AiVibeLimits and Fits (ISO 286)
Limits and fits is the ISO 286 system for specifying how mating holes and shafts are toleranced, using a letter for the fundamental deviation and a number for the tolerance grade, as in H7/g6, to give clearance, transition or interference fits.
In ISO 286, a tolerance class combines a fundamental deviation letter, capital for holes and lower case for shafts, with an international tolerance (IT) grade. The letter positions the tolerance zone relative to the nominal size and the grade sets its width. Combining a hole class with a shaft class defines a fit: clearance fits always leave a gap, interference fits always require force or heat to assemble, and transition fits may give either.
The hole-basis system, in which the hole uses the H deviation and the shaft class is varied, is the most common because holes are often made with fixed-size tools such as reamers. Typical examples are H7/g6 for a close sliding clearance fit, H7/k6 for a locational transition fit and H7/p6 for a locational interference fit. Bearing, bush, dowel, gear and coupling seats are specified this way, often following the bearing manufacturer's recommended shaft and housing classes.
ISO 286-2 tabulates the limit deviations for each class and size range. Fine grades such as IT6 and IT7 generally require precise processes such as boring, reaming or grinding. Surface roughness and temperature differences between parts change the effective fit, so they are considered for interference fits and precision assemblies.
Key points
- Tolerance classes combine a deviation letter with an IT grade
- Capital letters denote holes; lower-case letters denote shafts
- Fits are clearance, transition or interference
- Hole-basis fits such as H7/g6 are the most common
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory: edge devices plus AI agents that let people talk to CNC, PLC and robot controllers in plain language. Agents only propose changes, and a trained operator confirms each one.