Cylindrical Feature at RFS Primary

In Fig. 4-18(b), diameter A is the primary datum feature and RFS is applied; surface B is the secondary datum feature. The datum axis is the axis of the smallest circumscribed cylinder that contacts diameter A — that is, the actual mating envelope of diameter A. This cylinder encompasses variations in the size of A within specified limits. However, any variation in perpendicularity between surface B and diameter A, the primary datum feature, will affect the degree of contact of surface B with its datum plane.

4.5.6.2 Surface Primary. In Fig. 4- 18(c), surface B is the primary datum feature; diameter A is the secondary datum feature and RFS is applied. The datum axis is the axis of the smallest circumscribed cylinder that contacts diameter A and is perpendicular to the datum plane — that is, the actual mating envelope of a diameter that is perpendicular to datum plane B. In addition to size variations, this cylinder encompasses any variation in perpendicularity between diameter A and surface B, the primary datum feature.

4.5.6.3 Cylindrical Feature at MMC Secondary. In Fig. 4-18(d), surface B is the primary datum feature; diameter A is the secondary datum feature and MMC is applied. The datum axis is the axis of a virtual condition cylinder of fixed size that is peipendicular to the datum plane B. Variations in the size and perpendicularity of datum feature A are permitted to occur within this cylindrical boundary. Furthermore, as the actual mating envelope of datum feature A departs from its MMC size, a displacement of its axis relative to the datum axis is allowed. See para. 5.3.2.2.

4.5.7 Multiple Datum Features. Where more than one datum feature is used to establish a single datum, the appropriate datum reference letters and

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