the preceding explanation, a positional tolerance of some magnitude is specified for the location of features. The application of MMC permits the tolerance to exceed the value specified, provided features are within size limits, and the feature locations are such as to make the part acceptable. However, rejection of usable parts can occur where these features are actually located on or close to their true positions, but produced to a size smaller than the specified minimum (outside of limits). The principle of positional tolerancing at MMC can be extended in applications where it is necessary to provide greater tolerance within functional limits than would otherwise be allowed. This is accomplished by adjusting the minimum size limit of a hole to the absolute minimum required for insertion of an applicable fastener located precisely at true position, and specifying a zero positional tolerance at MMC. In this case, the positional tolerance allowed is totally dependent on the actual mating size of the considered feature, as explained in para. 2.8.3.
184.108.40.206 Example of Zero Positional Toler-ance at MMC. Figure 5-10 shows a drawing of the sams part with a zero positional tolerance at MMC specified. Note that the maximum size limit of the clearance holes remains the same but the minimum was adjusted to correspond with a 14 mm diameter fastener. This results in an increase in the size tolerance for the clearance holes, the increase being equal to the conventional positional tolerance specified in Fig. 5-9. Although the positional tolerance specified in Fig. 5-10 is zero at MMC. the positional tolerance allowed is in direct proportion to the actual clearance hole size as shown by the following tabulation:
Clearance Hole Diameter
(Feature Actual Positional Tolcrancc _Mating Size) Diameter Allowed
5.3.4 RFS as Related to Positional Tolerancing. In certain cases, the design or function of a part may require the positional tolerance, datum reference, or both, to be maintained regardless of feature actual mating sizes. RFS, where applied to the positional tolerance of circular features, requires the axis of each feature to be located within the specified positional tolerance regardless of the size of the feature. This requirement imposes a closer control of the features involved and introduces complexities in verification.
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