done only when necessary and in conjunction with datum targets.
4.6.7 Datums Established From Complex or Irregular Surfaces. The datum feature symbol should be attached only to identifiable datum features. Where datums are established by targets on complex or irregular surfaces, the datum feature symbol is not required. See Fig. 4-39. In this example, although the datum targets establish a proper datum reference frame (A,B,C), no surface of the part can be identified as a datum feature. Where a datum reference frame has been properly established but its planes are unclear, the datum feature symbol may be applied to appropriate extension or center lines as needed.
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