Where features of a part have been identified as datum features, the part is oriented and immobilized relative to the three mutually perpendicular planes of the datum reference frame in a selected order of precedence. This in turn makes the geometric relationships that exist between the features measurable. A true geometric counterpart of a feature used to establish a datum may be:
(b) a maximum material condition boundary (MMC concept);
(c) a least material condition boundary (LMC concept);
(d) a virtual condition boundary;
(ej an actual mating envelope;
if) a mathematically defined contour.
4.2.1 Application. As measurements cannot be made from a true geometric counterpart that is theoretical, a datum is assumed to exist in and be simulated by the associated processing equipment. For example, machine tables and surface plates, though not true planes, are of such quality that the planes derived from them are used to simulate the datums from which measurements are taken and dimensions verified. See Fig. 4-10. Also, for example, ring and plug gages, and mandrels, though not tme cylinders, are of such quality that their axes are used as simulated datums from which measurements are taken and dimensions verified. See Figs. 4-11 and 4-12. When magnified surfaces of manufactured parts are seen to have irregularities, contact is made with a simulated datum at a number of surface extremities or high points.
4.2.2 Datum Reference Frame. Sufficient datum features, those most important to the design of a part, or designated portions of these features are chosen to position the part in relation to a set of three mutually perpendicular planes, jointly called a datum reference frame. This reference frame exists in theory only and not on the part. Therefore, it is necessary to establish a method of simulating the theoretical reference frame from the actual features of the part. This simulation is accomplished by positioning specifically identified features in contact with appropriate datum simulators, in a stated order of precedence, to restrict motion of the part and to relate the part adequately to the datum reference frame. See Fig. 4-1.
188.8.131.52 Mutually Perpendicular Planes. The planes of the datum reference frame are simulated in a mutually perpendicular relationship to provide direction as well as the origin for related dimensions and measurements. Thus, when the part is positioned on the datum reference frame (by physical contact between each datum feature and its counterpart in the associated processing equipment), dimensions related to the datum reference frame by a feature control frame or note are thereby mutually perpendicular. This theoretical reference frame constitutes the three-plane dimensioning system used for datum referencing.
184.108.40.206 Number of Datum Reference Frames. In some cases, a single datum reference frame will suffice. In others, additional datum reference frames may be necessary where physical separation or the functional relationship of features requires that datum reference frames be applied at specific locations on the part. In such cases, each feature control frame must contain the datum feature references that are applicable. Any difference in the
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