Fig Zero Positional Tolerancing At Mmc For Coaxiality

congruent with the axis (or center point) of a datum feature.

5.12.1 Concentricity Tolerancing. A concentricity tolerance is a cylindrical (or spherical) tolerance zone whose axis (or center point) coincides with the axis (or center point) of the datum feature(s). The median points of all correspondingly-located elements of the feature(s) being controlled, regardless of feature size, must lie within the cylindrical (or spherical) tolerance zones. The specified tolerance and the datum reference can only apply on an RFS basis. See Fig. 5-54. Unlike the control covered by para. 5.11.1, where measurements taken along a surface of revolution are made to determine the location (eccentricity) of the axis or center point of the actual mating envelope, a concentricity tolerance requires the establishment and verification of the feature's median points.

NOTE: Irregularities in the form of an actual feature k> be inspected may make it difficult to establish the location of that feature's median points. For example, a nominally cylindrical surface of revolution may be bowed or out of round in addition to being displaced from its datum axis. In such instances, finding the median points of the feature may entail a time-consuming analysis of surface variations. Therefore, unless there is i definite need for the control of the feature's median points, it ii recommended that a control be specified in terms of a runout tolerance or a positional tolerance.

5.12.2 Difference Between Coaxlality Controls and Concentricity. The items shown in Figs. 5-56 and 5-57 are two possible acceptable configurations of the item depicted in Fig. 5-55. In Fig. 5-56, the axis of the controlled feature's actual mating envelope has been displaced 0.2 to the left, relative to the axis of datum feature A, and 0.5 material has been removed from the right side of the feature's surface. In Fig. 5-57, the axis of the controlled feature's actual mating envelope has also been displaced 0.2 to the left, relative to the axis of datum feature A, while 0.25 material has been removed from the upper side of the feature's surface and 0.25 material has been removed from the lower side of the feature's surface. Since the actual mating size of the controlled features in Figs. 5-56 and 5-57 is 25.0 diameter, the controlled features remain within acceptable limits of size. For coaxial positional tolerance, the location of the axis of the feature's actual mating envelope is controlled relative to the axis of the datum feature. Where checked for a coaxial positional tolerance relationship, the items depicted in Figs. 5-56 and 5-57 are acceptable. For concentricity, the locations of the midpoints of diametrically opposed (or correspondingly-located) feature elements are controlled relative to the axis of the datum feature.

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