## 3 Results  
### Surface Roughness  
The results of surface roughness measurements on the left slope of the specimen are listed in Table 3 and Figure 4. A comparison of the effects of different feed rates at the same spindle speed on the left slope of the specimen (Figure 1a) revealed that the surface roughness significantly differed between feed rates of 2000 mm/min and 3500 mm/min at a spindle speed of 20,000 rpm (\(p\) = 0.002), between feed rates of 2000 mm/min and 2500 mm/min at a spindle speed of 30,000 rpm (\(p\) = 0.04), between feed rates of 2000 mm/min and 3000 mm/min at a spindle speed of 30,000 rpm (\(p\) = 0.006), and between feed rates of 2000 mm/min and 3500 mm/min at a spindle speed of 30,000 rpm (\(p\) < 0.001). However, a comparison of the effects of different spindle speeds at the same feed rate revealed that surface roughness significantly differed between spindle speeds of 20,000 and 40,000 rpm at a feed rate of 2000 mm/min (\(p\) = 0.04). Although there were no significant differences in surface roughness at feed rates of 2500 mm/min, 3000 mm/min, and 3500 mm/min, the surface roughness tended to increase with the spindle speed.  
The results of surface roughness measurements on the left slope of the specimen (Figure 1a) varied when the milling conditions were varied in terms of the spindle speed and feed rate. The vertical axis shows the surface roughness and the horizontal axis shows the feed rate.  
The results of surface roughness measurements on the right slope of the specimen (Figure 1b) are listed in Table 4 and Figure 5. Surface roughness did not significantly differ among different feed rates at the same spindle speed. Further, a comparison of the surface roughness at the same feed rate but different spindle speeds revealed that it significantly differed between spindle speeds of 20,000 rpm and 30,000 rpm at a feed rate  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Feed Rate (mm/min)** & \multicolumn{3}{c}{**Spindle Speed (rpm)**} \\ \hline  & **20,000** & **30,000** & **40,000** \\ \hline  & **Median (IQR)** & **Median (IQR)** & **Median (IQR)** \\ \hline
2000 & 1.59 (1.48–1.78) a & 1.75 (1.71–1.78) ab & 1.88 (1.67–2.16) bcd \\
2500 & 1.74 (1.59–1.84) abc & 2.04 (1.75–2.33) cdf & 1.82 (1.74–1.88) abcd \\
3000 & 1.90 (1.84–1.90) abc & 1.99 (1.91–2.36) ef & 1.94 (1.93–2.01) def \\
3500 & 2.04 (2.00–2.12) ef & 2.32 (2.10–2.4) f & 2.04 (1.92–2.08) ef \\ \hline \hline \end{tabular} Statistical results of the Kruskal–Wallis test and [PERSON]’s multiple comparison test. Different letters indicate significant differences among the groups (\(p\) < 0.05).  
\end{table}
Table 3: Results of surface roughness measurements on the left slope (Figure 1a) using a surface roughness tester.  
of 2000 mm/min (\(p\) = 0.05); spindle speeds of 20,000 rpm and 40,000 rpm at a feed rate of 2000 mm/min (\(p\) = 0.001); spindle speeds of 20,000 rpm and 40,000 rpm at a feed rate of 2500 mm/min (\(p\) < 0.001); spindle speeds of 30,000 rpm and 40,000 rpm at a feed rate of 2500 mm/min (\(p\) = 0.001); spindle speeds of 20,000 rpm and 30,000 rpm at a feed rate of 3000 mm/min (\(p\) = 0.01); spindle speeds of 20,000 rpm and 40,000 rpm at a feed rate of 3000 mm/min (\(p\) < 0.001); spindle speeds of 20,000 rpm and 30,000 rpm at a feed rate of 3500 mm/min (\(p\) = 0.04); and spindle speeds of 20,000 rpm and 40,000 rpm at a feed rate of 3500 mm/min (\(p\) < 0.001). The surface roughness decreased as the spindle speed increased.  
The results of surface roughness measurements on the right slope of the specimen (Figure 1b) varied when the milling conditions were varied in terms of the spindle speed and feed rate. The vertical axis shows the surface roughness and the horizontal axis shows the feed rate.  
The results of surface roughness measurements on the flat surface of the specimen are listed in Table 5. On the flat surface (Figure 1c) neither the spindle speed nor feed rate affected the surface roughness.