A sustainability index for the micro-EDM drilling process

The increasing attention being paid to environmental protection has resulted in significant changes in all production systems. Moreover, system efficiency and productivity should be improved and manufacturing system design must take ecological constraints into consideration. Accordingly, the definition of an index of sustainability can help in assessing the different and sometimes conflicting aspects of a manufacturing process, such as manufacturing costs, energy consumption, waste management, personnel health, and operational safety. In this study, a sustainability index is defined and applied to a micro-electrical discharge machining (EDM) process in drilling operations. The index is composed of five sub-indices that evaluate a particular aspect: energy consumption, environmental impact, dielectric consumption, wear of electrode, and machining performance in general. Each sub-index is calculated by assigning an economic cost to the undesirable impact through a formulae of coefficients (weights) and variables (such as machining time and electrode wear). High index values mean that the process consumes a significant amount of resources and/or produces environmental pollution that is poorly sustainable. In contrast, lower values indicate that the process consumes fewer resources; therefore, it has a lower impact and is more sustainable. The developed sustainability index is applied to an experimental case: the drilling of micro holes in stainless steel (AISI 316L) and titanium alloy (Ti6Al4V) using two types of electrodes (brass and tungsten carbide). Several types of conventional and innovative dielectrics are used (both liquid and gas): kerosene, demineralised water, vegetable oil, compressed air, and oxygen gas. For each combination of workpiece, electrode material, and dielectric tested, the sustainability index is calculated and the outcomes are analysed. It is found that the dielectric plays a major role in the EDM process, because it significantly influences the machining performance. Moreover, it affects the environment in terms of the quantity of consumed resources and pollution created by the process. The type of electrode also affects the process. Within the limit of the present investigation, water and vegetable oil have the lowest environmental impact. For these dielectrics, brass electrodes halve the impact when stainless steel is processed, while tungsten carbide is better for titanium sheets. The use of kerosene as a dielectric coupled with tungsten carbide as an electrode makes the process less green and should be avoided. The combination of air as a dielectric and tungsten carbide as an electrode proves interesting. Oxygen as a dielectric shows instabilities during the process, thus performing poorly in terms of the index. The proposed model for measuring the sustainability of the micro-EDM drilling process is designed to be useful in industrial applications and is easily computable. The index provides a tool that can assist in the decision-making stage of selecting conditions aimed at minimising environmental impact.