Study On The Process And Mechanism Of Electrical Discharge Milling Based On Gas Dielectric

The research of this dissertation mainly focus on the technology of electrical discharge milling(ED-Milling) based on gas dielectric which is composed of three ED-Milling methods including conventional ED-Milling in gas,ultrasonic vibration aided ED-Milling in gas as well as gas medium ED-Milling submerged in liquid. Electrical discharge machining(EDM) in gas is suitable for ED-Milling as it owns numerous advantages such as safe and green dielectric,no fire trouble,low tool electrode wear,etc.So it provides a new path for ED-Milling.The principle of ultrasonic vibration aided ED-Milling in gas is that the machining performances and the machining status can be enhanced by means of ultrasonic vibration of the tool electrode. Gas medium ED-Milling submerged in liquid,a new-type ED-Milling process,immerse the machining area between the tool electrode and the workpiece during ED-Milling in gas to solve the machining debris adherence problem on the machining surface of the workpiece.The mechanism of ED-Milling in gas was studied through conducting single pulse discharge experimental research and analyzing the mechanism of EDM in gas.The following conclusions can be drawn from numerous single pulse discharge experiments. The single pulse discharge crater of the workpiece on the condition of the positive polarity is bigger than that on the condition of the negative polarity.The single pulse discharge crater of the workpiece is bigger than that of the tool electrode when the tool electrode is cathode and the workpiece is anode.The crater produced by single pulse discharge in the compressed air is bigger than that produced in the air and its form changes according to the fluid of the compressed air.The volume of the single pulse discharge crater increases as the pulse energy strengthens.The mechanism of ED-Milling in gas is further studied from the aspects of the breakdown of the dielectric, the spread of the discharging channel,the conversion and distribution of the discharging energy,the removal of the machining debris,the deionization as well as the integrated effect of the impulsive discharges.The results indicate that the discharging process during EDM in gas initially comes from Townsend breakdown process and then changes to streaming breakdown process.The discharging channel during EDM in gas expands more quickly than it does during conventional EDM.The removal of the machining debris during EDM in gas is concurrent results of the thermal expansion,the fluid of compressed gas,magnetic fluid dynamic force,while the thermal stress removal plays a dominant role in machining the hard-brittle material.The principles for ultrasonic vibration aided ED-Milling in gas and gas medium ED-Milling submerged in liquid were analyzed.According to the characteristics of the pressure status between electrodes,the medium between the tool electrode and the work piece for gas medium ED-Milling submerged in liquid is still gas dielectric.The mechanism of the effect of the tool electrode's ultrasonic vibration is that the machining efficiency and the machining status can be improved because ultrasonic vibration of the tool electrode can enlarge the practical discharge gap range.The fundamental machining performances such as material removal rate(MRR) and surface quality were studied systematically.The model of MRR for ED-Milling in gas was developed based on the characteristics of material removal of single pulse discharge from which the machining parameters can be divided into two groups including the parameters affecting the characteristics of material removal of single pulse discharge and the parameters affecting pulse usage factor.The model has been verified by process experimental research.By comparing the rule of MRR for gas medium ED-Milling submerged in liquid with the MRR for ED-Milling in gas,the MRR of the former is much greater than that of the latter when the large pulse energy is chosen, while the MRR of the former is close to the MRR of the latter when the small pulse energy is chosen.The model of surface roughness for ED-Milling in gas was developed based on the form of single pulse discharge crater and its overlap coefficient has been deduced.The model has been verified by the process experimental research.By comparing the surface of the workpiece of ED-Milling in gas,gas medium ED-Milling submerged in liquid,EDM in oil dielectric,the shape of the firth two is smoother than that of the last one,and the craters of the former two items are big and shallow while the craters of the last one are small and deep.A novel ED-Milling method,electrical discharge layered milling in gas (EDL-Milling in gas),was developed.The machine tool and process plan strategy of the technology were studied systematically.The machine tool can accomplish the following functions,X-axis and Y-axis feed movement of the workpiece,and Z-axis feed movement of the tool electrode,the generation movement of the rotary movement and ultrasonic vibration of the tool electrode.The machining process is controlled by an open system architecture CNC system based on "IPC+PMAC".The machine tool is applicable in these processes including EDL-Milling in gas,ultrasonic vibration aided EDL-Milling in gas as well as gas medium EDL-Milling submerged in liquid.Process plan strategies for EDL-Milling in gas were developed according to the machining features of the process.Bottom milling method,twice-layered slicing strategy,tool path planning algorithm with the couple of direction-parallel machining and contour machining path as well as the semi-online periodical compensation method were developed.CAD/CAM system can be accomplished through the following procedures. Drawing of the workpiece,process plan strategy,and initial tool path generation can be accomplished by the commercial software Pro/E.NC code accepted by CNC system can be acquired through post-processing procedure.3-D machining samples have verified the feasibility of the approach.Numerical solutions to the thermal model of EDM in gas were studied.The mathematical model of the thermo-physical procedure was built and the formula of distributed surface flux was deduced.Mathematical model can be calculated through FEM(finite element method) software ABAQUS.The results of the FEM analysis have verified the feasibility of the mathematical model of EDM in gas.