The procedure can start with the part tolerances and work through to the fixture tolerances, or it start with a set of assumed or selected fixture tolerances, calculating the resulting part tolerances, and comparing these with the required tolerances from the part drawing.
Close tolerances are expensive, thus tolerances are always selected as wide as possible, consistent with the proper functions of the part. A part that functions with wide tolerances will also function with close tolerances, but not necessarily vice versa.
An initially selected tolerance that is found to be too wide can be reduced without risk. In the search for fixture tolerances it is therefore recommended to start with those dimensions that offer the best possibility of using wide tolerances.
Tolerances are needed on dimensions relating to the following fixture elements