| A five-axis composite processing machine tool is designed by Shandong University, Tianjin University and Tsinghua University. The ultrasonic aided electric discharge milling system of the tool is consist of feeding system, ultrasonic vibration system, pulse power, pulse power control system, gap status detecting system, path planning system, tool wear detection and compensation system, etc.A numerical control high frequency pulse power is designed. The gap status is detected by the gap voltage collect system and controlled by the gap status control system. The data structure based on the hierarchical path planning strategy is designed to store the processing path data, which supports spindle back along the feeding path and processing breakpoints recovery. A process database is integrated in the pulse power control system, which is used to manage the process data. The tool wear detection and compensation system is a real time detection and compensation system. The tool wear and electrode diameter offset are compensated by tool path planning system. High dimensional accuracy and shape accuracy can be achieved.Electric discharge machining mechanisms, including the discharge channel formation and extension, material removal mechanism, polarity effect, etc, are system studied. Free electron plays a major role in the formation and expansion of the discharge channel. The mobility of ion is smaller in compared with electron, its contribution to the formation and expansion of the discharge channel is not obvious. The influence of standard voltage, open voltage, peak current, pulse during time, pulse off time on the expansion of the discharge channel is studied and mathematical model is established.The polarity effect on electrode coke protection, surface roughness, material removal rate is studied. The carbon on the electrode surface is mainly come from the dielectric fluid decompose and the material components. Part of the carbon pass through the gap is deposited on the electrode surface during the discharge explosion. Part of the carbon in the discharge channel becomes ion and moves towards the electrode when EDM with positive polarity machining.The influence of ultrasonic vibration on electric discharge machining is systematic studied. Although the energy of the ultrasonic vibration can't be strengthened to the energy of electric discharging, the ultrasonic vibration can increase spark region, decrease arc discharge and shot circuit, increase the material ejection rate, decrease the breakdown voltage of the dielectric fluid, thus increase the material removal rate and processing efficiency.Mathematical model of surface roughness and electric discharge parameters are established based on the relation of electric discharge energy and surface roughness theory. Mathematical model of surface roughness and single spark size are established based on the relation of single spark and surface roughness theory, which can reflect the alternation trends of surface roughness along single spark size well. The trends of surface roughness alternate along electric discharge parameters are analyzed based on the experimental data. The surface roughness decreases along the base voltage, increases along the peak current, increase along the pulse during time. The pulse off time has no influence on surface roughness when the pulse off time can satisfy the ionization require, else the surface roughness will decrease when arc discharge rate increases. The surfaces machined by negative polarity are better than that machined by positive polarity. The surfaces machined by ultrasonic vibration aided are better than that machined without ultrasonic vibration aided.Electric discharge milling technology is systematic studied. The influence of electrode base surface eccentric and uneven on milling machining is studied. The polarity effect on spark rate and machining efficiency is studied by simulation. The gap of negative machining is smaller than positive machining to achieve the same electric strength, and its spark rate is higher than positive machining. Verify experiments are carried out on the Archie EDM molding machine tool. Ultrasonic vibration aided electric discharge milling45steel orthogonal contrast experiments are carried out respectively. Mathematical model is established based on the theory relation between material removal rate and electric discharge parameters. The influence of ultrasonic vibration on electric discharge parameters'effects is studied by orthogonal analysis and regress analysis, same conclusion is obtained. Single factor electric discharge milling experiments are taken out to study the improve effect of ultrasonic vibration. The material removal rate increases, tool wear rate decreases, relative tool wear rate decreases when ultrasonic vibration is aided.Ultrasonic vibration aided electric discharge milling of engineering ceramic contrast experiments are taken out respectively and the influence of ultrasonic vibration and electric discharge parameters on material removal rate, tool wear rate, and relative material removal rate are studied. The improve effect of ultrasonic vibration on machining efficiency is very obvious. The melting points and boiling points of the engineering ceramic components are different, TiC and WC are separated out in melt solid mode, result in the uneven of the carbon content of the machined surfaces and their resistivity are effected. Insulation and low hardness TiO2and WO3are generated through the oxidation of TiC and WC with the oxygen released by the ionization of deionized water, which will separate out on the surface of the ceramic and decrease the conductivity of the ceramic. TiC and WC are complete oxidized on the serious burned surfaces, the surfaces are insulated and processing can't continue, the workpiece and electrode will crushed by collision. Peak current has the greatest influence on material removal rate. Because the ceramic is brittle materials, the main material removal mechanism is spalling, besides melting, evaporation, ejection when the processing energy is low. When the processing energy is high, oxygen, melting and ejection become the main material removal mechanisms; there is no crack on the surface.The electric field is simulated by Ansys and gap fluid field is simulated by fluent. Verify experiments are curried out and the influence of flushing mode on material removal rate, tool wear rate, and relative tool wear rate are studied, reciprocal effect of flushing mode, turning speed of the electrode, and ultrasonic vibration are studied. A double needles simulation model is established and simulation is curried out to study the near-point discharge phenomenon. The electric field strength and grads near the tip of the needles are very high, which is very beneficial to the breakdown of the dielectric. The press field, velocity field, particle distribution field and particle density of the gap center line in a vibration period are simulated to study the influence of ultrasonic vibration on them. The press field, velocity field, particle distribution field and particle density of the gap center line of four different depth are simulated to study the influence of depth on them.This work was financially supported by the the National863Major Project of the People's Republic of China (2009AA044204) and National Natural Science Foundation of China (50875157).
|
PDF Full Text Request