Chapter 3 Datums 105 Coaxial Datum Features Coaxial means two or more cylindrical shapes that share a common axis. Coaxial datum features exist when a single datum axis is established by two datum features that are coaxial. When more than one datum feature is used to establish a single datum, the datum reference letters are separated by a dash and placed in one compartment of the feature control frame. These datum reference letters are of equal importance and can be placed in any order. See Example 3-32. A datum axis established by coaxial datum features is normally used as a primary datum. Simulated pair of coaxial circumscribed cylinders Datum feature A Datum feature B Datum axis A-B The Drawing The Meaning Example 3-32. Coaxial datum features. The Datum Axis of Screw Threads, Gears, and Splines When a screw thread is used as a datum axis, the datum axis is established from the pitch cylinder unless otherwise specified. If another feature of the screw thread is desired, then a note such as “MAJOR DIA” or “MINOR DIA” is placed next to the datum feature symbol. For example, the note “MAJOR DIA” is specified when the datum axis is established from the major diameter cylinder. A specific feature such as the major diameter should be identified when a gear or spline is used as a datum axis. When this is done, the note “MAJOR DIA,” “MINOR DIA,” or “PITCH DIA” is placed next to the datum feature symbol as appropriate. The use of a screw thread, gear, or spline should be avoided for use as a datum axis unless necessary.