Milling Dynamics-Stability Analysis Methods And Applications

High-speed milling is widely utilized in key industries such as aerospace, shipping, die and mold, and automotive. It is one of the most important basic technologies for machining high precision complex surfaces, due to its well-known advantage, obtaining a large material removal rate while keeping relatively low cutting forces and maintaining a high quality level. Motivated by this benefit of high-speed milling, high performance machine tools and cutting tools are continually developed. However, high performance equipments are not equal to high performance machining. From the viewpoint of machining dynamics, chatter is one of the most severe limitations for surface quality and productivity in milling operations due to choosing improper machining parameters. Hence, stability analysis for the dynamic milling process is one of the most important prerequisites for the high speed milling technology. While, the chatter-free machining parameters cannot guarantee the surface quality either. To achieve high performance milling, we should predict the surface location error (SLE) on the basis of stability analysis. The key point is to conduct the dynamics-based process optimization with the stability and SLE constraints to obtain the optimal machining parameters to fulfill the aim of high performance machining.Since the regeneration effect of the dynamic chip thickness is the most powerful source of self-excitation, the researches on the stability of milling processes are mainly focused on this following problem, how to obtain the stability lobe diagrams from the governing dynamic equations of milling processes. This dissertation focuses on the problem and develops the semi-analytical tools for prediction of the milling stability and SLE. The main research content and achievements are as follows.A full-discretization method for prediction of milling stability is presented. The response of the system of the dynamic milling process considering the regenerative effect is calculated via a direct integration scheme. With the help of discretizing the time period and simultaneously approximating the involved system state, time-periodic and time delay items, the transition matrix of the system on one time period is constructed and the milling stability is then predicted based on Floquet theory. Compared with the well-known semi-discretization method, this method has much higher computational efficiency without loss of any numerical precision. Thereafter, the basic idea of the full-discretization method is generalized to calculate the SLE. Also, a second-order full-discretization method is proposed on this basis.Based on the numerical methods for integral equations, this dissertation then presents the numerical integration method for milling stability prediction. As one of the bases of gradient-based optimization algorithms, the sensitivity analysis is usually required to calculate the derivatives of the system response with respect to the machining parameters. On the basis of the numerical integration method, the semi-analytical expressions of derivatives of the Floquet transition matrix with respect to the machining parameters are obtained. Compared with the existing methods in the literature, the unique merit of the proposed method is that it can be used for analytically computing the sensitivity of the stability boundary in milling, without employing any finite difference methods. Therefore, the high accuracy and high efficiency are both achieved. Also, the numerical integration method is generalized to the spectral method for prediction of chatter stability.On the basis of the experimental modal analysis technique, the dissertation then develops a comprehensive milling dynamic model considering the regeneration effect of the dynamic chip thickness and the structural mode coupling effect of the cutter. Thereafter, by using the full-discretization method, the stability of the model is analyzed and one experiment is conducted to validate the predicted stability lobe diagram. Furthermore, the full-discretization method is utilized to predict the chatter stability and the surface location errors due to the different machining parameters and the other experiments are performed to verify the predicted results. The theoretical and experimental results both demonstrate that the structural mode coupling effect of the cutter cannot be neglected for accurate and reliable prediction of the milling stability and surface location error.The work on prediction of the milling stability and surface location error will be useful and helpful for developing next-generation high-performance machine tools, investigating the machining process dynamics and choosing the optimal machining parameters.