# An Accuracy-Efficiency-Power Consumption Hybrid Optimization Method for CNC Milling Process
## 4 Results and Discussions
### Verification  
#### 4.3.1 Power Consumption without Cutting Load  
To verify the effectiveness of the proposed system, the electricity unit price and machining cost (hourly-rate) were set as 2.5 NTD/degree and 25 NTD/hour, and a 3-D straight line machining trajectory from (X2000, Y1500, Z1500) to (Z-1500, Y-1500, X-2000) was planned. The cutting path with the same feedrate was repeated twice. The original feedrate for machining was 1000 mm/min, and expected to improve for 3-times faster. Based on those parameters, the system calculated and recommend using a feedrate of 3000 mm/min as shown in Figure 15. Because the power consumption of three servo motors was quite small for a feedrate range of 1000 to 3000 mm/min (as shown in Figure 6, Figure 9, Figure 10, Figure 12), the electricity cost (about only 0.005 kW) is much less than the machining cost. Therefore, short machining time will be more dominant for the overall cost. Meanwhile, to remain the same chip load, the rotation speed of spindle was also increased from 300 rpm to 9000 rpm.  
Figure 14: X-axis motor power consumption vs. Spindle speed range of 1000 to 2250 rpm.  
Figure 15: The proposed parameters information for w/o cutting load.  
Power consumption amounts of the three axes servo motors with a feedrate of 1000 mm/min are shown in Figures 16-18. In the figures, those with very low power consumption (almost zero) represent that machine was moving at that time. The total machining time was about 20 min, the power consumption of X-, Y-, and Z-axis motor were 2.396, 3.619, and 2.178 kW, respectively. The total power consumption was 8.193 kW, and the power consumption cost was about 20.4825 NTD. The machining cost for 20-minutes is about 83.333 NTD. By summing up the power consumption cost and machining cost, the total cost for this machining process was about 103.833 NTD.  
Figure 16: Power consumption data of X-axis with F1000.  
Figure 17: Power consumption data of Y-axis with F1000.  
Figure 18: Power consumption data of Z-axis with F1000.  
The comparison of power consumption between actual measured data and the estimated calculation is shown in Table 3. It can be seen that the actual power cost is 2 to 3 NTD (or equivalent to 1 to 2 kWH) higher than the estimated calculation. Due to the error between the generated characteristic curves and the actual values of the three axes servo motors, it is quite acceptable. It was also noted that the generated characteristic curves had good agreement with the actual measured values (difference between the actual values and the estimated values were only about 0.1 kWH).  
As it was noted, when the feedrate was adjusted from 1000 mm/min to 3000 mm/min, the total power consumption cost was reduced from 21 NTD to 8.5 NTD, and the machining cost decreased from 83.3 NTD to 27.8 NTD; the total cost was lowered to 36.3 NTD from 104.4 NTD as well. It proved that when feedrate increased, the motor power consumption may also increase. However, if the machining time can be significantly shortened, then the total power consumption cost will still be significantly reduced.