# Interaction of Machining Parameters on MRR of Sintered NdFeB Processed by EDM-Milling
## 4 Interaction of Machining Parameters on the MRR  
### Interaction of Current and Voltage on the MRR  
#### 4.1.1 Significance of Current on the MRR  
It is obvious in Figure 2 that the influence of current on MRR gradually increases with the increase of voltage. When the value of voltage is 70 V, the change in MRR caused by the change in current reaches a maximum, i.e., the \(\triangle_{U}(I)\) is 34.22 mg/min. This means that the influence of current on MRR is maximum, i.e., the significance of current on MRR is strongest. When the value of voltage is 30 V, \(\triangle_{U}(I)\) is at its minimum of 19.68 mg/min. This means that the significance of current on MRR is weakest. The difference between the 34.22 mg/min and 19.68 mg/min is 14.54 mg/min, i.e., \(\triangle_{U}I\) is 14.54 mg/min, which is listed in Table 2 and used to analyse the significance of MRR. To interpret the above results, the size of the crater on the machined surfaces under different values of current and voltage are observed comparatively, as shown in Figure 3.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{2}{c}{_U-I_} & \multicolumn{2}{c}{\(T_{\text{on}}\)-\(T_{\text{off}}\)} \\ \hline \(\triangle_{y}(x)\) & \(\triangle_{U}(I)\) & & \(\triangle_{U}(I)\) & \(\triangle_{\text{Ten}}(T_{\text{off}})\) & \(\triangle_{\text{Toff}}(T_{\text{on}})\) \\ (mg/min) & 34.22 & 11.93 & 32.86 & 27.16 \\ min & 19.68 & 3.36 & 1.50 & 4.47 \\ \(\triangle_{y\,x}\) & \(\triangle_{U\,I}\) & & \(\triangle_{U\,I\,U}\) & \(\triangle_{\text{Ten}\,I\,U}\) & \(\triangle_{\text{Toff}\,I\,T\,\text{on}}\) \\ (mg/min) & 14.54 & 8.57 & 31.36 & 22.69 \\ sum (mg/min) & 23.11 & & & 54.05 \\ product (mg\({}^{2}\)/min\({}^{2}\)) & 124.61 & & & 711.56 \\ \hline  & \multicolumn{2}{c}{\(T_{\text{on}}\)-\(U\)} & \multicolumn{2}{c}{\(T_{\text{off}}\)-\(U\)} \\ \hline \(\triangle_{y}(x)\) & \(\triangle_{\text{Ten}}(I)\) & & \(\triangle_{U}(T_{\text{on}})\) & \(\triangle_{T\,\text{off}}(I)\) & \(\triangle_{U}(T_{\text{off}})\) \\ max & 20.24 & 27.18 & 16.21 & 28.84 \\ min & 3.35 & 4.47 & 3.52 & 5.51 \\ \(\triangle_{y\,x}\) & \(\triangle_{\text{Ton}\,U}\) & & \(\triangle_{U\,I\,U\,U}\) & \(\triangle_{U\,I\,U\,T\,\text{off}}\) \\ (mg/min) & 16.89 & 22.71 & 12.69 & 23.33 \\ sum (mg/min) & 39.6 & & & 36.02 \\ product (mg\({}^{2}\)/min\({}^{2}\)) & 383.57 & & & 296.06 \\ \hline  & \multicolumn{2}{c}{_1-T\({}_{\text{on}}\)_} & \multicolumn{2}{c}{\(I\)-\(T_{\text{off}}\)} \\ \hline \(\triangle_{y}(x)\) & \(\triangle_{I}(T_{\text{on}})\) & & \(\triangle_{\text{Ton}}(I)\) & \(\triangle_{I}(T_{\text{off}})\) & \(\triangle_{\text{Toff}}(I)\) \\ max & 12.00 & 26.87 & 17.18 & 26.87 \\ min & 12.00 & 26.87 & 17.18 & 26.87 \\ \(\triangle_{y\,x}\Table 3 lists the mean and difference of crater diameters under different values of current and voltage. It is well known that MRR is positively associated with crater diameter. Thus, the effect of voltage on MRR is more significant when the current is 3 A. However, the significance of the effect of voltage on MRR is weaker when the current is 6 A. This is consistent with the results of the interaction curve. This results in significant effects of discharge energy variations on MRR [9]. The reasons for this can be explained as follows. When the discharge energy is low, the deionisation of the dielectric is good, and the diffusion condition of the inter-polar dielectric is good. A change in the value of voltage will result in a change in the distance between the electrode and the workpiece. The energy density of the plasma channel varies significantly from its centre to its circumference along the radius. The effective radius of the plasma channel that can be used for the removal of material varies significantly, and as a result, the diameter of the crater varies significantly. Moreover, there are fewer residual particles in the discharge channel, which means less consumption of discharge energy due to secondary discharge. Therefore, the effect of voltage on MRR is greater under low current.