Ordliste
Tolerance class — definition
A standardised class stating how much dimensional deviation is permitted, graded by the size of the dimension.
A tolerance class is not a single number but a table: the larger the dimension, the greater the deviation permitted within the same class. That is logical, because shrinkage and die wear act proportionally.
For castings, NS-EN ISO 8062-3 applies, working with DCTG grades. A lower grade number means a tighter tolerance.
For machined dimensions, ISO 2768 applies, with the general classes fine, medium, coarse and very coarse.
A common source of misunderstanding is applying the same tolerance requirement to the whole part. That is neither necessary nor cheap. Specify a tight tolerance where the function requires it, and let the rest follow the general class.