gaging tolerances apply. Figure 4-30 illustrates a part where datum target points are located by means of basic dimensions. In this example, the three mutually perpendicular planes of the datum reference frame are established by three target points on the primary datum feature, two on the secondary, and one on the tertiary.
4.6.3 Datum Planes Established by Datum Targets. A primary datum plane is established by at least three target points or areas not on a straight line. See Fig. 4-31. A secondary datum plane is usually established by two targets. A tertiary datum plane is usually established by one target. A combination of target points, lines, and areas may be used. See Fig. 4-32. For irregular or step surfaces, the datum plane should contain at least one of the datum targets.
220.127.116.11 Stepped Surfaces. A datum plane may also be established by targets located on step surfaces, as in Fig. 4-33. The basic dimension defines the offset between the target points (the toleranced linear dimension in this example controls the dis tance between the surfaces). Profile tolerancing may be used on the offset surface in lieu of the toleranced dimension and dimension origin symbol. Curved or free-form surfaces may require datum planes completely offset from the datum targets. See Fig. 4-39.
4.6.4 Primary Datum Axis. Two sets of three equally spaced targets may be used to establish a datum axis for a primary datum feature (RFS). See Figs. 4-34 and 4-35. The two target sets are spaced as far apart as practicable and dimensioned from the secondary datum. The centering device used to establish the datum axis has two sets of three equally spaced contacting features capable of moving radially at an equal rate from a common axis. Where two cylindrical datum features are used to establish a datum axis, as in Fig. 4-35, each datum feature is identified with a different datum feature symbol. Each target set contains different datum identifying letters.
Was this article helpful?