The paper focuses on the problem of choosing the manufacturing route in metal removal processes, a highly important element in computer aided process planning (CAPP) systems. At present, decisions concerning the cutting parameters used in metal removal operations are based on experience. The object of this work has been to design a system to help select the parameters in the cutting process in the case of milling. The algorithm is based on optimising the cost of the operation. The result is the selection of parameters that take into account all the existing restrictive factors (material, geometry, roughness, machine and tool). A wide range of parts have been evaluated, using group technology in order to choose representative cases. We compared the results of our system with the values proposed in the reference manuals. The experiment has served to determine the different relationships between the factors that have an influence on milling operations and the features which allow us to take into account the wide range of geometrical possibilities existing for all the pieces we studied. Applying our method helps to make the right decisions on the optimal parameters in mill operations as applied to the machining processes, and, in our experiments, leads to a reduction of about 35% in processing times while ensuring optimal finishing conditions. (C) 2004 Elsevier Ltd. All rights reserved.