## Parts With Cylindrical Datum Features

A cylindrical datum feature is always associated with two theoretical planes intersecting at right angles on the datum axis. The datum of a cylindrical surface is the axis of the true geometric counterpart of the datum feature (for example, the actual mating envelope or the virtual condition boundary), and simulated by the axis of a cylinder in the processing equipment. This axis serves as the origin of measurement from which other features of the part are located. See Figs. 4-5. 4-11, and 4-12.

4.4.2.1 Cylindrical Datum Feature. Figure 4-5 illustrates a part having a cylindrical datum feature.

Primary datum feature K relates the part to the first datum plane. Since secondary datum feature M is cylindrical, it is associated with two theoretical planes, the second and third in a three-plane relationship.

4.4.2.2 Datum Axis and Two Planes. These two theoretical planes are represented on a drawing by center lines crossing at right angles, as in Fig. 4-5(a)- The intersection of these planes coincides with the datum axis. See Fig. 4-5(b). Once established, the datum axis becomes the origin for related dimensions while the second and third planes indicate the direction of measurements.

4.4.2.3 Orientation of Two Planes. In Fig. 4-5, the rotational orientation of the second and third planes of the datum reference frame is not specified, as rotation of the pattern of holes about the datum axis has no effect on the function of the part. In such cases, only two datum features are referenced in the feature control frame:

(a) primary datum feature K, that establishes a datum plane; and

(b) secondary datum feature M, that establishes a datum axis perpendicular to datum plane K. This axis is the intersection of the second and third datum planes.

4.4.3 Rotational Orientation. To establish rotational orientation of two planes about a datum axis, a third or tertiary datum feature is referenced in the feature control frame.

(a) Figure 4-6 illustrates rotational orientation of the two planes intersecting through shaft B, the secondary datum feature, established by the center plane of slot C, the tertiary datum feature. Figure 4-7 iDus-trates the development of the theoretical datum reference frame for the positional tolerance of the three holes in Fig. 4-6.

(b) Figure 4-8 illustrates rotational orientation of the two planes intersecting through hole B, the secondary datum feature. Orientation is established by the width of hole C, the tertiary datum feature. Figure 4-9 illustrates the development of the theoretical datum reference frame for the positional tolerance of the other holes applied in Fig. 4-8.