Fig Positional Tolerancing At Mmc Relative To Datum Feature Center Planes

5.3.1 Material Condition Basis. Positional tol-erancing is applied on an MMC, RFS, or LMC basis. When MMC or LMC is required, the appropriate modifier follows the specified tolerance and applicable datum reference in the feature control frame. See para. 2.8.

5.3.2 MMC as Related to Positional Toler-ancing. The positional tolerance and maximum material condition of mating features are considered in relation to each other. MMC by itself means a feature of a finished product contains the maximum amount of material permitted by the toleranced size dimension for that feature. For holes, slots, and other internal features, maximum material is the condition where these features are at their minimum allowable sizes. For shafts, bosses. lugs, tabs, and other external features, maximum material is the condition where these features are at their maximum allowable sizes.

5.3.2.1 Explanation of Positional Tolerance at MMC. A positional tolerance applied at MMC may be explained in either of the following ways.

(a) In Terms of the Surface of a Hole. While maintaining the specified size limits of the hole, no element of the hole surface shall be inside a theoretical boundary located at tnie position. See Fig. 5-5.

(b) In Terms of the Axis of a Hole. Where a hole is at MMC (minimum diameter), its axis must fall within a cylindrical tolerance zone whose axis is located at true position. The diameter of this zone is equal to the positional tolerance. See Figs. 5-6(a) and (b). This tolerance zone also defines the limits of variation in the attitude of the axis of the hole in relation to the datum surface. See Fig. 5-6(c). It is only where the hole is at MMC that the specified tolerance zone applies. Where the actual mating size of the hole is larger than MMC, additional positional tolerance results. See Fig. 5-7. This increase of positional tolerance is equal to the difference between the specified maximum material condition limit of size (MMC) and the actual mating size of the hole. Where the actual mating size is larger than MMC, the specified positional tolerance for a hole may be exceeded and still satisfy function and interchange-ability requirements.

NOTE: In certain cases of extreme form deviation (within limits of iize) or orientation deviation of the hole, the tolerance in terms of the axis may not be exactly equivalent to the tolerance in terms of the surface. In such cases, the surface interpretation shall take precedence.

5.3.2.2 Displacement Allowed by Datum Features at MMC. In many instances, a group of features (such as a group of mounting holes) must

Datum Target Center Plane

Theoretical boundary-(Virtual condition) minimum diameter of hole (MMC) minus the positional tolerance.

Hole position may vary but no point on its surface shall be

Inside theoretical boundary.

True position

Theoretical boundary-(Virtual condition) minimum diameter of hole (MMC) minus the positional tolerance.

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