Positional tolerancing for symmetrical relationships is that condition where the center plane of the actual mating envelope of one or more features is congruent with the axis or center plane of a datum feature within specified limits. MMC, LMC, or RFS modifiers may be specified to apply to both the tolerance and the datum feature.
5.13.1 Positional Tolerancing at MMC for As-ssmblability. A symmetrical relationship may be controlled by specifying a positional tolerance at MMC as in Fig. 5-59. The explanations given in paras. 5.10.1(a) and (b) apply to the considered feature. The datum feature may be specified either on an MMC, LMC, or RFS basis, depending upon the design requirements.
5.13.2 Zero Positional Tolerancing at MMC for Symmetrical Relationships. Where it is necessary to control the symmetrical relationship of related features within their limits of size, a zero positional tolerance at MMC is specified. The datum feature is normally specified on an MMC basis. Boundaries of perfect form are thereby established that are truly symmetrical where both features are at MMC. Variations in position are permitted only where the features depart from their MMC size toward LMC. This application is the same as that shown in Fig. 5-50 except that it applies a tolerance to a center plane location.
5.13.3 Positional Tolerancing RFS for As-semblability. Some designs may require a control of the symmetrical relationship between features regardless of their actual sizes. In such cases, both the specified positional tolerance and the datum reference apply on an RFS basis. See Fig. 5-60.
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