Research On Delay-Time Varying Dynamics Behavior And Stability Of Variable Pitch End Mills For Milling Thin-Walled Parts

Variable pitch end mills have excellent vibration damping performance and are widely used in the processing of key components.As a typical difficult-to-machine parts,the large material removal and low structural rigidity make thin-walled parts prone to cutting vibration during processing.As a kind of unsteady vibration,chatter is the key factor to destroy the stability of the system,which reduces the surface quality and machining efficiency of the workpiece.In this paper,the machining chatter mechanism of variable pitch end mills for milling thin-walled parts is analyzed.Considering the delay of excitation force caused by the change of intertooth angle and the time-varying effect of dynamic parameters of thin-walled parts,a delay-time varying dynamic model of the machining system is established to predict the stability of cutting process,or optimize the tool structure and process parameters.It is a necessary means to effectively improve cutting stability,and achieve high precision and high efficiency machining performance.The vibration model of with machining system with the variable pitch end mills is the theoretical basis for studying the dynamic behavior and stability of the system,first,by analyzing the characteristics of the contact area between the variable pitch end mill and the workpiece,the dynamic cutting thicknesses under different tool path conditions are determined,and the dynamic milling force model of variable pitch end mill is established.Then,considering the effect of delay caused by unequal pitch on the milling force,the cutting vibration mechanism under multi-source excitation is studied,and the delay dynamic model of the process system is determined.Finally,for different milling process systems,the regeneration effect and the contact loss of dynamic cutting thickness are analyzed,and the delay-time varying dynamic model of the process system is established.It provides a basic model for the research on the cutting stability of machining system.Based on the theoretical model of the delay dynamics of machining system,the cutting stability of variable pitch end mills is studied.First,the time-frequency and semi-analytical method is used to solve the delay dynamic differential equation of variable pitch end mill,and to determine the cutting stability criterion of process system.Secondly,according to the experiment of milling aluminum alloy with variable depth of cut,the dynamic model parameters and the limit cutting depth are obtained,which provide the key parameters for the prediction of the delay dynamic model.Then,based on the single-frequency method and the semi-discrete method,the cutting stability of the delay system is studied,and the reliability of the stability prediction model is verified with experiments.At the same time,the frequency domain characteristics of milling force with time-varying delay are analyzed,and the effect of pitch angle on vibration spectrum is obtained.Finally,by considering the influence of tool-workpiece contact dimension,a nonlinear milling force coefficient model is established,and the influence of interaction milling parameters on the cutting stability of the process system is clarified.The vibration characteristics of delay feedback under different working conditions are revealed,which provides theoretical support for the prediction of cutting stability of thin-walled parts.Based on the delay-time varying dynamic model,based on the solution method of the delay dynamic differential equation,the mechanism of the time-varying effect of modal parameters on the dynamic behavior of thin-walled parts is revealed.First,considering the influence of workpiece material removal on modal parameters,the modal parameters of thin-walled parts under different working conditions are corrected based on the combined approximation method and the first-order perturbation method respectively,so as to improve the accuracy of dynamic model parameter prediction.Secondly,aiming at the time-varying effect of dynamic behavior of thin-walled parts,considering the multi-modal coupling and positiondependent characteristics,a delay-time varying multi-point contact dynamic model is established to obtain the rigidity parameters fields of different orders.Then,the cutting stability of variable pitch end mill in milling thin-walled parts under the action of multiple factors is studied,and the output response of forced vibration is predicted synchronously.Finally,combined with the experiment of milling thinwalled parts with variable pitch end mill,the completeness of the proposed delaytime varying dynamic model is verified,which lays a foundation for further evaluation and optimization of system parameters.By determining the delay-time varying dynamic behavior and stability of the machining system,the machining system parameters are evaluated and optimized to improve the cutting stability of the process system.First,based on the delay-time varying dynamic model of the machining system,the delay and time-varying effects are evaluated.A method to improve the limit depth of cut is proposed,and the minimum envelope method is adopted to draw the multi-point contact stability lobe diagram of the tool-workpiece.Then,by studying the relationship between the process parameters and machining system eigenvalues,the dependence of the eigenvalues on the system delay is determined.An improved self-adaptive particle swarm optimization algorithm is proposed to optimize the inter-tooth angle of the tool,and the cutting stability domain is predicted to clarify the time-varying characteristics effect of the thin-walled parts on the cutting stability of the delay system.Finally,combined with the vibration damping performance test experiment of optimized tool,the machined surface state and vibration signal of the workpieces are analyzed to verify the effectiveness of optimizing the inter-tooth angle to improve the cutting stability of the system.At the same time,the process parameters of thinwalled parts are optimized based on the minimum envelope cutting stability domain,and a multi-level milling process is proposed to meet the machining accuracy and quality requirements of thin-walled parts,which provides guidance for the improvement of thin-walled parts milling technology.The related research work carried out in this paper aims to characterize the dynamic behavior of delay-time varying machining system,and improve the cutting stability of efficiency machining of thin-walled parts,as well as to play an important role in the application and development of high-performance tools.