Smallest pair of coaxial circumscribed cylinders-
ders. as explained in para. 4.5.3(a). A datum axis established by coaxial datum features is normally used as a primary datum. For one possible method of controlling the coaxiality of these diameters, see para. 188.8.131.52.4
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.
4.5.9 Screw Threads, Gears, and Splines.
Where a screw thread is specified as a datum refer ence, the datum axis is derived from the pitch cylinder, unless otherwise specified. See para. 2.9. Where a gear or spline is specified as a datum reference, a specific feature of the gear or spline must be designated to derive a datum axis. See para. 2.10. In general. these types of datum features should be avoided.
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