Geometric error evaluation and improvement in UEDM for deep microholes

A geometric error evaluation methodology for deep microholes, incorporating roundness error and taper assessment through an edge contour extraction algorithm based on image processing is presented. The proposed approach applies filtering, segmentation, closure operation, and contour detection to the obtained image, with roundness error quantified via the least squares circle method. Ultrasonic-assisted electrical discharge machining (UEDM) was used with a single-sided electrode guiding device, and a deep microhole was fabricated in aluminum alloy with a diameter of 0.228 mm and an aspect ratio of 30:1. The resulting microhole exhibited significant geometric inaccuracies, including pronounced roundness errors and taper. Analysis indicates that centrifugal force generated by high-speed spindle rotation, which leads to electrode deflection, constitutes the primary mechanism for taper formation in deep microholes. To mitigate geometric deviations caused by electrode displacement, a double-sided electrode guidance strategy was developed for the machining of microholes with high aspect ratios. Comparative experimental results demonstrate that the double-sided electrode guidance method reduces roundness error by 75.2% and taper by 97.7% relative to single-sided guidance. The double-sided electrode guidance method not only significantly enhances geometric accuracy but also minimizes repetitive positioning and clamping errors during subsequent electrode-based reaming operations, thereby improving processing efficiency in post-wire EDM processes.