# Interaction of Machining Parameters on MRR of Sintered NdFeB Processed by EDM-Milling
## 4 Interaction of Machining Parameters on the MRR
### Interaction of Pulse on Time and Voltage on the MRR
#### 4.3.2 Significance of Pulse on Time on the MRR  
As shown in Figure 7, the significance of the influence of pulse on time on MRR decreases and then increases as the voltage increases. When the value of voltage is 70 V, \(\bigtriangleup_{U}(T_{\text{on}})\) reaches a maximum of 27.18 mg/min. When the value of voltage is 35 V, \(\bigtriangleup_{U}(T_{\text{on}})\) is at its minimum of 4.47 mg/min. The \(\bigtriangleup_{U}T_{\text{on}}\) is 22.71 mg/min, as listed in Table 2. In addition, when the value of voltage is 30 V, the \(\bigtriangleup_{U}(T_{\text{on}})\) is 6.74 mg/min.  
When the voltage is 30 V, the discharge gap is small. The residual heat and corrosion products are dissipated not sufficiently. In this case, if the corrosion products and residual heat are appropriate in the discharge gap, the breakdown of the dielectric is easy, which will be favourable to the improvement of the MRR. However, if there are too many residual corrosion products and heat in the discharge gap, the utilization of the discharge energy will deteriorate, which will go against the improvement of the MRR. Figure 8a,c show discharge waveforms under a voltage of 30 V. It can be seen that the breakdown delay time is longer, which might be due to the weak breakdown effect when the pulse on time is 30 \(\upmu\)s (Figure 8a), caused by less remaining heat and corrosion products. However, the breakdown delay time is shorter when the pulse on time is 90 \(\upmu\)s, as shown in Figure 8c. This might be due to the obvious breakdown effect of the remaining heat and corrosion products after the previous pulse discharge. The pulse on time influences the pulse discharge energy, the residual heat, and the residual corrosion product. The pulse discharge energy increases with the increase of pulse on time; however, the utilization of the energy will become insufficient due to the poor condition of the dielectric. Likewise, the decrease of pulse on time will result in a decrease of energy and an improvement of its sufficient utilization. Therefore, the influence of pulse on time on MRR is not significant under a voltage of 30 V.  
When the voltage increases to 35 V, the discharge gap increases slightly. The dissipation of discharge energy increases slightly. The breakdown delay time increases slightly. The significance of the effect of pulse on time on MRR becomes less significant.  
As the voltage increases to 70 V, the discharge gap becomes larger. The positive effect of the previous pulse discharge on the subsequent one becomes weaker. Furthermore, to maintain the average voltage at a higher level, the control system of the machine tool increases the breakdown delay time, which shortens the duration of a single pulse discharge and decreases the effective utilization of the energy. In this case, pulses without breakdown are more likely to occur. Comparing Figure 8b,d, it can be seen that the utilization of the pulse energy is not sufficient due to the continuous waveform without breakdown in the case of high voltage and short pulse on time (Figure 8b). However, the discharge frequency is higher under the condition of high voltage and long pulse on time (Figure 8d). In conclusion, the significance of the effect of pulse on time on MRR increases as it affects deionisation, effective breakdown frequency, and the effective time of a single pulse discharge.