Manufacturing Tolerances: The ISO System Explained

A manufacturing tolerance specifies the allowable deviation of a manufactured dimension from the nominal value. No manufacturing process can produce an exact dimension, there is always a variation. Tolerances define how large this variation may be for the part to function.

Tolerances are specified as:

• ± tolerance: e.g., 50 ±0.02 mm → dimension may be 49.98-50.02 mm
• ISO tolerance field: e.g., 50 H7 → defined via ISO 286 (tolerance class + position)
• General tolerance: e.g., DIN 2768-m → applies to all dimensions not individually toleranced

The international tolerance system according to ISO 286 defines tolerance classes through two elements:

• Tolerance field position (letter), Offset from the nominal dimension. Capital letters (H, G, F...) = holes. Lowercase letters (h, g, f...) = shafts.
• Tolerance grade IT (number), The width of the tolerance. IT1 = extremely tight, IT16 = very coarse.

A fit describes the dimensional relationship between two mating parts (hole + shaft). Depending on the application:

• Clearance fit (e.g., H7/f7), Always clearance, parts slide into each other. For plain bearings, guides.
• Transition fit (e.g., H7/k6), Clearance or interference possible. For gears on shafts, bearings with light seat.
• Interference fit (e.g., H7/p6), Always interference, parts must be pressed together. For fixed connections without screws.

The most common combination in mechanical engineering: H7/g6 (clearance fit with minimal clearance) and H7/p6 (interference fit).

Tolerances have a direct impact on manufacturing costs:

Practical tip: Only tolerance the dimensions tightly that functionally require it. All other dimensions should fall under the general tolerance DIN 2768-m. This saves significant manufacturing costs.