Grinding aided electrochemical discharge drilling (G-ECDD): A theoretical analysis and mathematical modelling of material removal rate

Grinding aided Electrochemical Discharge Drilling (G-ECDD) is a triplex hybrid machining process in which the various physical and chemical aspects of Electrochemical Discharge Machining (ECDM) and grinding actions are combined for selective material removal. An attempt has been made to develop a mathematical model for Material Removal Rate (MRR) by considering the MRR in G-ECDD as the weighted sum of MRR by ECDM action and grinding action of diamond core drill tool. Three distinct machining conditions depending on the predominant material removal mechanism are identified from the microscopic images of the machined surface and by the critical analysis of the machining parameters. The significant machining parameters like discharge energy, discharge frequency, tool feed rate, tool rotational speed and grit density are correlated with MRR for the three different machining conditions in G-ECDD. The weights for determining the individual contribution of ECDM and grinding action in the overall MRR are found from the experiments conducted using different machining conditions. A parameter called penetration ratio (P-ratio) is identified to be effective for predicting the performance of G-ECDD and to distinguish the different machining conditions. The dimensionless factor P-ratio can be defined as the ratio of the downward feed rate of the tool to the depth of the thermally softened layer developed per unit time during G-ECDD. The grinding action predominates for the process with P > 1.2. The combined grinding and thermal action results for MRR when 0.8 ≤ P ≤ 1.2. The thermal melting action of discharges pre-dominates for P < 0.8 and mainly contributes for MRR. The G-ECDD performed with P-ratio in the range 0.8 ≤ P ≤ 1.2 exhibited high MRR with superior machined hole quality as the thermally softened material and the recast layers are fully removed by the abrasive action of the grinding tool thereby developing a clear hole. It is recommended that the machining parameters like tool rotational speed, tool feed rate, voltage and pulse duration need to be pre-determined using the developed model with the P-ratio in the range 0.8 ≤ P ≤ 1.2 to obtain the full potential and benefits of the G-ECDD process. The model has been validated using further experiments and found that the model is capable of predicting the MRR with a mean percentage error of 8.38%.