Process optimization by transferring conventional electrical discharge machining to near dry proceedings for precise bore hole in CoCrMo

Conventional electrical discharge machining (EDM) is processed in a full bath of dielectric media such as deionized water or mineral and synthetic oil products based on carbon compounds. Nowadays, it is also common to perform the process with a mist spray, consisting of a liquid-air mixture, called near dry EDM. This work presents the results of a process transferring from conventional electrical discharge machining to near-dry machining using a mist nozzle with a defined mist precipitation rate to supply a mixture of deionized water and air as dielectric medium. In the presence of a gas at the process site, air mainly consisting of nitrogen and oxygen, other physical laws apply, which are not fully understood until today. For the experimental investigation, holes were drilled in the workpiece, dental alloy CoCrMo, using a brass electrode and then analyzed and optimized by using methods of design of experiments. The results show that the influence of the edge zones is much greater with conventional cooling than with mist cooling, since the larger surface area of the individual drops makes the evaporation enthalpy much larger and thus the process heat is better dissipated. Furthermore, it was observed that oxygen has a process stabilizing effect on the process and the chemical reaction products of the oxidation at the bore hole wall could be detected. This paper shows that the transferring from conventional EDM to near dry EDM has achieved better results in terms of erosion time, relative electrode wear and bore inlet.