Limits and fits
A tolerance class is a letter (the fundamental deviation, i.e. the position of the zone) and a number (the IT grade, i.e. the size of the zone). Capital letters are holes, small letters shafts (ISO 286).
Deviations
- a…h shafts lie below the zero line (upper deviation es; h: es = 0);
- js is symmetric, ±IT/2;
- j…zc lie above (lower deviation ei).
Holes mirror shafts: EI = −es for A…H, and the special rule ES = −ei + Δ for K…N up to IT8 and P…ZC up to IT7.
Fits
max clearance = ES − ei, min clearance = EI − es (negative means interference). Fit tolerance = hole tolerance + shaft tolerance.
Example: Ø40 H7/k6
H7: 0 / +25 µm; k6: +2 / +18 µm. Maximum clearance 23 µm, maximum interference 18 µm — a transition fit, hole-basis.