## 4 Results and Discussion  
### Non-Stationary Characteristics of Corner Milling  
When cutting parameters such as cutting width, depth of cut and feed rate remain unchanged, changes in the tool path will lead to significant changes in cutting force caused by the special shape at the corners. The average cutting force will increase when the machined surface is concave along the tool side, and the average cutting force will decrease when the machined surface is convex along the tool side. Since the cutting amount of the tool to the workpiece, the angle of the tool axis and the tool path are constantly changing with the machining process, the sensor signal shows a highly non-stationary characteristic.  
It can be seen from Figure 6 that in the curve section of each corner, the cutting force changes rapidly, and the maximum amplitude is significantly higher than that of the straight section. The non-stationarity of the cutting force in the straight segment can be eliminated to a certain extent by calculating the resultant force of the forces in all directions, but it is invalid at the corners. Therefore, for corner milling, the cutting force is highly non-stationary. It can be seen from Figure 7 that the system stiffness of the tool in the corner section is significantly greater than that in the straight section, resulting in significantly smaller vibration signals in the corner section than in the straight section. The vibration signals in the three directions all show strong non-stationary characteristics with the cutting  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Metrics**} & **Workpiece** & **Cutter** & **Cutter** & **Cutter** & **Cutter** & **Standard** \\  & **Serial Number** & **Tooth A** & **Tooth B** & **Tooth C** & **Tooth D** & **Value** \\ \hline \multirow{4}{*}{\(VB_{\text{mm}}\)} & 1 & 0.03 & 0.02 & 0.03 & 0.02 & 0.025 \\  & 2 & 0.06 & 0.05 & 0.06 & 0.04 & 0.053 \\  & 3 & 0.08 & 0.09 & 0.08 & 0.06 & 0.078 \\  & 4 & 0.10 & 0.09 & 0.10 & 0.07 & 0.09 \\ \hline \multirow{4}{*}{\(VB_{max}\)(mm)} & 1 & 0.08 & 0.12 & 0.15 & 0.18 & 0.18 \\  & 2 & 0.15 & 0.17 & 0.17 & 0.24 & 0.24 \\ \cline{1-1}  & 3 & 0.18 & 0.20 & 0.23 & 0.26 & 0.26 \\ \cline{1-1}  & 4 & 0.21 & 0.23 & 0.26 & 0.27 & 0.27 \\ \hline \hline \end{tabular}
\end{table}
Table 8: The wear values of the experimental tool for corner circumferential milling.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**UTH0804**} & **Diameter** & **Overall** & **Blade Length** & **Number of** & **Helical** \\  & **D (mm)** & **Length** & **L\({}_{1}\) (mm)** & **Tenth** & **Angle** \\  & & **L (mm)** & **N** & **\(\beta\) (\({}^{\circ}\))** \\ \hline \hline \end{tabular}
\end{table}
Table 7: Specific parameters for corner milling tools.  
path. It can be seen from Figure 8 that the time-domain waveform of the acoustic emission signal is far less sensitive to the path than the cutting force signal and the vibration signal.  
Figure 6: Influence of corner milling path on cutting force.  
Figure 7: Influence of corner milling path on vibration signal.  
To sum up, both the force signal and the vibration signal show strong non-stationary characteristics with the corner milling path, while the corner milling path has little influence on the acoustic emission signal. Therefore, compared with the other two sensor signals, the acoustic emission signal has a great advantage in monitoring corner milling.