Thus far we have discussed the planer datums, where a planer surface is used to establish a datum plane. Contrary to this, when a datum symbol is associated with a planer FOS or a cylindrical surface, the center-plane or the centerline of the FOS is used to establish the datum. Additionally, an MMC modifier can be used in the datum reference frame to specify the condition of the FOS at which the datum will be established. When this type of datum does not contain any modifier, by default RFS modifier is assumed. In some older drawings S symbol within a circle is used to denote RFS modifier. When RFS modifier exists by default or by (S) symbol, the datum axis or center-plane is established through physical contact between the datum feature surface and the inspection gage. Measuring devices, which are variable in size, such as chucks, collates or other centering devices are used to simulate the true geometric counterpart of the datum feature to establish the datum axis or center-plane. (next)

Figure 10. Internal and external center-plane datum

For example, for an internal planner feature-of-size (see figure Figure 10-A), or an external FOS (see Figure 10-B), the datum plane will be established by the center-plane of two parallel planes, which will contact the high points of the feature-of-size surfaces.(next)

Figure 11. Internal and external cylindrical surface datums

Similarly, for an exteranl FOS (Figure 11-A) or an internal FOS (Figure 11-B), the datum axis will be established by the axis of the smallest or largest cylinder touching the high points of the exteranl or internal surface, respectively. (next)

Figure 12. Application of 3 datums

Figure 12 illustrates an application, where the primary datum is a feature, and the secondary and tertiary datums are axis and center-plane datums respectively. Datum axis B will be established by a largest cylinder touching the high points of the hole and at the same time perpendicular with the datum plane A. Similarly, the datum center-plane C, will be established by the center-plane of two largest separated parallel planes, which are perpendicular to datum plane A and datum axis B simultaneously.