Optimisation of CNC wire EDM process parameters and establishment polynomial process model for multiple performance characterestics

Introduction Industries have witnessed a rapid growth in the development of harder and difficult to machine materials such as hastalloy, nitralloy, carbides, stainless steel and many other high strength temperature resistant alloys (HSTR). These materials find wide applications in aerospace, nuclear engineering and other industries owing to their high strength to weight ratio, hardness and heat resisting qualities. For such materials, the conventional edged tool machining is difficult, time consuming and sometimes impossible. Considering the seriousness of the problem, Merchant in 1960s emphasized the need for the newer concepts in metal machining called Electrical Discharge Machining (EDM). EDM is the process of material removal from any electrically conductive material by the initiation of rapid and repetitive (Non Stationary) electrical sparks between a tool and work piece connected in an electrical circuit and separated by a flowing dielectric medium. The fundamental capabilities of EDM are possible to machine any conductive material irrespective of its hardness and toughness. Thus, hard metals and alloys which cannot be machined by conventional machine can be machined by EDM. One of the types of EDM is Wire EDM (WEDM). WEDM involves a series of complex heating and cooling processes. In this case, the relative motion between the electrode and the work piece is non axial. Cutting can be done with wire, ribbon, blade or a rotating disc. Cutting using the wire is more popular and the wire is continuously fed from spools. WEDM is always carried out with wire electrode and work piece supplied with dielectric fluid. The electrode and the work piece are separated by a small gap varying from 20 micron to 1mm which is always maintained in order to initiate the spark. Because of the spark localized heating takes place increasing the temperature to the extent of 10000oC. Thus each spark removes a small, discrete portion of material from the work piece leaving a small crater. Gokler et al. [1], made an attempt to select the most suitable cutting and offset parameter combination for the WEDM process on 1040 steel to get the desired surface roughness value. Rozenek et al. [2], studied the effect of pulse-on time, pulse offtime, voltage on the surface roughness and machining feed rate during WEDM of metal matrix composite. Puri et al. [3], determined the influencing parameters on performances such as average cutting speed and surface finish characteristics. Tosun et al. [4], investigated the influence of voltage, wire speed and dielectric pressure on the metal removal rate. From the literature survey, it is found that the Material Removal Rate (MRR), Surface Roughness (Ra), of WEDM is an important performance characteristic and Spark Gap (SG) and Dimensional Deviation (DD) has not been addressed by many researchers. Hence, in this paper an attempt has been made to identify the influencing (significant) parameters on the multiple machining performances and optimize these machining parameters for minimizing the variations in multiple machining