Stability And Tool Parameters Optimization Of Multi-Delays Milling Process For Hybrid Machine Tool-Tool-Workpiece

Milling is a commonly used processing method in manufacturing.Machine tool manufacturing and processing technology are the key factors that affect the processing quality and precision.With the development of science and technology,parallel / hybrid machine tools are used in milling processing due to their reconfigurability,high flexibility,and high efficiency.However,the self-excited chatter during processing is still a key factor restricting machining efficiency and workpiece quality,which is affected by milling parameters and the dynamic characteristics of the system.Therefore,it is of great significance to establish accurate and efficient stability prediction methods and study the passive control strategy of milling chatter by considering the effects of the machining posture of the hybrid machine tool and the type of tools and the type of workpieces on the stability of the milling system.This dissertation focuses on stability of multi-delays milling process of hybrid machine tool-tool-workpiece by means of theoretical,numerical simulation and experiment.We develop a mechanical model of milling process,and study the stability of muti-delays milling system and the optimization of cutter structure parameters.Furthermore,we analysis the dynamic characteristics of hybrid machine tools in the working space.The main research contents are as follows:Firstly,a milling force model is established to predict of the milling force of various types cutters in the milling process by considering the spatiality of the cutter geometry.The model considers the radius changes of different cutting edge shapes,and variable pitch angles and variable helix angles.The chip thickness at the machining position is represented as a space vector.The results of the milling force experiments reveal that the simulated and measured cutting forces are essentially consistent in the peak and the approximate trend.Then,taking into account the regeneration effect,a model of dynamic milling force is established,which provides a theoretical basis for the subsequent prediction of the milling stability.Secondly,in order to predict the stability of multi-delays milling,the accurate and efficient approaches are proposed to predict the stability of multi-delay milling.Through the axial layering technique and the delay approximation method,the variable pitch/variable helix cutter is approximated into several variable pitch cutters in the axial direction.Further,the system is transformed from the variable time-delay to multidelays.Based on the semi-discretization method,a multi-delays stability prediction model for variable pitch/variable helix cutter is proposed.Subsequently,in order to obtain a more efficient model for predicting the multi-delays milling stability,the Lagrange interpolation algorithm and Hermite interpolation algorithm are used to interpolate the time-delay displacement terms and displacement terms of the system equations,respectively.The transfer matrices of two adjacent discrete periods are obtained,and then the prediction method of the multi-delays milling stability based on Hermite interpolation is determined.The simulation results verify the correctness of the proposed two methods.Through the analysis,it can be seen that the stability limit of the variable pitch and variable helix cutter changes significantly compared with the regular cutter.However,it is still necessary to select the optimal pitch angle and helix angle to achieve the optimal solution of stability through the cutter parameter optimization.Thirdly,based on the multi-delays milling stability prediction model,an optimization strategy is proposed for the parameters of the variable pitch / variable helix cutter.This optimization strategy aims to improve the absolute stability region of multidelays milling system and defines a stability multiplier that can express stability quantitatively.Taking the pitch angle and helix angle as design variables,and we chose the stability multiplier the optimization objective and find the best combination of pitch angle and helix angle to achieve the maximum material removal rate during process.The dynamic models of two-degree-of-freedom milling and single-degree-of-freedom milling of thin-walled workpiece are established respectively.Specifically,the multidelays effect of variable pitch and variable helix cutter as well as the multi-modal effect of thin-walled workpiece in the dynamic model of thin-waller workpieces are considered.The genetic algorithm is used to obtain the optimal combination of pitch angle and helix angle under different milling conditions.The advanced semidiscretization method and Hermite numerical method are utilized to predict the stability lobe diagrams(SLDs).The experiment results show that the optimized cutters can improve the absolute stability region under two milling conditions,and the optimized cutter can suppress the chatter caused by the first two-order modes of thin-walled workpiece.The effectiveness of the optimization strategy and the correctness of the multi-delays stability model are verified.Finally,for the case that the dynamic characteristics of the Tri Mule hybrid machine tool in the workspace change depending on the posture of the hybrid machine tool,a theoretical model is established for predicting the dynamic characteristics of the hybrid machine tool in the workspace quickly.Considering the influence of the elastic deformation of the hinges and limbs on the dynamic characteristics of the machine tool,the beam theory and the sub-structure synthesis method are used to establish an elastic dynamic model of the hybrid machine tool.The natural frequency of the hybrid machine tool in the workspace and the frequency response functions(FRFs)at the end position of the hybrid machine tool are obtained.Further,the stability of the Tri Mule hybrid machine tool in the workspace is further analyzed using the Zero-order Approximation(ZOA)method.In addition,the response surface theory is used to establish the mathematical relationship between the minimum critical cutting depth of the hybrid machine tool and the posture of the machine tool.The distribution of the minimum critical axial cutting depth of the machine tool in the workspace is described quickly and accurately.