# An Accuracy-Efficiency-Power Consumption Hybrid Optimization Method for CNC Milling Process
## 4 Results and Discussions
### Verification
#### 4.3.2 Power Consumption with Cutting Load  
The condition used for verification was assumed as electricity fee is 2.5 NTD/degree and the machine cost is 250 NTD/hour. For machining, a 4-mm depth of cut, and 5-mm width of cut, and 25-mm diameter end mill were used for machining iron. The original spindle speed and feedrate were 1000 rpm and 600 mm/min, respectively, the expected improvement of feedrate was at least 1.5 times faster. A straight line machining trajectory from (X565, Y-120,), (X-560, Y115,) was planned. The cutting path was repeated six times, and the total cutting length was 7560 mm. The system analyzed for the optimal feedrate according to motor power consumption characteristics curves. A feedrate of 1035 mm/min was finally chosen as the optimal value for the process (as shown in Figure 22). \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Processing SourceInformation** & **Power Costs (NTD)** & **Machining Costs (NTD)** & **Total Costs (NTD)** \\ \hline Calculated by program (F1000) & 17.85 & 83.333 & 101.183 \\ \hline Calculated with measured data (F1000) & 20.482 & 83.333 & 103.833 \\ \hline Calculated by program (F3000) & 8.3 & 27.778 & 36.078 \\ \hline Calculated with measured data (F3000) & 8.3 & 27.778 & 36.078 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Actual and estimated comparison for power consumption without cutting load. Figure 22: The proposed parameters information for w/cutting load. When a feedrate of 600 mm/min was used, the total machining time was about 12.6 min, and the total power consumption was about 12.663 kW. After converting, the power consumption cost was about 31.657 NTD, the machining cost was about 52.5 NTD, and total cost became 84.157 NTD. When the recommended feedrate of 1035 mm/min was used, the total machining time reduced to 7.3 min, the total power consumption was about 5.5056 kW. After converting, the power consumption cost was about 13.764 NTD, and the machining cost is 30.42 NTD. By summing up power consumption cost and machining cost, the total cost for this process was about 44.184 NTD. Figure 23 shows the power consumption data for using 600-mm/min feedrate. The X-axis actual measured power consumption was 8.485 kW (blue line), and the Y-axis actual measured power consumption was 4.178 kW (red line). When a 1035-mm/min feedrate was used, the X-axis actual power consumption (measured) was 4.543 kW (blue line in Figure 24), and the Y-axis actual power consumption (measured) was 0.9626 kW (red line in Figure 24). The comparison of actual measurement and estimated calculation of power consumption is shown in Table 4. It was noted that the estimated power consumption in X-axis and Y-axis were smaller than the actual measured data. For a feedrate of 600 mm/min, the actual total power consumption  
Figure 23: Actual power consumption with cutting load using 600-mm/min feedrate. Figure 24: Actual Power consumption with cutting load using 1035-mm/min feedrate. for X-axis (3.325 kW) was higher than the estimate, also the Y-axis (1.226 kW) was higher than the estimated calculation value. For a 1035-mm/min feedrate, the actual total power consumption for X-axis (3.047 kW) was higher than the estimated value. In addition, the actual total power consumption of Y-axis (0.5809 kW) was higher than the estimated calculation value. The actual power consumption was higher than the estimation calculated by characteristics curves which may be because of the 24 starts and stops of the machine for each machining experiment due to repeating the machining process for six times. More energy was consumed for the machine starts and stops. In general, the recommended optimal parameters by the system could improve the machining efficiency and reduce the total power consumption cost. As it can be seen in Table 4, when the feedrate was adjusted from 600 mm/min to 1035 mm/min, the power consumption cost was reduced from 31.657 NTD to 13.764 NTD, and the total machining time also dropped from 12.6 min to 7.3 min.