A pattern of holes CAN be specified as a datum feature. The concept is described in ASME Y14.5M-1994, paragraph 4.5.8 as follows:
"4.5.8 Pattern of Features. Multiple features of size, such as a pattern of holes at MMC, may be used as a group to establish a datum when part function dictates. See Fig. 4-22. In this case, individual datum axes are established at the true position of
each hole. These are the axes of true cylinders that simulate the virtual condition of the holes. When the part is mounted on the primary datum surface, the pattern of holes establishes the second and third datum planes of the datum reference frame. Where the secondary datum feature is referenced at MMC in the feature control frame, the axis of the feature pattern established by all the holes may depart from the axis of the datum reference frame as the datum feature departs from MMC."
The method for identifying the pattern as a datum feature on the field of the drawing is specified in paragraph 3.4.6 as follows:
"3.4.6 Combined Feature Control Frame and Datum Feature Symbol. Where a feature or pattem of features controlled by a geometric tolerance also serves as a datum feature, the feature control frame and datum feature symbol are combined. See
Fig. 3-23. ....."
Of course, the viewing the figures referenced in the paragraphs will also help out a lot. Hope this answers your questions.
GDTGUY
