INVESTIGATIONS ON ECDM FOR SUBTRACTIVE MICRO-FABRICATION ON GLASS

Abstract

Miniaturization has been one of the driving forces of technology during last 30 years. One of the best examples in miniaturization can be seen in semiconductor industry for automation, where number of components on a chip has approximately been doubled in every 18 months. The technology has moved into the nano-processing era and even for precision processes submicron precision is achievable. The manufacturing industry uses two main approaches for micro-fabrication, i.e., to produce micro-features of well-defined shape and size. The top-down approach of micro-fabrication consists of subtractive processes that removes materials from a bulk material to produce a desired topographical feature. Whereas bottom-up approach involves additive processes that involves assembling together precursor building blocks to obtain the desired features. The subtractive micro fabrication is preferred for large runs of parts, simpler design, and for parts that can be manufactured exactly the same way many times without changes. Whereas additive manufacturing is best suited for single prototypes or very small batches, and for parts that have a specialized function. It is imperative to understand that subtractive processes would be economically superior to additive processes for mass production. In recent years, the need for micro-mechanical systems has increased, for example, drug delivery systems and lab-on-chip systems, etc. The consequence is that traditional engineering materials such as metals, polymers, ceramic and glass are seen more often in micro-products. Amongst engineering materials, glass can be a preferred material for micro-fabrication. Its special properties like transparency, chemical resistance and easy bonding with semi-conductive materials make it a viable material in area of micro-fluidics, biomedical systems etc. However, glass being hard and brittle is difficult-to- vi machine using conventional used subtractive processes. Thus, a need exists to identify, design or develop a subtractive process for machining of glass with capabilities to machine in micro regime. A solution for fabrication of micro-features on glass can be use of unconventional machining processes. Electro chemical discharge machining (ECDM) is one such unconventional machining process. It is an advanced hybrid process comprising the techniques of electrochemical machining (ECM) and electro discharge machining (EDM) together. In this process the material is removed due to melting of work piece material and chemical etching. ECDM has proven its efficacy for the machining of hard, brittle and non-conductive materials. Further, ECDM can be used for machining of conductive materials. Irrespective of many advantages available with ECDM, its mechanism is yet to be fully explored. The need of accuracy and precision required in micro-fabrication using ECDM insists further investigations. In view of above, the prominent objectives of the present research focus on the following aspects: • Development of ECDM setup for subtractive micro-fabrication on glass. This may involve machining of different shapes like holes, micro-channels and complex profiles in micro regime. • To develop scientific theory with explanation on the mechanism of the proposed process by conducting detailed study of the surfaces produced, geometrical dimensions and material removal rate (MRR) relating other process parameters. • To investigate the effect of various process parameters on the response characteristics (e.g., surface quality and material removal rate). vii • To provide additional assistance of mechanical grinding to ECDM termed as grinding assisted electro chemical discharge machining (GAECDM) for enhancement of process performance. • To characterize the machined features in terms of dimensional accuracy, surface integrity using different characterization techniques like, voltage time wave forms, optical microscope etc. The experimental work of the present research endeavour was divided into five phases. An exhaustive experimentation has been carried out to investigate the influence of different process parameters on the output responses during ECDM of borosilicate glass. In first phase, three different types of electrolytes namely, NaCl, KOH and NaOH were used for drilling of micro-holes. The measured response characteristics were material removal rate (MRR) and hole overcut (HOC). The effect of pulse duration on the aspect ratio of the work material was investigated. An effective range of pulse duration was identified to achieve better control on the response characteristics. The response characteristics measured were machining depth, surface damage, aspect ratio and tool wear. In second phase, experiments were performed for the selection of ranges of the input process parameters using one-factor-at-time (OFAT) approach in context of fabrication of holes and micro-channels. The selected process parameters were applied voltage, pulse-on-time (Ton), pulse-off-time (Toff), electrolyte concentration and feed rate. The response characteristics measured for holes were MRR and HOC, while for micro channels were MRR and width overcut (WOC). Further, investigation was carried out using cylindrical tool having diamond abrasive coating over it for drilling of holes. Such viii kind of drilling operation was termed as Grinding Assisted Electro Chemical Discharge Machining (GAECDM). In third and fourth phase, second order non-linear mathematical models were developed for establishing the relationship between the input process parameters and response characteristics for drilling of holes and fabrication of micro-channels respectively. The Design of Experiments (DOE) was based on Response Surface Methodology (RSM). The input process parameters were applied voltage, Ton, Toff, electrolyte concentration and feed rate. The response characteristics for hole were HOC, taper and MRR, while for micro channels, the responses were WOC, surface roughness (SR) and MRR. The process performance can be further improved by optimizing the input process parameters. The optimized performance can lead to effective management of resources and reduce the production cost. Therefore, multi-criteria optimization was also performed using desirability approach. In fifth phase, the optimal parameters were used to assess the process performance. The process performance of GAECDM was assessed over conventional ECDM in terms of MRR, hole over cut, taper and heat affected zone. The effect of the optimal process parameters on the machining performance has also been investigated during the fabrication of three different intricate profiles of micro-channels. ECDM is a complex machining process because the process is a hybrid machining process, which is a combination of ECM and EDM processes. The understanding of the material removal mechanism is area of research of paramount importance. Therefore, an effort was made to realize the process mechanism of ECDM by visualization technique. The nature of process mechanism was investigated by high speed images ix and voltage time waveform. The different zones of process mechanism was identified for electro chemical discharge phenomenon.