# 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
#### 4.1.2 Significance of Voltage on the MRR  
It is also obvious in Figure 2 that the significance of the influence of voltage on MRR decreases and then increases as the current increases. When the value of current is 3 A, \(\triangle_{I}(U)\) reaches a maximum of 11.93 mg/min, indicating that the influence of voltage on MRR is strongest. When the value of current is 5.4 A, \(\triangle_{I}(U)\) is at its minimum of 3.36 mg/min, indicating that the significance of voltage on MRR is weakest. The \(\triangle_{I}U\) is 8.57 mg/min and is listed in Table 2. Furthermore, \(\triangle_{I}(U)\) is 4.89 mg/min at a current of 6 A, which is less than that under a current of 3 A.  
As shown in Table 3, the difference in crater diameter between 3 A and 6 A is 54.96 \(\upmu\)m when the voltage is 30 V, and 57.12 \(\upmu\)m when the voltage is 70 V. This means that the change in crater diameter caused by the change in current is smaller when the voltage is 30 V. Namely, the effect of current on crater diameter is less significant under a voltage of 30 V. However, the change in crater diameter caused by a change in current is greater when the voltage is 70 V, indicating the effect of current on crater diameter is more significant at 70 V. Considering that crater diameter significantly affects MRR, the effect of current on MRR is more significant under the voltage of 70 V than at 30 V, which is consistent with the results in Figure 2. The reasons for this phenomenon can be analysed as follows.  
When the voltage is 30 V, the distance between the electrode and the workpiece is small. The deionisation of the dielectric between the poles is poor. The residual heat and residual corrosion products of the previous pulse discharge are retained in the dielectric between the electrode and the workpiece, which has a great effect on the next discharge. When the current changes, the expansion or contraction process of the plasma channel from the centre of the channel to the circumferential edge of the channel is affected because of changes in the diffusion conditions of the inter-polar dielectric. However, its radial expansion or contraction is limited, and the change in the radial dimension of the plasma channel is not obvious, which results in a smaller change in crater diameter. Therefore, the change in crater diameter on the surface of the workpiece caused by the change in current is less under a voltage of 30 V.  
When the voltage is 70 V, the distance between the tool electrode and the workpiece is larger [28]. The deionisation effect of the inter-polar dielectric is better. The expansion resistance of the plasma discharge channel is smaller along the radius direction. Meanwhile, the residual heat and residual corrosion products of the previous pulse discharge are sufficiently dispersed. As a result, the radius of the discharge channel varies greatly with the change in current. Consequently, the change in crater diameter is larger. Thus, the significance of the effect of current on MRR increases with increasing voltage.  
Figure 4 shows the measured waveform of discharge voltage and discharge current. Compared with the case of 3 A and 70 V (Figure 4b), the breakdown delay time is shorter in the case of 3 A and 30 V (Figure 4a). Under the conditions of low voltage, the distance between the poles is small. The residual heat and residual corrosion products of the previous pulse discharge are more easily retained between the two poles. The deionisation of the dielectric between the poles is not sufficient. As a result, there is an influence of the previous pulse discharge on the dielectric breakdown process of the next pulse discharge. The variation in the diameter of the plasma discharge channel is affected. Consequently, the change in crater diameter is influenced.