Tolerances

The tolerance on a dimension is the variation tolerated and may be considered as a numerical expression of the desired quality of workmanship. It is the difference between the high and low limits of size for that dimension. Thus, a part that should be exactly 25 mm nominal diameter will be accepted for a certain purpose if it is within the limits 25.1 mm (the high limit) and 24.9 mm (the low limit). The difference between the two (0.2 mm) is the tolerance.

FIGURE

The following table shows that tolerances may be stated in one of three ways, using a hole that has a nominal size of 100 mm diameter and a tolerance of 0.02 mm as an example.

TABLE

With sheet material (such as patch plates) used in certain repairs, the dimensions quoted in the repair scheme usually have a tolerance in one direction only, the nominal size being the lower limit. In effect, the patch plate must never be below the nominal size, although it can be slightly over in accordance with the repair scheme in the manual.