| Stainless steel materials are widely used in various fields of industrial production and daily life due to their excellent corrosion resistance and durability.The high temperature and vibration generated by traditional machining methods when machining micro-hole parts made of difficult-to-cut stainless steel materials often lead to severe wear of machining tools and continuous deterioration of machining conditions.In contrast,electrical discharge machining(EDM)is a non-traditional machining method that utilizes spark discharges generated between the tool electrode and the workpiece to corrode and remove excess metal materials.Due to the non-contact between the tool electrode and the workpiece,there are almost no macroscopic cutting force during the machining process,and the fact that the machining is not influenced by the strength and hardness of the workpiece material,which make it is considered to be one of the best methods for machining difficult to machine material workpiece with three-dimensional complex cavity structure.Micro-electrical discharge machining(Micro-EDM)is the miniaturization of its processing parameters and process characteristics in conventional EDM.However,micro-EDM limits its machining area to a very small range around the tool electrode,which making it very hard for its corrosion product particles to be discharged from the discharge gap in a timely manner.At the same time,it is also prone to the frequent occurrence of side discharges,secondary discharges and short circuits,which seriously affects the machining efficiency and processing accuracy after the completion of micro-EDM.In this paper,304 stainless steel material is taken as the research object.The research on micro-electrical discharge hole machining technology and its influence mechanism aims to improve the processing efficiency of micro-electrical discharge hole machining parts while simultaneously improving the dimensional accuracy and shape and position accuracy of the processed micropores.In order to research the influence of the polarity effect,the study of energy conversion on electrodes in micro-EDM was carried out.The theoretical analysis of the energy and distribution during the inter-electrode discharge of an electric spark is conducted.A semiinfinite body heat source conduction model for single-pulse discharge is established and its analytical solution is obtained.Using the crater radius and depth obtained from the single-pulse discharge experimental results,the energy distribution ratios of the discharge plasma radius and the cathode anode are obtained,respectively.The results show that within the current range of single-pulse discharge energy,the energy distribution ratio of the anode is significantly bigger than that of the cathode.The sum of the energy distribution ratios of the anode and the cathode during single-pulse discharge machining increases with the increase of the discharge energy.The material removal volume of both the anode and the cathode is highly linearly correlated with the discharge energy,and both increase with the increase of the discharge energy.At the same time,the energy utilization rate of the anode during single-pulse discharge is obviously higher than that of the cathode.The research results show that the comprehensive processing performance of positive polarity processing will be obviously higher than that of negative polarity processing within the current machining parameter setting range.In order to research the effect of electrode shape on the flow field in rotating electrode machining gap at a micro-scale,a simulation study was carried out.The preliminary experimental calculation and estimation of the discharge gap size for micro-EDM were carried out.The geometric and theoretical models of the gap flow field in rotary cylindrical electrode machining and rotary helical electrode machining were established.And the simulation analysis of the gap flow field was performed using the Fluent software.The axial profile velocity field program,axial profile pressure field program,and bottom profile pressure field program of the rotating cylindrical electrode machining gap flow field and the rotating helical electrode machining gap flow field were obtained,respectively.The velocity vector distributions in the helical groove during forward and reverse machining of rotating helical electrodes were obtained through simulation.Compared to rotating cylindrical electrodes,rotating helical electrodes facilitate the flow of working fluid in the flow field and the discharge of electroetching product particles when processing deep and small holes,thereby improving the machining efficiency of EDM.At the same time,through simulation analysis and comparison,it is found that the automatic liquid pumping mechanism of helical electrode forward machining and the automatic liquid flushing mechanism of helical electrode reverse machining are both effective,and helical electrode forward machining can achieve better machining accuracy than reverse machining.In order to research the scale effect of processing parameters,the effect of peocessing parameters on the machining performance of micro-electrical discharge holes was studied.The effects of electrode rotation and rotational speed changes on the distribution of forces acting on corrosion products and fluid micro-clusters in the discharge channel were theoretically analyzed.The effect of electrode rotation speed on processing efficiency and processing accuracy in micro-EDM was studied through single-factor experiments.The evaluation indicators of processing efficiency specifically include material removal rate(MRR)and relative tool wear rate(RTWR).And the evaluation indicators of processing accuracy specifically include the inlet and outlet diameters of the machining holes,the inlet and outlet roundness of the machining holes,and the taper angle of the machining holes.The effects of electrode diameter on machining efficiency and accuracy were studied through skin effect,capacitance effect,area effect and single-factor experiments.The relationship between discharge energy and discharge gap was analyzed,and the effects of discharge energy on machining efficiency and accuracy were studied through single-factor experiments.A comprehensive optimization research of micro-EDM parameters was carried out.The effects of processing parameters on MRR,RTWR,hole-inlet roundness,and hole taper angle in micro-EDM were analyzed through orthogonal experiments.The biggest processing factor affecting the MRR is the electrode diameter,followed by the discharge energy,and then the electrode rotation speed;The machining factor that has the greatest impact on the RTWR is the electrode diameter,followed by the discharge energy,and the rotational speed has the smallest impact on the relative electrode wear rate;The machining factor that has the greatest impact on the hole-inlet roundness is the electrode diameter,followed by the rotational speed.The discharge energy has almost no impact on the hole-inlet roundness.The machining factor that has the greatest impact on the taper angle is the discharge energy,followed by the electrode diameter,and the electrode rotation speed has the smallest impact on the taper angle.The optimal process parameter combinations for single objective optimization purposes are obtained.The processing efficiency and precision indicators were comprehensively optimized through comprehensive evaluation methods,and the processing system was comprehensively evaluated through the grey correlation analysis method.The signal-to-noise ratio index was used as the weight of various parameters in the comprehensive evaluation,establishing a comprehensive process index that took into account both processing efficiency and processing accuracy.Among them,the diameter has the greatest impact on the comprehensive process indicators,followed by the discharge energy,and finally the electrode rotational speed.The optimum combination of machining parameters under the comprehensive optimization of multiple indicators was founded finally.A prediction method for electrode wear in micro-electrical discharge hole machining based on the total normal discharge time was established.The variation trend of electrode loss length and discharge time,as well as the variation trend of machining hole-depth and discharge time under different discharge energy and electrode diameter processing conditions,were obtained by the electrical contact method.A mathematical model of discharge time electrode loss length based on normal discharge time and a mathematical model of discharge time machining holedepth has been established.The effectiveness of the proposed electrode loss length prediction method has been verified through micro-electrical discharge through-hole and blind-hole experiments. |
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