Research On The Scale Effects In Micro Electrical Discharge Machining

With the rapid development of micro electro mechanical system(MEMS),the application of microscale components and microstructures is more and more widely used,and the precision requirements are getting higher and higher,which puts forward higher requirements for micro-machining technology.Traditional micro-cutting technologies are all contact-type machining with a large cutting force,which would result in deformation of the workpiece and affect the machining accuracy.As one of the non-traditional machining technologies,micro electrical discharge machining(micro EDM)belongs to the non-contact machining,which means that the tool does not contact with the workpiece in machining process and the macro-force is extremely small.The material removal is realized by the local high temperature generated by spark discharges.Micro EDM is very suitable for the processing of small parts.Micro EDM technology is generally considered as the application of macro electrical discharge machining(macro EDM)technology in the field of micro-scale applications.However,a large number of studies have indicated that micro EDM shows many different characteristics from the macro EDM in terms of processing mechanism and processing performance due to the effect of scale effect.To make systematic research on the scale effect in micro EDM and reveal the formation mechanism,action mechanism and influence rule would be conducive to the application of micro EDM technology and improve the predictability and controllability of processing performance.This paper has systematically studied the scale effect in micro EDM from three aspects:formation mechanism,action mechanism and influence rule.The formation mechanism of scale effect in micro-scale discharge was studied from the perspective of dielectric breakdown and discharge channel expansion.A comparison of existing dielectric breakdown mechanisms was made and a new bubble-electron breakdown mechanism suitable for micro EDM based on the field emission theory was proposed.Based on the force state of the charged particles in discharge channel,the expansion phase of discharge channel was theoretically analyzed and the regularity of expansion acceleration of discharge channel was deduced.Based on the discharge channel equilibrium conditions,the expressions of the magnetic flux density and pressure distribution in the discharge channel were studied.The theoretical analysis shows that there are great differences between micro and macro EDM in terms of dielectric breakdown and discharge channel expansion,which would cause different energy transfer and processing performance.This is one of the major reasons of the scale effect in micro-scale discharge.The energy distribution during micro-scale discharge is studied and the mechanism of scale effect was analyzed.The difference of material removal methods between micro and macro EDM was analyzed.An electro-thermal model suitable for micro EDM was proposed.Based on the electro-thermal model,a method for determining the energy distribution and discharge channel diameter during discharging was proposed by comparing the crater size of single discharge and the temperature distribution regularity calculated.The micro energy pulse power with the minimum pulse width of 1 ?s was developed.And the single discharge experiments with positive and negative polarity were made respectively to research the influences of discharge duration and discharge current on the ratio of the depth of discharge pits,discharge channel diameter and energy distribution.The results show that at the micro-scale discharge conditions,the radius of discharge channel on anode is greater than that of cathode.And the shorter discharge duration would lead the greater difference.In addition,the percentage of energy transferred to electrodes would increase with the increase of discharge duration.Moreover,the smaller the discharge current is,the more obvious the effect of discharge duration on energy distribution.The influence of electrical parameter scale on energy distribution is an important reason for the scale effect during the micro-scale discharge.The effect of tool electrode shape and rotation on micro EDM performance was investigated.The force analysis of discharge debris in the discharge gap was carried out and the influence of electrode shape and direction of rotation on the flow characteristics of dielectrics was simulated.Compared with the cylindrical electrode,the helical electrode can promote the dielectric flow in discharge gap,improve the chip evacuation ability,improve the overall processing performance and obtain larger material removal rate(MRR),smaller relative tool wear ratio(RTWR)and higher processing accuracy.There is also a great difference in machining performance obtained by positive rotation and inversion of helical electrode.Higher machining accuracy can be obtained by helical electrode with positive rotation and larger MRR can be obtained by helical electrode with reversal.In addition,a significant scale effect appears at the optimum rotation speed with the miniaturization of discharge energy.The optimum rotation speed decreases with the decrease of discharge energy.Low discharge energy results in a poor ability to maintain the discharge channel.Thus,the discharge energy would be pull cut easily.So,lower rotation speed is suitable for lower discharge energy.The scale effect caused by the miniaturization of tool electrode in micro EDM was studied.The influence of tool electrode diameter on electric field strength and discharge energy was analyzed theoretically,and the electric field intensity between the electrodes was simulated.The micro-scale(diameter is less than 500?m)and macro-scale(diameter is more than 500?m)tool electrodes were used in EDM to research the effects of tool electrode diameter on MRR,RTWR,surface roughness and material migration.The results show that the miniaturization of electrode diameter leads to the increase of local electric field intensity and the decrease of discharge energy,which results in the decrease of MRR and tool wear rate(TWR).RTWR showed a significant scale effect.In the field of micro EDM,RTWR decreases with the decrease of the electrode diameter.While in the macro EDM range,RTWR would increase with the decrease of the electrode diameter.In addition,surface roughness and material migration also show the phenomenon of scale effect.The smaller the discharge energy is,the more obvious the scale effect is.The single factor scale effect and the system scale effect in micro EDM were quantitatively evaluated.Based on orthogonal experiment and similarity theory,the single-factor scale effect and the system scale effect evaluation method in micro EDM were proposed.The effectiveness of this method is verified by orthogonal experiments.The type and degree of scale effect were also analyzed.The results show that the miniaturization of open circuit voltage,capacitance and electrode diameter leads to scale effect with different degrees in terms of MRR,RTWR and overcut.The open circuit voltage is the most significant factor that affects the scale effect in micro EDM,With the miniaturization of processing scale,the machining mode is gradually changed from macro EDM to micro EDM.The scale effect produced is a positive scale effect.Moreover,the higher the degree of miniaturization is,the more significant the scale effect behaves.