Evaluation of graphite as tool electrode material in low-energy electrical discharge machining processes

In sinking electrical discharge machining (S-EDM), graphite is a well-established tool electrode material for roughing processes as it enables high material removal rates V̇W while maintaining low relative tool wear ϑ. For finishing processes, other tool electrode materials, most notably copper, are regarded as superior due to lower achievable surface roughness Ra. With current process understanding, graphite tool electrodes reveal significant relative tool wear ϑ and low material removal rates V̇W when used in finishing processes. The achievable surface roughness is typically limited to Ra = 0.8 μm using graphite as tool electrode material with grain sizes Dg < 3 μm. In the upstream preparation of tool electrodes by cutting, graphite offers significant advantages over copper since it enables higher aspect ratios φ and exhibits lower susceptibility to burr formation potentially diminishing the need for subsequent deburring processes. Due to these advantages, the development of finishing processes utilizing graphite tool electrodes promises to be highly advantageous. Expanding the application scope of graphite tool electrodes to include finishing processes could help reduce operational complexity in industrial manufacturing. There is currently no comprehensive understanding of the lower technological boundaries of graphite tool electrodes with respect to the achievable surface roughness Ra and the corresponding tool electrode wear behavior. Therefore, the transition regime from macro- to micro-EDM, around the threshold discharge energy of Ed = 100 µJ, is explored. In this study, three tool electrode materials with grain sizes 1 μm ≤ Dg ≤ 9 μm were investigated. To allow for a causal explanation of the observed process performance and wear behavior, their thermo- and electrophysical properties were characterized. Subsequently, experiments with discharge durations 1 µs ≤ te ≤ 19 µs and varying tool electrode polarity P were conducted under otherwise constant process conditions, resulting in measured average discharge energies 36.49 µJ ≤ E̅d ≤ 302.72 µJ. Tool electrode polarity P was identified as a decisive factor for wear behavior with negative tool electrode polarity P leading to excessive relative tool wear ϑ. Cavities with surface roughness as low as Ra = 0.45 μm were produced while the lowest relative tool wear achieved was ϑ = 72.92%, which is on the same order of magnitude as that of copper under standard finishing conditions. This works aims to outline the capabilities of graphite as a tool electrode material in finishing processes and to motivate further research towards a coherent process understanding of its application in micro-EDM.