We present an automatic algorithm of digitization on a CNC coordinate measuring machine for reverse engineering of an unknown free-formed surface. This algorithm supervises the movement of a contact type ball-tip probe in an autonomous manner in that the surface is discretely sampled to be fitted by a polyhedron surface model of triangular plane patches. Vertex points of the polyhedron are measured so that the surface is approximated with a minimum number of point data satisfying a specified geometric tolerance. An effective compensation algorithm of ball-tip radius is also proposed to obtain an actual surface profile from the digitized data. (C) 1997 Published by Elsevier Science Ltd.