GENERAL

This Section establishes the principles of tolerances of location. Included are position, concentricity, and symmetry used to control the following relationships:

(a) center distance between such features as holes, slots, bosses, and tabs;

(b) location of features [such as in (a) above] as a group, from datum features, such as plane and cylindrical surfaces;

(c) coaxiality of features;

(d) concentricity or symmetry of features — center distances of correspondingly-located feature elements equally disposed about a datum axis or plane.

5.2 POSITIONAL TOLERANCING

A positional tolerance defines:

(a) a zone within which the center, axis, or center plane of a feature of size is permitted to vary from a true (theoretically exact) position; or

(b) (where specified on an MMC or LMC basis) a boundary, defined as the virtual condition, located at the true (theoretically cxact) position, that may not be violated by the surface or surfaces of the considered feature.

Basic dimensions establish the true position from specified datum features and between interrelated features. A positional tolerance is indicated by the position symbol, a tolerance value, applicable material condition modifiers, and appropriate datum references placed in a feature control frame.

5.2.1 Method. The following paragraphs describe methods used in expressing positional tolerances.

5.2.1.1 Basic Dimensions and General Tolerances. The location of each feature (hole, slot, stud, etc.) is given by basic dimensions. Many drawings are based on a schedule of general tolerances, usually provided near the drawing title block. Dimensions locating true position must be excluded from the general tolerance in one of the following ways:

(a) applying the basic dimension symbol to each of the basic dimensions [see Figs. 5-1(a) and (b)J;

(b) specifying on the drawing (or in a document referenced on the drawing) the general note: UN-TOLERANCED DIMENSIONS LOCATING TRUE POSITION ARE BASIC. See Fig. 5-1(c).

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