# 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
#### 4.3.1 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 on time increases. When the value of pulse on time is 30 \(\upmu\)s, \(\triangle_{\text{Ton}}(U)\) reaches a maximum of 20.24 mg/min. When the value of pulse on time is 69 \(\upmu\)s, \(\triangle_{\text{Ton}}(U)\) is at its minimum of 3.35 mg/min. The \(\triangle_{\text{Ton}\,U}\) is 16.89 mg/min, as listed in Table 2. In addition, when the value of pulse on time is 90 \(\upmu\)s, the \(\triangle_{\text{Ton}}(U)\) is 11.24 mg/min.  
When the pulse on time is set to a certain value, changing the reference voltage leads to a change in the distance between the poles. As a result, the deionisation of the dielectric and the breakdown delay time of a single pulse discharge will be affected.  
When the pulse on time is 30 \(\upmu\)s, the discharge energy is low, and the corrosion product and residual heat are small. Their dissipation is sufficient, and the dielectric is pure. When the voltage is varied between 30 V and 70 V, the dielectric can respond to the diffusion of corrosion product and residual heat quickly. On the other hand, the breakdown delay time is large due to the weak breakdown effect of the previous discharge. When the voltage becomes larger, the control system of the EDM machine tool increases the breakdown delay time to make the average voltage between poles equal to the set value. As a result, the effective machining time within a period of discharge will be decreased further. Pulse periods without breakdown are more likely to occur under high voltage conditions. Figure 8 shows the waveform of discharge voltage and discharge current under different pulse on times and voltages. Figure 8a,b shows waveforms at a pulse on time of 30 \(\upmu\)s. The discharge is continuous and uniform in the case of low voltage (Figure 8a); however, continuous pulse periods without breakdown can be observed in the case of high voltage (Figure 8b). Thus, the effect of voltage on MRR is significant under conditions with a low pulse on time.  
Figure 8: Waveform of discharge voltage and discharge current under different pulse on times and voltages.  
As the pulse on time gradually increases to 69 \(\upmu\)s, the energy of the pulse discharge gradually increases [31]. The residual corrosion products and residual heat gradually increase. The dissipation ability of the dielectric weakens gradually. The dielectric deteriorates gradually. The energy consumed by the corrosion products gradually increases. The utilization of the energy is reduced gradually. As a result, the significance of the effect of voltage on MRR is gradually weakens with the gradual increase in the pulse on time to a dielectric value.  
Thereafter, with the increase of pulse on time up to 90 \(\upmu\)s, the heat in the inter-polar dielectric increases significantly, and the breakdown effect due to the previous discharge becomes obvious. As shown in Figure 8, the breakdown delay time is clearly shorter at higher pulse on times (Figure 8c,d) compared with the corresponding low pulse on time (Figure 8a,b). Moreover, discharge concentration is limited due to the poor diffusion conditions of the dielectric under low voltage. However, it can be improved significantly due to improvement of the heat diffusion conditions under a higher voltage and a larger distance between poles. Consequently, the significance of the effect of voltage on MRR increases gradually as the pulse on time increases.