Research On The Technology And Mechanism Of Vibration Assisted Electro-arc Milling

Electro-arc machining(EAM)is a new kind of electrical discharge machining(EDM)which adopts high energy density arcs to erode materials.The characteristics of EAM include high material removal rate(MRR),low relative electrode wear ratio(REWR)and high efficiency.Moreover,the dielectric medium of EAM is environmental friendly.As a result,EAM processes have gained extensive attention in recent years.However,EAM processes have low open circuit voltage,high peak current,and long pulse duration,which lead to narrow gap width,high bulges,and large debris.So short circuits are easily caused,which affect the stability and machining results of EAM processes Vibration assisted electro-arc milling(V-EAM)is proposed which improves machining process and MRR,and reduces REWR and surface roughness(Rz)by the vibration of workpiece which enlarging gap width,improving gap status,and disturbing arc plasma.V-EAM is a prominent,fast,high efficiency,and environmental friendly EDM processing technology.The mechanism of V-EAM is studied based on the formation,existence,and disturbance of arc plasma and crater geometry.An arc plasma model is built based on knowledge of Plasma Physics.The formation,existence,and disturbance of arc plasma have been studied.The relationships between arc pressure,arc temperature,arc voltage,arc current,arc diameter,arc length,and energy distribution are analyzed.Moreover,the relationships between arc plasma,crater geometry,and machining results are studied and the model of arc plasma-crater geometry-machining results is built then.It is proposed that the power source determines the characteristics(arc pressure,are temperature,arc current,arc diameter,the maximum arc length,arc power,and polarity effect)of arc plasma,the arc plasma determines the crater geometry(crater diameter,crater depth,the ratio of depth to diameter,and removal volume),and the machining results(MRR,REWR,and Rz)are determined finally.The ranges of peak current and pulse duration of conventional EDM(C-EDM)and EAM are compared,based on which single pulsed power source is developed,which can generate spark and arc discharges.Single pulsed discharge experiment with experimental parameters of peak current and pulse duration is carried out adopting spherical electrodes.It is found that a short circuit can be led by a single pulsed discharge when peak current is high or pulse duration is long.EAM has been discriminated between C-EDM according to whether a short circuit appears.The explanation is that the high power makes arc plasma expand,which enlarges the arc diameter,crater diameter,and bulge.The growing bulge leads to a short circuit finally,when the machining parameters of peak current and pulse duration get to the ranges of EAM.The effects of vibration on average gap width,flushing rate,jointing of bulges and arc plasma are analyzed.The mechanism of vibration in V-EAM is concluded as following:the vibration lengthens arc plasma,improves power of arc plasma,and strengthens polarity effect which makes the anode acquire more energy than the cathode,enlarges anode craters,reduces cathode craters,improves MRR,and reduces REWR at last;the average gap width is enlarged by the vibration,which improves the moving of arc roots on both polarities,prevents energy concentration,and improves gap status finally.The characteristics when C-EDM becoming EAM are studied by needlelike electrodes according to the ranges of C-EDM and EAM.The results show that the transformation is gradual.The growing peak current enlarges crater in diameter and height,which increases removal volume and Rz,the lengthened pulse duration reinforces polarity effect by decreasing crater depth of cathode,which reduces REWR,and both the high current and long pulse duration improve MRR and reduce REWR simultaneously.The flushing arc breaking method of EAM are studied by singled pulsed discharges,the purpose of which is to disturb arc plasma and reduce energy concentration,not to break normal arc plasma.A new method to measure the impulse force of EAM is proposed.The changing and composition of the impulse force of EAM is revealed.The impulse force is generated by the expanding and shrinking of bubbles firstly,and the arc plasma pressure takes effect then.The impulse force is growing with the increasing of peak current,while the impulse force is irrelevant to pulse durationAn independent discharge status detection system(IDSDS)is proposed,and following equipments,including a servo system,an electro-arc machining direct current power source(EAMDC),a milling spindle,dielectric circulation system,and a vibration platform,are developed.An experimental prototype has been developed adopting them.Arc discharge breakdown experiments are carried out The gap width is less than 20?m when open circuit voltage is 80V and the dielectric medium is tap water.It is suggested that the breakdown is leaded by electrical field and heat generated by leak current.The experimental prototype is tested by experiment and the results show that the experimental prototype fulfills its design requirements.The feed rate can be adjusted by gap status and the machining process is stable and high efficiency.Additionally,most of debris are solid balls which suggests that the material of workpiece is eroded in liquid formSingle factor experiments with the experimental parameters of peak current,tool rotation speed,flushing pressure,and comparative experiments with the experimental parameters of workpiece polarity and vibration frequency are carried out.The results show that the growing of peak current leads to the growing of MRR and Rz,and the decreasing of REWR.Discharge concentration appears when the tool rotation speed or the flushing pressure is low,which decreases MRR and increases REWR.Arcs are broken frequently in contract,which decreases the duration of discharges,reduces MRR and increases REWR at last.The MRR of positive polarity is higher than that of negative polarity.Both the amplitude and the frequency of vibration take effects.More MRR,less REWR and Rz are acquired if larger amplitude is adopted when the frequency is the same Low frequency will decrease discharge duration and high frequency will reduce amplitude seriously which makes the effects of vibration on gap width be neglectable,both of which are to the disadvantages of machining results,The experimental results show that vibration can improve gap status,reduce short circuit,improve machining process and results effectively,just as that the theory predicts above.Orthogonal experiment and analysis of means are conducted.Machining parameters are optimized.The relationships between input parameters(peak current,tool rotation speed,frequency,and flushing pressure)and output parameters(MRR,REWR,WLT,and Rz)are studied systematically.Regression and analysis of variance(ANOVA)are carried out then.Regression equations are verified to be accurate by the verification test.It has been proven that the process model is reliable.Samples are machined by the experimental prototype.The optimized machining parameters are adopted,which make the machining processes fast and stable.Cavities have been milled by V-EAM successfully.The validity and advantages of V-EAM are verified.