Tolerances and fits

Tolerance is how far off is still good

No part is made exactly to size, so the print says how much it may vary. 1.000 ±.005 means anything from .995 to 1.005 is good. The tolerance is the whole band: .010. Some prints give the two limits instead (1.005/.995), or only one direction (1.000 +.005/−.000).

Tighter tolerances cost more: more careful setups, more passes, better tools, more inspection. Hold what the print asks, not more.

Fits: how a hole and a shaft go together

A fit is the relationship between two mating sizes. With clearance the shaft always slides in. With interference the shaft is always bigger and has to be pressed or shrunk in. A transition fit can land either way.

ISO fits name each size by a letter and a number, like H7. The letter places the band: an H hole’s smallest size is exactly the nominal size. The number is the IT grade, the width of the band: the higher the number, the wider the tolerance. A shaft letter early in the alphabet (f, g) sits below the size and gives clearance; one late in it (p, s) sits above and gives interference.

So H7/g6 is a sliding fit for parts that must move and locate accurately, and H7/p6 is a press fit for parts that must stay put.