This paper presents a CNC machine tool interpolator with a path compensation method for a repeated contour machining process. In addition to a conventional contour interpolation algorithm, the proposed interpolator also includes contour-error calculation, data extraction and contour-error interpolation algorithms so that the previous contour machining result can be introduced to improve the accuracy of the subsequent repeated machining. Mathematical analysis and experimental evaluation are presented in this paper. (C) 1998 Elsevier Science Ltd. All rights reserved.