Datum Targets

Datum targets designate specific points, lines, or areas of contact on a part that are used in establishing a datum reference frame. Because of inherent irregularities, the entire surface of some features cannot be effectively used to establish a datum. Examples are nonplanar or uneven surfaces produced by casting, forging, or molding; surfaces of weldments; and thin-section surfaces subject to bowing, warping, or other inherent or induced distortions. Datum targets and datum features (as described earlier) may be combined to establish a datum reference frame.

4.6.1 Datum Target Symbols. Points, lines, and areas on datum features are designated on the drawing by means of a datum target symbol. See Fig. 3-6. The symbol is placed outside the part outline with a radial (leader) line directed to the target. The use of a solid radial (leader) line indicates that the datum target is on the near (visible) surface. The use of a dashed radial (leader) line, as in Fig. 4-38, indicates that the datum target is on the far (hidden) surface. The datum feature itself is usually identified with a datum feature symbol.

4.6.1.1 Datum Target Points. A datum target point is indicated by the target point symbol, dimen-sionally located on a direct view of the surface. Where there is no direct view, the point location is dimensioned on two adjacent views. See Fig. 4-27.

4.6.1.2 Datum Target Lines. A datum target line is indicated by the target point symbol on an edge view of the surface, a phantom line on the direct view, or both. See Fig. 4-28. Where the length of the datum target line must be controlled, its length and location are dimensioned.

4.6.1.3 Datum Target Areas. Where it is determined that án area or areas of contact is necessary to assure establishment of the datum (that is, where spherical or pointed pins would be inadequate), a target area of the desired shape is specified. The datum target area is indicated by section lines inside a phantom outline of the desired shape, with controlling dimensions added. The diameter of circular úreas is given in the upper half of the datum target symbol. See Fig. 4-29(a). Where it becomes impracticable to delineate a circular target area, the method of indication shown in Fig. 4-29(b) may be used.

4.6.2 Datum Target Dimensions. The location and size, where applicable, of datum targets are defined with either basic or toleranced dimensions. If defined with basic dimensions, established tooling or

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