High-spatial Resolution Figuring by Pulse Width Modulation Controlled Plasma Chemical Vaporization Machining

Numerically controlled plasma chemical vaporization machining (NC-PCVM) is an ultra-precision figuring technique using chemical reactions between radicals generated in atmospheric pressure plasma and surface of the workpiece. Figure error consists of waviness which has long spatial wavelength and roughness which has short spatial wavelength. In the case of NC-PCVM using the cylindrical electrode with a diameter of 3-6 mm, it is difficult to remove the small wavelength components (several millimetres in wavelength). Therefore, improvement of spatial resolution in figuring is required for NC-PCVM. To resolve this issue, pulse width modulation (PWM) control unit was developed to control the removal volume distribution instead of scanning speed control of the worktable. Removal error at the position where the set removal depth steeply change drastically decreased to less than 4% by applying PWM controlled figuring, while the maximum removal error of figuring which applied the scanning speed control of the worktable to control the removal depth distribution was 20%. And a new electrode, which limits the irradiation area of plasma by installing of a alumina ceramics cover having a small orifice with a diameter of 1.0 mm, was also developed. By applying this electrode, the full width at half maximum (FWHM) of the removal spot was reduced from 4.68 mm to 1.10 mm. Small spatial wavelength components of 4 mm were successfully corrected by NC-PCVM which was combined PWM control and orifice electrode.