Numerical Simulation And Machining Parameter Optimization Of Micro Hole Drilling Aided With Planetary Movement Of Electrode By Micro EDM

Micro-hole drilled by Micro electrical discharge machining (EDM) has the advantages of non mechanical contact with workpiece during machining. Due to its ability of machining any electrically conductive material regardless of hardness of workpiece material, it is widely used to drill micro holes with high aspect ratio. However, it is difficult to obtain micro holes with high aspect ratio because gaseous bubbles and debris generated by discharges restricts the flow of liquid into the narrow discharge gap. The method of micro-hole drilling aided with planetary movement of electrode is able to improve the aspect ratio of micro hole. However, little theoretical analysis has not been done yet related to the mechanism of improvement of micro hole aspect ratio and the relationship between the aspect ratio of micro hole and the radii of planetary movement.A control system has been developed based on a horizontal Micro EDM system. The control system has the function of quick positioning, manual feed, electrode machining, micro hole machining, surface electrical contact and planetary movement of electrode. Using this control system, extensive experiments of micro-hole drilling aided with planetary movement of electrode are conducted.In this thesis, two phase fluid dynamics is used to explain the effect of planetary movement of electrode on improvement of the aspect ratio of micro hole drilled by Micro EDM. Due to uneven flow field caused by planetary movement of electrode, analytical solution can’t be achieved directly. Therefore, the qualitative analysis is obtained from numerical simulation tool. The motion state of bubble and fluid is analyzed by volume of fluid (VOF) method through the simulation. Meanwhile the viscous resistance in the widest gap surface between of electrode and hole wall is calculated in the simulation. It is found that the dielectric flow rate increases in the narrow gap when planetary movement exists because viscous resistance and surface tension is reduced in the widest gap. This is the reason that the aspect ratio of micro hole increases when the planetary movement of electrode is applied in micro hole drilling by micro EDM. The effect of the radius of planetary movement on the aspect ratio of micro-hole is qualitatively analyzed and experimentally verified under planetary radius of0,5,10,15,20um. It is revealed that the viscous resistance decrease with the increase of radius of planetary movement. The aspect ratio of micro hole increase with the increase of radius of planetary movement. Experimental results agree with the simulation results.In experiments, it was observed that unmatched velocity, radius of planetary movement and electrode feed rate lead to unstable machining process, resulting in low machining efficiency. These three machining parameters in planetary movement of electrode need to be optimized. A theoretical model was proposed to describe the relationship among velocity, radius of planetary movement and electrode feed rate. Based on the model, the optimal velocity of planetary movement can be calculated when the planetary radius and feed velocity are known. This model has been verified by experiments. The experiments show that there is an optimal velocity of planetary movement when the radius of planetary movement and electrode feed rate are fixed. The maximum material removal rate was achieved at the optimal velocity of planetary movement. The experimental results agree with theoretical values of the proposed model.