Fundamental research for the application of carbon fibers as tool electrodes for micro-ED drilling

Electrical discharge machining (EDM) is an established process used for the machining of complex geometries in hard, high-temperature-resistant and electrically conductive materials. Nevertheless, electrical discharge micro-drilling (micro-ED drilling) still poses the challenge of machining holes with diameters dh ≤ 50 µm, which are commonly present in systems such as fuel injection systems, printing heads, optical diaphragms, and cooling-holes in turbine blades. The preparation of even smaller pin tool electrodes demands a great amount of non-productive time which often involves the use of additional electrical discharge dressing processes for its manufacturing. A possible solution is the use of single carbon fibers as tool electrodes given its small diameters 5 µm ≤ del ≤ 10 µm. Main challenges of using single carbon fibers for this purpose include the necessity separating single fibers from a filament yarn as well as its handling before and during the machining process. In this paper the fundamental research on the geometrical, mechanical and electrical properties of single carbon fibers of the types mesophase pitch-based fibers (MP) and polyacrylonitrile-based fibers (PAN) for its use as tool electrodes in micro-ED drilling as well as the necessary steps to be conducted for the fiber separation are presented. The aim is to establish a set of process parameters and techniques that allow for the systematic and reproducible use of single carbon fibers as tool electrodes and the determination of the most appropriate carbon fiber type. For the separation of carbon fibers pyrolysis processes were carried out. The geometrical properties of the carbon fibers were analyzed using a scanning electron microscope (SEM). Furthermore, a tensile test was performed for all carbon fiber types and diameters. In order to find the specific electrical resistance ρeW of single carbon fibers while using different dielectric fluids, a four-terminal sensing method was carried out. After analyzing the results obtained, the mechanical and electrical limits of each carbon fiber type and fiber diameter could be successfully determined.