Optimization of Dielectric Oil Viscosity for High-precision Wire EDM

Deionized water has been conventionally used as dielectric fluid in wire EDM, while oil in die-sinking EDM. The gap distance using dielectric oil is smaller than that using deionized water, since the electrical resistivity of dielectric oil is higher. In addition, EDM in oil can prevent oxidization and anodization of machined surface. For these reasons, dielectric oil has been recently used also in wire EDM for achieving high-precision machining. However, the influence of dielectric oil properties on wire EDM characteristics has not been clarified sufficiently. This study fundamentally investigated the influence of dielectric oil viscosity on wire EDM characteristics. Three types of dielectric oil differing only in viscosity were examined in this study. Linear cutting experiments were carried out using these oils at first. The experiment results showed that the removal rate increases with dielectric oil viscosity. Then, the influence of viscosity on crater removal volume was investigated to discuss the cause of the change in removal rate, and it was found that crater removal volume increases with the viscosity. Next, the difference in machining accuracy with dielectric oil viscosity was investigated. It was found that the corner shape accuracy deteriorates with the increase of dielectric oil viscosity while the machined surface waviness and straightness improve slightly. In addition, CFD analysis of hydrodynamics force distribution on wire surface, structural analysis of wire deflection, and high-speed observation of wire vibration during machining were carried out. The results showed that the wire deflection increases, and the wire vibration decreases in the case of higher viscosity. From these results, it can be considered that the hydrodynamics force acting on the wire increases with dielectric oil viscosity. Then, the wire deflection in the machining direction increases, leading to the deterioration in corner shape accuracy. On the other hand, the wire vibration is somewhat restrained, leading to the improvement in the machined surface accuracy.