case is the hole-basis system H, in which the minimum hole size is the nominal size. Example with H7/g6 on a 25 mm diameter: the H7 bore measures from 25.000 to 25.021 mm, the g6 shaft from 24.980 to 24.993 mm. The clearance is therefore between 7 and 41 µm: precise guidance, with no interference. With the same bore, k6 gives a transition fit and p6 an interference fit.
These values call for precise means. An H7 bore is obtained by boring or reaming, a g6 shaft by fine turning or grinding, and inspection is done with a plug gauge (go and no-go ends) or by direct measurement. Surface treatments add a thickness that uses up clearance: anodising a shaft, for example, reduces its clearance with the bore.
In a quote request
State the complete fit (for example Ø25 H7/g6) and not only isolated dimensions, specify the mating part if you are making it elsewhere, and say whether dimensions apply before or after treatment. Do not impose a tighter fit than needed: it makes the part more expensive. The guide on machining tolerances explains how to read ISO tolerances in detail.