INVESTIGATIONS ON PERFORMANCE ENHANCEMENT OF ECDM PROCESS WHILE MACHINING GLASS

Abstract

Glass is extensively used in laboratory appliances, optical instruments, fabrication of electro-mechanical systems, micro-electronics, domestic and fancy articles due to its excellent properties such as transparency and chemical inertness. Machining of brittle, hard and non-conducting materials like glass is, however, a challenging activity and requires special processes and techniques. The popular non-conventional methods used in processing such materials include ultrasonic, laser and Lithographie Galvanoformung and Abformung (LIGA). However, limitations like high set-up costs, issues in processing reflecting materials and lower surface finish in some of these processes make them a difficult choice. Electro chemical discharge machining (ECDM), on the other hand, is a non-traditional machining process suitable for hard, brittle and non conducting materials. It is primarily used in making micro holes, micro-grooves, micro-slots, micro-channels and complex shaped micro-contours on materials like glass, ceramics (pure Al203), composites etc. Significantly, the process is capable of providing reasonable material removal rate (MRR) and better surface finish. The ECDM process is being investigated since 1968; however, owing to the complexity of the process that involves multiple variables, the technology is still in the nascent stage and, more often, known as a laboratory process. The use of rotational tool and electrolyte stirring methods in ECDM has rarely been attempted; which creates an opportunity to explore this technique for further enhancement in performance. In the present work, a set up for implementing ECDM has been fabricated with attachments for rotary tool and electrolyte stirring with an aim to improve the process performance. Three different types of glass materials (soda lime, borosilicate and optical) were machined with three electrolytes (NaOH, NaNO3 and NaCl solution) and two electrodes, namely - copper and stainless steel (SS-304). Design of experiments (DOE) technique has been used taking upto seven process (input) parameters and three response parameters (material removal (MR), tool wear (TW) and surface roughness (SR)). Further, possible mechanisms of material removal and tool wear modes have also been investigated using scanning electron micrography. The experimental results indicated that, the material removal rate (MRR) while using copper tool was upto 47 % higher than the MRR obtained with the help of SS-304 tools. The tool wear rate vi (TWR) in SS-304 tools, on the other hand, was much less as compared to Cu tools. In case of optical glass, higher material removal rates were recorded even at lower electrolyte concentrations (10%) than borosilicate glass. The performance enhancement studies through electrolyte stirring and tool rotation arrangement showed considerable improvement in MRR (average upto 41.9 %); the surface quality was also showed a significant improvement. In optical glass, considerable improvement in MRR (upto 17.4%) with respect to soda lime glass was noticed; however, the tool wear was marginally more in optical glass (upto 2%) as compared to soda lime glass. The increase in Ra was recorded upto 74.7% with the stirring attachment and 86.1% with the rotational tool attachment in optical glass. Better surface roughness (values upto Ra = 0.22μm) were observed in optical and soda lime glass than borosilicate glass even at lower electrolyte concentration (10%). Modeling of process using various approaches was also carried out. The derived mathematical model predicted the results within ±11% while compared to the actual trials. The developed regression models for MR and TW predicted results with errors in the range of 19.5–38 % for various trial conditions. However, a good correlation with the simulation results was obtained using Adaptive Neuro Fuzzy Inference System (ANFIS) and the experimental results could be obtained in which errors remained within ± 0.5 %. The work indicates that further investigations can well be attempted to develop environment-friendly electrolyte for ECDM which will address some environment related issues associated with the process. Further, finite element simulation of the thin gas layer and the mechanism of thermal erosion under various parametric conditions could throw more light in developing better understanding of the process.