No abstract

While machining the quartz glass in micro ECDM (electrochemical discharge machining) scanning process, the wear of tool electrode is adverse to machining accuracy. In this research, the developed spindle with servo feedback control of flexible force is used to avoid the instantaneous change of contact force that leads to the bending of tool electrode when the machining surface is uneven. The basic experiments are carried out through optimizing the processing parameters of open-circuit voltage, scanning speed, and scanning time, and then the mechanism and key influential factors of tool wear are analyzed. It is found that the tool wear is caused by the chemical reaction, the heat from discharges, and the friction between tool-electrode tip and workpiece surface. Therefore, three methods for reducing the tool wear are proposed and experimented. Method I: the low temperature electrolyte is applied in increasing the heat dissipation ratio of the tool-tip. Method II: the side wall of tool electrode is insulated by an oil film to constrain spark discharges only at the tool-tip bottom. Method III: more bubbles are pumped to promote the formation of gas film and the electrolyte renewal. Experimental results show the tool wear can be obviously reduced, so the machining accuracy of micro structures on quartz glass is improved in micro ECDM scanning process.

In micro electrical discharge machining (micro EDM) using the non-hollow circular cross-section tool electrode with the side flushing technique, when the aspect ratio of machined micro-hole is expected to be further increased, the discharge debris expelling speed and the working fluid renewal efficiency are weakened, which hinders the improvement of machining efficiency and accuracy with increased machining depth. In order to reveal the flow behavior of the working fluid in the micro EDM gap, so as to realize the high-precision and high-efficiency machining of micro-hole with high aspect ratio, a three-phase flow simulation model of fluid, bubble and debris is established in Fluent under the ideal assumption that the spark discharges occur continuously to generate high-pressure bubbles. The simulation results show that when the boundary condition of the flushing pressure at the side gap entrance is set to 0, the pressure wave emitted when the high-pressure bubble expands, which is formed by the instantaneous gasification of the working fluid between electrodes under high temperature, is the source of pneumatic force that drives the working fluid flow at the micron scale. Affected by the gap flow channel structure and the viscous resistance from inner wall, the flow velocity direction of the fluid dragging the discharge debris to rise up and expel will change, forming a dynamic alternation process of flowing into and out of the side machining gap entry. As the machining depth increases, due to the energy attenuation of the pressure wave propagating from the bottom gap to the side gap entrance, the expelling speed of the discharge debris decreases exponentially at the side gap entrance, resulting in the reduced machining efficiency and accuracy. However, when the simulated bubble generation frequency is increased to the MHz level, the expelling efficiency of debris has a step-like improvement. The continuous and high-frequency generation of high-pressure bubbles can maintain a high pressure gradient in the bottom gap, and the discharge debris is able to continuously move upward without falling back to accumulate in the bottom gap, which is beneficial to the stable and smooth machining process, realizing the high-precision and high-efficiency machining of micro-hole with high aspect ratio.