# Interaction of Machining Parameters on MRR of Sintered NdFeB Processed by EDM-Milling
## 4 Interaction of Machining Parameters on the MRR
### Interaction of Pulse off Time and Voltage on the MRR
#### 4.4.1 Significance of Voltage on the MRR  
It can be seen that the significance of the influence of voltage on MRR decreases and then increases as the pulse off time increases. \(\triangle_{\text{Toff}}(U)\) reaches a maximum of 16.21 mg/min when the value of pulse off time is 190 us, and \(\triangle_{\text{Toff}}\) (\(U\)) is at its minimum of 3.52 mg/min when the value of pulse off time is 112 us. The \(\triangle_{\text{Toff}}\)\(u\) is 12.69 mg/min, as listed in Table 2. In addition, when the value of pulse off time is 80 us, the \(\triangle_{\text{Toff}}(U)\) is 7.85 mg/min.  
The pulse off time affects the deionisation time and discharge frequency [28]. The deionisation time affects the residual corrosion products and heat from the previous discharge. If the deionisation time is appropriate, proper corrosion products and heat will be conducive to the formation of the plasma channel, and the discharge energy will be used effectively. However, excessive residual corrosion products and heat will seriously deteriorate the dielectric, and the energy will not be used effectively.  
Figure 9: Interaction curve of pulse off time and voltage on MRR.  
Voltage affects the distance between the poles and the breakdown delay time. The distance affects the deionisation of the dielectric and the diffusion of the pulse energy. When the distance increases, it helps to reduce the density of the corrosion products and residual heat in the inter-polar dielectric and increases diffusion. The breakdown delay time affects deionisation and the utilization of discharge energy. When the breakdown delay time is increased by the control system of the machine tool, the effective pulse discharge time is shortened, while the deionisation time is prolonged.  
Based on the above, an analysis is conducted on the significance of the effect of voltage on the MRR with the variation of the pulse off time, considering the waveform of discharge voltage and discharge current, as shown in Figure 10.  
When the pulse off time is 80 \(\upmu\)s, the deionisation time is short. The material removal process will be affected by the previous discharge due to the residual heat and corrosion product. The breakdown delay time is short, and the diffusion resistance of the discharge energy is large. The dielectric will be affected significantly by the voltage. As a result, the MRR will be significantly affected by the voltage.  
When the pulse off time increases from 80 to 112 \(\upmu\)s, the deionisation time prolongs. The residual heat and corrosion products are gradually reduced, and the diffusion condition of the dielectric improves. The effect of voltage on MRR decreases with of the increase in pulse off time.  
When the pulse off time increases from 112 to 190 \(\upmu\)s, the discharge frequency is reduced further and deionisation becomes more sufficient. The effect of voltage on the residual heat and corrosion products becomes weakened. However, the average voltage between the poles becomes significantly lower than the reference voltage as pulse off time increases. The machine tool increases the breakdown delay time to prolong the average  
Figure 10: Waveform of discharge voltage and discharge current under different pulse off time and voltage.  
voltage. This will result in a decrease in discharge time in a single pulse. Moreover, the discharge frequency will be reduced significantly due the dielectric not being broken down effectively. As shown in Figure 10d, pulses without breakdown can be easily observed under conditions of high voltage and long pulse off time. Thus, the significance of the influence of voltage on MRR increases with the increase in pulse off time from 112 to 190 \(\upmu\)s.