Variable-parameter high-precision electrochemical discharge drilling method for 440C-Nb without recast layer

Engine nozzle orifices are broadly used in the automotive field, and their manufacture entails high drilling accuracy, consistency, and excellent machined surface quality. Electrochemical discharge drilling (ECDD) is an innovative compound machining technique, which can be used to machine small holes of difficult-to-cut materials. Electrical discharge machining (EDM) as well as electrochemical machining (ECM) can occur simultaneously in ECDD, owing to which it can obtain not only good surface quality but also high processing speed. However, the machining accuracy, taper, and surface quality achieved using ECDD do not satisfy the requirements of engine nozzle orifices, which are of a higher standard. For improving the machining accuracy and surface quality, a variable-parameter electrochemical discharge drilling (VPECDD) has been proposed. The machining process is divided into two consecutive stages, including perforation and shape modification, while different parameters are used in different stages. In this study, the finite element method was applied to study the mechanism of VPECDD, and comparative experiments were performed among EDM, ECDD, and VPECDD. The results indicate that VPECDD can achieve very high machining accuracy and consistency, obtain an inlet and outlet diameter difference at 0.002 mm, maintain an average hole taper at 0.0004°, and remove the recast layer completely. Finally, it is verified that VPECDD is suitable for machining microholes with high precision and no recast layer.