Adaptive planetary control with auxiliary magnetic field method for fast EDM drilling

The existence of a thick recast layer on the surface of film cooling holes after fast electrical discharge machining (EDM) drilling is a critical problem in the manufacture of turbine blades. To suppress the generation of a recast layer and improve the machining efficiency, an adaptive planetary control with an auxiliary magnetic field (APCAMF) method is proposed to improve the machining quality of film cooling holes. The auxiliary magnetic field is used to concentrate the discharge energy and accelerate the removal of molten materials. Planetary movements are used to decrease the thickness of a recast layer that solidifies on the hole wall during machining. To reduce unnecessary planetary movements, planetary machining is combined with adaptive servo control. The adaptive control method uses a model that combines online identification with an indirect self-tuning regulator. The controller obtains the discharge status through a novel discharge state detector based on a programmable System-on-Chip (PSoC). While the discharge status is used as the input to the adaptive controller, the servo feedrate is used as the output. Parameters of planetary machining are determined by the effective feed distance and the proportion of short-circuit pulses. With a simple structure, the APCAMF scheme is easy to be implemented. Experiments showed that a rotating auxiliary magnetic field can effectively reduce the thickness of a recast layer as large as 33.53 μm in fast EDM drilling. The recast layer reduction rate of the APCAMF scheme is 73.47%, with the machining efficiency increased by 18.52%.