A datum is defined in ISO 5459:1981 [2] as:A theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which toleranced features are related. Datums may by based on one or more datum features of a part...
When measuring a part with a datum feature requirement that requires you to use a cylinder as the spatial orientation datum (base plane datum) there are several considerations you must keep in mind. The surface of the datum cylinder will never be ‘true’. When measuring a part surface form, waviness, and roundness deviations will cause some variation in the calculation of the cylinder thus changing the orientation of the cylinder.
The orientation is the angular deviation of 3d line through the center of the cylinder. It will be imperative that you take as many points when measuring a cylinder on a part that are equally distributed around each circle segment of the cylinder to create a more consistent best-fit cylinder.
A datum simulator is the best method to get consistent measurement values.
