## 5 Experimental Results
### Machine Tool Thread Milling Experiment  
This study utilized the five-axis CNC machining center, as shown in Figure 11, for the thread milling experiment to validate and evaluate the feasibility and performance of the PCB-ILCC approach developed in this study under the cutting force disturbance. The machining center mainly comprises a vertical axis (Z feed drive axis) and two planar axes (\(X\) feed drive axis and \(Y\) feed drive axis). The vertical axis and planar axes are actuated by PMSMs so that the thread milling tool can perform linear and circular motions. Each feed drive axis is equipped with a ballscrew lead of 10 mm and adopts a 1.5 kW PMSM pack. Table 2 shows the PMSM-equivalent parameters of the feed drive axes, respectively, identified by the constant speed experiment and estimation experiment [28]. The thread milling experiment used a 90 mm long thread milling tool with a tool holder diameter of 12 mm. The material of the disk-type thread milling blade was tungsten steel. The blade diameter is 20 mm with a blade angle of 60 degrees. The workpiece material is AL6061. Figure 11: Five-axis CNC machining center used in thread milling experiment. Figure 10: Control performance comparison in PMSM motion control experiment. Figure 12 shows the experimental results while performing thread milling operations on the five-axis CNC machining center; here, because the PMSM-DC has been validated to provide superior control results with small contour errors, the PCB-ILCC was compared with the PMSM-PPIC to clarify the control performance between the proposed and conventional control methods under the adverse influence of external disturbances. The vertical axis denotes the RMS contour error of each cycle, and the horizontal axis represents the cycle number of the helical path. Figure 12 illustrates that the RMS contour error of the PMSM-PPIC is almost unchanged and remains at a large value. The AVG of the RMS variation is 10.6930 \(\upmu\)m, the SD is 0.0286 \(\upmu\)m, and the ratio between the SD and AVG is 0.27%. Compared with the PMSM-PPIC, although the cutting force makes the RMS contour error fluctuate, the PCB-ILCC still significantly reduces the RMS contour error. The PCB-ILCC achieves that the RMS contour error starts to converge at the fifth cycle and is reduced from 3.0734 \(\upmu\)m (the first cycle) to 1.1055 \(\upmu\)m (the tenth cycle) with a rate of reduction of 64.03%. Figure 13 shows the magnitude of the CEVs in the last cycle (the tenth cycle) of Figure 12. Figure 14 compares the control performance of the PMSM-PPIC and PCB-ILCC. Because of the influence of friction disturbance on each feed drive axis, the contour error of the PMSM-PPIC has been significantly increased at quadrants of circular motion, with the MAX error of 21.7032 \(\upmu\)m. In addition, because of the influence of the cutting force and motion dynamics of each feed drive axis, the suppression of the contour error is limited, and there were larger contour errors while using the PMSM-PPIC; the AVG error is 10.4015 \(\upmu\)m and the RMS error is 10.7086 \(\upmu\)m. Compared with the PMSM-PPIC, the PCB-ILCC can effectively suppress the adverse influence of the cutting force and friction disturbance, and improves the motion dynamics of each feed drive axis. Therefore, the PCB-ILCC can significantly reduce the contour error; the MAX error is reduced to 3.6132 \(\upmu\)m at a rate of 83.35%, the RMS error is reduced to 1.1055 \(\upmu\)m with a rate of 89.68%, and the AVG error is reduced to 0.9818 \(\upmu\)m with a rate of 90.56%. Therefore, the machine tool thread milling experiment indicated that, compared with the PMSM-PPIC, the PCB-ILCC could