| Integral turbine disk with complex structure is core part of modern aerospace engine.Generally speaking,it is made of difficult-to-machine high temperature alloy.The traditional machining methods have difficulties in machining these par ts.At present,large die-sinking EDM machine tools have gradually become the indispensable machining equipments for large and complex part s such as integral turbine disks.These parts with high precision and long machining cycle put forward high requirements for accuracy,efficiency and stability of EDM machine tools.For small and medium-sized EDM machine tools,thermal error and vibration of which are less obvious than that of metal cutting machine tools.Therefore,less attention has been paid to the thermal and dynamic characteristics of EDM machine tools.However,due to large machining area,long machining time and heavy moving parts,the caused thermal error and vibration of spindle lead to the change of displacement and dynamic behavior of spindle nose.It plays a vital role in decreasing machining accuracy and stability of machine tool,thereby suggesting the utmost attention.On thermal characteristics,heat accumulated after long-time continuous machining with large machining area induces spindle thermal error.In addition,especially for high-speed jump motion,large inertia of transmission system leads to significant temperature rise in spindle components,and thus impairing the machining accuracy.On dynamic characteristics,during high-speed jump motion,the spindle nose would be subject to great hydraulic power,resulting in vibration and impact especially for large electrode.Moreover,the vibration will change the gap discharge state and bad for machining efficiency and stability.Therefore,in order to improve the machining accuracy and stability of machine tool,the thermal and dynamic characteristics of large die-sinking EDM machine are studied combined with characteristics of EDM.The A2190 large Ngau–Tau-type precision six axis linkage CNC EDM machine tool is taken as the research object in this dissertation.The heat source in EDM area and spindle drive system,as well as fluctuation of environment temperature have been modeled.And effects of these heat sources on temperature rise and thermal deformation of large EDM machine tools have been investigated.In order to improve the calculation efficiency of thermal characteristics analysis of machine tool,an equivalent heat source model for machining zone has been proposed and its validity has been verified.Based on the equivalent heat source model,the steady-state heat transfer process of long-time machining has been studied.Besides,the thermal deformation of spindle and worktable have been revealed.For the thermal behavior of spindle drive system,a test system for temperature rise and deformation of spindle have been built through simulating jump motion of spindle.In terms of environment temperature,horizontal and vertical gradient of temperature,average temperature,frequency and a mplitude of temperature fluctuation have been discussed.The influence of main parameters in heat transfer model of discharge area and environment temperature on temperature rise of spindle nose have been studied.Based on the boundary condition of spark discharge heat,a method of simulating thermal equilibrium experiment on EDM machine tool has been proposed.The transient temperature field,thermal deformation and thermal balance time of machine tool spindle and worktable have been obtained and verified by the experiment.Combining fuzzy clustering analysis with correlation theory,the thermal sensitive points of machine tools have been selected,the thermal deformation prediction model based on RBF neural network has been established.What's more,the influence of jump motion period on thermal deformation of spindle has been studied through the model.Based on the established prediction model of thermal deformation,semi-closed loop feed forward compensation method has been chosen and verified under different working conditions for the control of thermal deformation.High-speed jump motion is an important reason for impact and vibration of EDM machine tools.In view of Lagrange equation,the model of spindle feed system has been established.Afterwards,modal analysis was carried out for spindle column of machine tool,laying foundation for dynamic analysis.Kinematic characteristics of control strategies including trapezoidal velocity profile,constant jerk profile and sinusoidal jerk profile have been compared.Numerical model of transient load of spindle nose during jump motion has been set up.The expression of differential pressure resistance has been deduced in detail especially for the adsorption and extrusion effect from working fluid during electrode rising and falling.The change law of pressure difference resistance is obtained,and then the numerical analysis method is verified.Under these strategies,dynamic model of spindle periodic feed system has been established,and the relationship between electrode movement and inertial force as well as hydraulic force on spindle nose is recognized.Furthermore,the spindle nose displacement during electrode movement is measured,from which the influence of jump velocity on damping vibration parameters of spindle is figured out.According to discharge waveform characteristics,detection method of voltage and current rising edge and falling edge has been proposed to identify and count the discharge waveform and ignition delay.In view of the detection method,the discharge rate and ignition delay time under different jump speed and machining time have been discussed.Hence,the effect of spindle vibration on the discharge state is obtained.In addition,the flow field and particle distribution in discharge gap when entering machining state until the end of vibration have been simulated to determine whether low-frequency vibration of spindle caused by jump motion has an effect on flow field and debris removal.Finally,active damping control method for spindle is put forward.What's more,the simulation platform of feed drive system has been established and verified. |
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