Effect of form errors in datum features on evaluated geometries

In coordinate metrology a common task is to align the frame of reference of the workpiece, as represented by the coordinates gathered by a coordinate measuring system, with that associated with its specification. The positions of geometric features of a work piece are often specified relative to datum features that define an axis system or frame of reference for the work piece. for an ideal geometry and ideal measuring system, subsequent calculations will not depend on which datum features are used to define the frame of reference. For real geometries, the presence of form error associated with the datum surfaces means that different measurement strategies will lead to different definitions of the frames of reference and therefore different estimates of the positions of other geometric features associated with the work piece. In this paper we define approaches for evaluating the contribution of form error associated with datum features to the uncertainties associated with geometric features derived from coordinate data.