Specifying Datum Features In An Order Of Precedence

Datum features must be specified in an order of precedence to position a part properly on the datum reference frame. Figure 4-2 illustrates a part where the datum features are plane surfaces. The desired order of precedence is indicated by entering the appropriate datum feature reference letters, from left to right, in the feature control frame. In Fig. 4-2(a), the datum features are identified as surfaces D, E, and F. These surfaces are most important to the design and function of the part, as illustrated by Fig. 4-2(b). Surfaces D, E, and F are the primary, secondary, and tertiary datum features, respectively, since they appear in that order in the feature control frame.

NOTE: When necessary to relate linear and angular dimensions to > datum reference frame, the desired order of precedence may be Indicated Sv • note wch at UNLESS OTHFRW15F SPfC«-

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