More than one datum reference frame may be necessary for certain parts, depending upon functional requirements. In Fig. 4-24, datum features A, B, and C establish one datum reference frame, while datum features D, B, and C and datum features D, E, and B establish different datum reference frames.
18.104.22.168 Functional Datum Features. Only the required datum features should be referenced in feature control frames when specifying geometric tolerances. An understanding of the geometric control provided by these tolerances (as explained in Sections 5 and 6) is necessary to determine effectively the number of datum references required for a given application. Additionally, functional requirements of the design should be the basis for selecting the related datum features to be referenced in the feature control frame. Figure 4-25 illustrates a part where three geometric tolerances are specified, each having the required number of datum references. Although common datum identifying letters appear in each frame, each combination is a different and independent requirement.
4.5.12 Simultaneous Requirements. Where two or more features or patterns of features are located by basic dimensions related to common datum
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