Development and calibration of a cutting force model for multi-axis ball-end milling

Due to the development of CNC machining centre and automatic programming software, the multi-axis ball-end milling has become the most widely used machining process to remove unwanted material in 3D sculptured surface generation. As improved quality and high productivity are the two main competing factors in the modern manufacturing world, it becomes necessary to develop models that can accurately characterize the performance of machining process.; Cutting force acting on the cutter is one of the important variables that give significant machining process information. An improved mechanistic cutting force model is presented in this research work for accurate prediction of cutting forces in multi axis ball-end milling process. The main feature of this improved model is the new concept of the undeformed chip thickness, which is calculated from the actual trochoidal tool path trajectory. In multi-axis ball-end milling, the chip thickness is calculated individually for each of the horizontal, non-horizontal and rotational cutting motions using a generalized concept. For horizontal cut, the undeformed chip thickness is expressed two dimensionally, whereas the same is expressed three dimensionally for both non-horizontal and rotational cuts.; Accurate evaluation of the empirical cutting force coefficients is critical to the reliability of the predicted cutting forces. A simplified and efficient method to determine the cutting force coefficients of the newly developed model is proposed in this research work. The unique feature of this new method is that only a single half-slot cut is to be performed to calibrate the force coefficients that are valid over a wide range of cutting conditions. Instantaneous cutting forces are used with the established helical cutting profile on the ball-end mill. The half-slot calibration cut enables successive determination of the lumped discrete values of the varying cutting mechanics parameters along the cutter axis, whereas the size effect parameters are determined from the known variation of the undeformed chip thickness with cutter rotation.; The non-rigid structure of the machine tool often results in noisy cutting force measurement data which is captured by the force dynamometer. This noisy force data compromises the calibration accuracy of the empirical force coefficients, which affects reliable cutting force prediction. The noisy force data is first fitted to a polynomial function to reduce signal fluctuation. Two distinct solution methods, namely forward and backward solution methods, are proposed to solve the force coefficients. The former is a direct solution method and applicable to cutting force data with relatively low levels of noise. The latter is an approximate solution method which is more tolerant to increased noise magnitude in the force data.; A series of steady state horizontal, non-horizontal and rotational cuts are performed to validate the capability of the developed force model along with the proposed efficient calibration method. It is shown that the force model using the newly proposed 3D chip thickness concept give significantly better predictions of cutting forces compared to the existing chip thickness concepts for non-horizontal and rotational cuts.