Impact Analysis of Electrolyte Pressure on Shape Accuracy of Micro Holes in ECM with Hollow Electrodes

In electrochemical machining (ECM) of micro holes with high aspect ratio, hollow electrodes are commonly utilized to remove electrolytic products from the inter-electrode gap. In this paper, the effect of electrolyte pressure through hollow electrode on shape accuracy is studied by both numerical simulations and experimental tests. The multi-physics models consisting of electric current field, electrolyte flow, gas diffusion and heat transfer are established by using COMSOL Multiphysics software. With the inlet pressure range of 0.25-1.15 MPa employed, the material removal rate of workpiece is predicted. At different machining depths, the material removal rate of micro-hole bottom is in an increasing trend with the pressure increasing, which has a positive effect on machining stability. With the pressure increasing, the material removal rate of side wall is fluctuating. And the area of vortex zone in the machining gap increases. Consequently, the shape accuracy is deteriorated. From experimental results of reverse tapered holes, at a small machining depth, the shape accuracy is deteriorated (straightness error > 20 μm) with excessive electrolyte pressure. When the hole is deep, the shape accuracy is deteriorated with low pressure because of electrolytic products blocking. The shape accuracy can be improved by changing the electrolyte pressure in optimal rules obtained from the simulated results. The proposed simulation model can be used as a predictive tool to provide optimal parameters for better shape accuracy in micro ECM. On 1.0 mm thick 18CrNi8 workpiece, a reverse tapered hole with diameter of 175 μm and taper angle of 1.12°is machined. The deviation between designed shape and experimental results is 4.2 μm.