104 Geometric Dimensioning and Tolerancing • The symbol can be placed on the outside surface of a cylindrical feature. • The symbol can be centered on the opposite side of the dimension line arrowhead, but it must line up and be centered with the arrowhead. • The symbol can replace the dimension line and arrowhead when the dimension line is placed outside of the extension lines. • The symbol can be placed on the dimension line. • The symbol can be placed on a leader line shoulder. • The symbol can be placed below and attached to the center of a feature control frame. Simulated Datum Axis The simulated datum axis is the axis of a perfectly cylindrical inspection device that contacts the datum feature surface. For an external datum feature, as shown in Example 3-30, the inspection device is the smallest circumscribed cylinder. The inspection device for an internal datum feature is the largest inscribed cylinder, as shown in Example 3-31. This application is valid only when datum features are specified at RMB. RMB is described in Chapter 4. For a review on simulated datums and datum feature simulators, refer to the discussion earlier in this chapter. Datum feature simulator Datum axis Datum feature (part) Simulated datum Smallest circumscribed cylinder Example 3-30. Simulated datum axis for an external datum feature only when a datum feature is specifi ed at RMB. Datum feature (part) Datum feature simulator Datum axis Simulated datum Largest inscribed cylinder Example 3-31. Simulated datum axis for an internal datum feature only when a datum feature is specifi ed at RMB.