Gap profile model of electrical discharge machining process to predict material removal rate

Abstract

Electrical discharge machining (EDM) is a non-traditional material removal technique using an electrical spark-erosion process in the presence of dielectric liquid where electrode and workpiece are not in physical contact. Although EDM is widely implemented in the manufacturing industry, knowledge about the process is still at an early stage, which poses many challenges for further development. Experimental analysis is time-consuming and costly due to the highly stochastic and complex nature of the process. Thus, research efforts are directed toward process modeling to study EDM behaviour by eliminating experimental difficulties. Developed models are mostly centred on only one sparking phase, especially the discharge or ignition phase. In order to achieve a complete understanding of machining behaviour, it is essential to consider all sparking phases. This research presents a mathematical model of EDM gap profile by introducing an equivalent circuit of gap spark. This is to reach precise insight into the interactive behaviour of the machining process regarding ignition, discharge and recovery phases. The equivalent circuit model is designed based on the sparking phases and pulse power generator. Buck converter and transistor-based switching circuits are used to provide suitable pulsed voltage. Spark circuit is employed to obtain mathematical equations of gap profile for studying the time-varying behaviour of the EDM process through Matlab simulation. In order to validate the model, simulated data are first compared with previous experimental data and then with data from the EDM operation manual, both in term of Material Removal Rate (MRR). It is shown that the simulated model can predict the dynamic behavior of the EDM process with an average simulated error of about 8.27% for steel workpiece and copper electrode and about 7.93% for steel workpiece and graphite electrode. Comparison with MRR from the EDM manual also showed an average error of 10.10%, which is acceptable to standardize the validation process. Besides, the consistency range of the model is confirmed at noise power np≤10−5J with an average error of 11.15% for steel workpiece and copper electrode. Then, a parametric study of simulated MRR is carried out to investigate the effect of pulse on-time and peak gap current on MRR. Research conducted shows that the MRR increased by increasing pulse on-time and peak gap current up to peak value of pulse on-time for each peak discharge current. Finally, based on the EDM discharge self-sustaining condition, gap discharge closed-loop structure is formed via discharge model to evaluate the discharge stability. The Influence of peak discharge current on the response time of the system is analyzed using frequency response method. It is found that increasing peak discharge current results in slower system time response and improves the discharge stability. This study can be helpful to reveal the mechanism of EDM, predict the machining time, maintain the discharge stability, and select the process parameters.