Research On Micro Electrical Discharge Machining In Atmospheric Pressure Cold Plasma Jet

Characteristics of dielectric significantly influence machining performance of micro electrical discharge machining(EDM).A high material removal rate(MRR)and large tool electrode wear are obtained in micro EDM in liquid dielectric,such as kerosene-based oil or deionized water.Although tool electrode wear in gas dielectric is small,MRR is extremely low due to the small discharge gap.Atmospheric pressure cold plasma jet(APPJ)is proposed as the dielectric for micro EDM in this dissertation.A series of theoretical analysis and corresponding experiments of micro EDM in APPJ are conducted.Experiments of micro EDM in other types of dielectric,such as deionized water and gas jets are also conducted for comparison.The study results provide new insights in the decrease of tool electrode wear,the increase of MRR and improvement of surface quality in micro EDM process.The main contents and results of the dissertation are as follows:(1)In order to select the generation mode of APPJ suitable for micro EDM,some typical APPJ generation modes and their characteristics are reviewed.It is found that the APPJ generated by bare electrodes corona discharge is suitable for micro EDM,according to the requirements of dielectrics for micro EDM.The feasibility of micro EDM in APPJ is also verified by experiments.(2)It is found that the number density of charged species in APPJ is much higher than that in gas jets.A larger number density of charged species in APPJ leads to a higher electrical conductivity in APPJ than that in gas jets,which is helpful to obtain a larger breakdown distance in APPJ than that in gas jets.Experimental results of breakdown distance detection indicate that the average breakdown distances in APPJ are larger than 4 ?m,while in the case of gas jets all breakdown distances are less than 2 ?m.(3)Micro grooves are machined by micro EDM milling in APPJ,nitrogen jet,air jet,oxygen jet and deionized water.Machining characteristics of micro EDM in APPJ are compared to those in other types of dielectric.It is found that a better surface quality is obtained by micro EDM in APPJ than those in gas jets and deionized water.Straighter side walls and flatter bottoms of micro grooves are obtained in APPJ than those in deionized water.Tool electrode wear ratio(TWR)in APPJ is less than that in deionized water.A higher MRR is also achieved in APPJ than those in gas dielectrics.When the open voltage is 100 V and capacitance is 8200 pF,MRR in APPJ is 200 ?m3/s,which is higher than that in air jet(121 ?m3/s)and oxygen jet(156 ?m3/s).The surface roughness Ra in APPJ is only 417 nm,much lower than that in air jet(539 nm),oxygen jet(636 nm)and deionized water(672 nm).The results in the open voltage of 80 V and capacitance of 3300 pF show the same trend.Pule with high peak current and short discharge duration is helpful to improve MRR in APPJ.In the case of deionized water,this conclusion is totally on the contrary.(4)The dry type and near-dry type composite dielectrics based on APPJ are used to improve the MRR of micro EDM in APPJ,respectively.It is found that MRR in the dry type composite medium of APPJ and coaxial high-velocity air jet(APPJ+HVAJ)is more than 1.25 times of that in APPJ.MRR in the near-dry type composite dielectric of APPJ and coaxial mist jet(APPJ+MJ)is more than 6.26 times of that in APPJ,which is even higher than that in deionized water under the same conditions.Meanwhile,better surface quality,lower TWR and better profile are also achieved in APPJ+MJ than those in deionized water.A complex 3D micro cavity is also generated in APPJ+MJ without compensation of tool electrode wear.A 1/8 sphere was machined in the cavity.The bottom of the machined cavity is uniform.The tool electrode wear ratio is only 0.043%,which proves that the dielectric APPJ+MJ has potential application in micro EDM milling.