184.108.40.206 Identifying Features to Establish Datums. It is necessary to identify features on a part to establish datums for dimensions locating true positions. For example, in Fig. 5-2, if datum references had been omitted, it would not be clcar whether the inside diameter or the outside diameter was the intended datum feature for the dimensions locating true positions. The intended datum features are identified with datum feature symbols, and the applicable datum references are included in the feature control frame. For information on specifying datums in an order of precedence, see para. 4.4.
5.2.2 Application to Base Line and Chain Dimensioning. True position dimensioning can be applied as base line dimensioning or as chain dimensioning. For positional tolerancing, unlike plus and minus tolerancing as shown in Fig. 2-4, basic dimensions are used to establish the true positions of features. Assuming identical positional tolerances are specified, the resultant tolerance between any two holes will be the same for chain dimensioning as for base line dimensioning. This also applies to angular dimensions, whether base line or chain type.
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