Reverse microelectrical discharge machining (R-MEDM) process is a recent variant of microelectrical discharge machining process capable of fabricating high aspect ratio arrayed microfeatures and textured surfaces. Efficient flushing of the debris particles from the interelectrode gap is essential for process stability, but extremely small interelectrode gaps (∼5 µm) make the dispelling of debris difficult, rendering the R-MEDM process infeasible for machining difficult-to-erode materials and creation of engineered/textured surfaces. It has been experimentally observed that the electrode vibrations facilitate the flushing of debris particles and improve the erosion rate, surface morphology, and dimensional accuracy of the machined features. Despite the obvious advantages, the vibration-assisted R-MEDM process, specifically the debris motion and dielectric flow under the effect of vibration, is not very well understood. Consequently, this paper is focused on computational modeling of the debris motion and its interaction with the dielectric fluid under low-amplitude vibrations imparted via a magnetorestrictive actuator. The effects of frequency and amplitude of the electrode vibration on the debris motion have been quantified. The higher local debris velocities and oscillatory motion due to flow reversal potentially reduce the debris agglomeration. As a result, the normal discharge duration, which is responsible for the material erosion, is increased and fabrication of arrayed features on difficult-to-erode materials and creation of surface texture over large areas become feasible.[ 2013-0394]Index Terms-Reverse micro electrical discharge machining (R-EDM), vibration assisted EDM, debris simulation, dielectric flow, textured surfaces, micromachining.

The objective of this paper is to study the time-evolution of the process mechanics for micro-electrical discharge machining (MEDM) and reverse-micro-electrical discharge machining (R-MEDM), as a function of key system parameters, viz., voltage, capacitance, and threshold of the spark circuit. Full factorial experiments have been performed to quantify the aforementioned system parameters on the MEDM and R-MEDM processes. The process monitoring voltage and current signals, material erosion rate and the surface roughness values are the machining responses of interest. The voltage and current (V-I) signals reveal information about the material erosion rate and the extent of debris-interference associated with the corresponding process. Analysis of the V-I signals shows that R-MEDM is more stable than MEDM and can therefore be operated at aggressive conditions of capacitance and voltage. R-MEDM also results in higher material erosion rates but the resulting surface has a higher surface roughness value than that generated by MEDM. A debris deposition mechanism is proposed for R-MEDM that suggests debris entrapment and subsequent welding to the machined feature to be the reason for the increased surface roughness.

There are several examples in nature where the biological surfaces exhibit unique functional response, such as velcro, fish scale and lotus leaves. The texture on lotus leaf exhibits super-hydrophobicity and self cleaning properties. Lotus leaf has hemispherical protrusions of 20–30 μm in diameters which are randomly distributed over the surface. This work is focused on creating similar textured surfaces on Ti6Al4V rods via a vibration assisted reverse micro Electrical Discharge Machining (R-MEDM) process. Textured surfaces containing micropillars of 40–50 μm in diameter spaced at 35 μm have been created during the process. These textured surfaces are expected to exhibit hydrophobicity and hemocompatibility. To experimentally characterize the process, a full factorial design of experiments has been conducted to analyze the effects of voltage, capacitance, amplitude and frequency of the anode (plate electrode) vibrations on the erosion rate and process stability. The process stability is expressed in terms of the percentages of the normal, open circuit and the short circuit durations in the voltage-current (VI) signature obtained during the process. It has been observed that the normal discharge durations increase with an increase in the amplitude and the frequency of the vibrations. Fabricated texture exhibits hydrophobicity and the measured contact angles in a sessile drop test with water varied between 110 and 115°. Also, the textured surface was subjected to hemotoxicity tests which yielded positive results. Based on these results, it can be seen that the machined textured surface are hydrophobic and biocompatible in nature which could potentially find applications in cardiovascular biomedical implants. In addition, this process has been used to create hierarchical structures comprising of primary and a secondary structure over it to mimic the hierarchical structures found on lotus leaves.

High aspect ratio arrayed micro-structures and textured surfaces are required in diversified applications such as electrical contacts, printing heads, electrodes for micro-batteries, injection nozzles, nano-material delivery systems, biomedical implants, and hydrophobic surfaces. Reverse micro-electrical discharge machining (R-MEDM) process has a capability to fabricate such arrayed features on a variety of workpiece materials irrespective of their mechanical properties. R-MEDM is a variant of micro-electrical discharge machining (MEDM) process, key difference being, extruded arrayed features are fabricated in the R-MEDM process against the micro-cavities that are machined in MEDM. This article highlights the recent advances in process characterization and modelling of mechanics of the R-MEDM process. The focus of discussion is on comparing the process with the other micromachining processes presently available for the fabrication of arrayed micro-features. In addition, R-MEDM process characteristics in the fabrication of arrayed features on ‘easy’ and ‘difficult’ to erode materials are presented. It is understood that R-MEDM has comparable or in some cases better performance in the fabrication of arrayed features than the processes like micro-milling, micro-wire EDM, micro-wire electrical discharge grinding (EDG) and block EDG.

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