This section describes the five methods of specifying a dimensional tolerance.
The different tolerance specification methods are discussed below.
Standard Tolerances
With this method, you leave the tolerance definition up to general specifications (you do not need to specify tolerances individually). The real precision is defined in the CAM system.
Symmetrical Tolerances
Symmetrical tolerance are normally used within the final machining specifications as its value specifies the most probable value of the machined parts. However, it is not easy to specify and understand plus/minus specifications in cases of tight and lose fits.
The plus/minus tolerance specification leads to a dimension label with the plus/minus specification.
If you switch from plus/minus tolerances to upper/lower or limit tolerances, the tolerance specification is taken as a default. For example, a value 70 with a +/- 0.5 tolerance will be transformed into a 70 [-0.5 | +0.5] tolerance respectively [ 69.5 | 70.5 ]. The square brackets are used in the mathematical sense of closed intervals as in 69.5 <= value <= 70.5.
Upper/Lower Tolerances
This tolerance type requests two individual delta values.
The upper/lower tolerance specification leads to a dimension label with the upper/lower specification.
Limit Tolerances
This tolerance type requests two limit values.
The limit tolerance specification leads to a dimension label with the limits specification.
ISO Tolerance Fields and Qualities
The most common method to specify tolerance values of parts fitting together is doing it on a quality level (for example g6/H7 where the shaft has a tolerance according to g6 and a hole a tolerance according to H7 ISO tolerance fields and quality. As the ISO tolerance specification is derived from the manufacturing effort, manufacturing a 20H7 will need similar effort as a 10H7 although 10H7 means 10.000 - 10.018 and 20H7 is 20.000 - 20.021 and not 20.000 - 20.036).
Creo Elements/Direct Machining knows the tolerance fields and tolerance qualities allowed by the ISO standard, but neither the knowledge that 10H7 means 10.000 - 10.018 has been implemented (due to copyright reasons) nor the fact that there are recommendations for using certain combinations of tolerances and avoid others completely.
