Form and orientation tolerances critical to function and interchangeability are specified where the tolerances of size and location do not provide sufficient control. A tolerance of form or orientation may be specified where no tolerance of size is given, for example, in the control of flatness after assembly of the parts.

6.3.1 Form and Orientation Tolerance Zones.

A form or orientation tolerance specifies a zone within which the considered feature, its line elements. its axis, or its center plane must be contained. Cylindrical Tolerance Zone. Where the tolerance value represents the diameter of a cylindrical zone, it is preceded by the diameter symbol. In all other cases, the tolerance value represents a total linear distance between two geometric boundaries and no symbol is required. Limited Area and Length. Certain designs require control over a limited area or length of the surface, rather than control of the total surface. In these instances, the area or length, and its location are indicated by a heavy chain line drawn adjacent to the surface with appropriate dimensioning. Where so indicated, the specified tolerance applies within these limits instead of to the total surface. See para. 4.5.10 and Fig. 4-23. Identifying Datum References. It is necessary to identify features on a part to establish datums from which dimensions control orientation, runout, and when necessary, profile. For example, in Fig. 6-22, if datum references had been omitted, it would not be clear whether the larger diameter or the smaller diameter was the intended datum feature for the dimensions controlling profile. 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.

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