Micro convexity elimination of tungsten carbide in electrical discharge milling for lens moulds

Electrical discharge milling (ED milling) using spherical electrode is considered to fabricate tungsten carbide (WC) lens moulds for high volume and cost efficient mass production comparing with hemispherical electrode whose wear is noticeably larger. However, micro convexity with dome shape generating in the center of the mould leads to poor surface quality of moulds when using spherical electrode. Therefore, based on boundary layer theory, the forming mechanism of the micro convexity is analyzed in detail by measuring its height and profile by virtue of 3D laser scanning microscope and scanning electron microscope (SEM). The relationship between the boundary layer thickness and the discharge gap is subsequently established, which can be utilized to reduce the convexity to improve the surface quality of lens mould. Then a method to eliminate the micro convexity is proposed. In addition, experiments of different discharge gaps were carried out to confirm the boundary layer theory. Results show that the height of micro convexity can be reduced to 4 μm when the discharge gap was specified to the boundary layer thickness at certain rotational speed.