Optimization of pocket milling tool path for chatter avoidance and reduced machining time

Abstract

Modern Computer Numerical Control (CNC) machines and Computer Aided Manufacturing (CAM) systems have become quite sophisticated and can machine geometrically feasible pocket profiles. More t han 8 0% o f a ll mechanical parts which are manufactured by milling machines can be cut by Numerical Control (NC) pocket machining, which involves the removal of material within a closed boundary. Pocket milling is applied in the manufacture of dies and molds, and in the manufacture of aerospace and aircraft parts. Most of the current Computer Aided Design and Computer Aided Manufacturing (CAD/CAM) systems used in CNC machines, such as Computer Aided Threedimensional Interactive Application (CATIA) and Mastercam, provide the capability of generating NC pocket milling instructions based on the geometric definition o f a work p iece a nd t he c utter, t hus a utomating p art programming. However, the generation or selection of tool path is, in most cases, based on operator’s experience and intuition. There is no consideration of the dynamic effects of the cutting process. This presents shortcomings that limit the productivity of a CNC machining system since the efficiency of the tool path strategy impacts on the overall efficiency of the machining process. In this work, mathematical models for predicting static and dynamic cutting forces in pocket milling were developed in MATLAB R and verified experimentally. In the dynamic cutting force model, the instantaneous undeformed chip thickness was modelled to include the dynamic modulations caused by the tool vibrations so that the dynamic regeneration effect which leads to chatter is taken into account. This model was found to be more accurate in predicting cutting forces than the commonly used static models. A chatter stability prediction criterion was proposed in which a force ratio defined xiii as the ratio of maximum dynamic cutting force to maximum static cutting force was employed as an indicator of chatter occurrence. This ratio has a limit value beyond which the machining process becomes unstable and is dependent on the cutter and workpiece materials. A weighting criteria was applied so as to optimize the tool path generated for chatter avoidance, minimizing forces acting on the tool and minimizing machining time. Experiments were carried out using an 8mm HSS helical cutter and Aluminium 7075-0 workpiece and the limiting value of force ratio was 1.238. The experimental results were in good agreement with the prediction model. The influence of roughing tool path strategy on machining time, cutting force and chatter vibration was investigated and analysed, for zigzag, parallel spiral and true spiral tool paths. The zigzag tool path was found to have the least machining time, while the true spiral tool path was found to have the least average cutting forces. The thesis presents an investigation and analysis of end milling of pockets and the influence of roughing tool path strategy on the resultant cutting forces, chatter vibrations and machining time.