## 3 Results and Discussions  
### Surface Quality Measurements  
#### 3.1.1 Surface Quality Resulting from CNC Milling of Wood  
The analysis of Rk has shown an increase in processing roughness with the feed speed and this was valid for all angles with respect to the grain tested (Figure 5a). An exception was the 15\({}^{\circ}\) processing angle, for which Rk had the highest magnitude for the feed speed of 2 m/min, then almost linearly decreasing. Changing the cutting direction from longitudinal (0\({}^{\circ}\)) to 15\({}^{\circ}\), Rk has increased approximately 2-4 times with the greatest amount for the feed speed of 2 m/min, and this was statistically significant when checked with the Duncan test (Table 1). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Cutting Angle, in \({}^{\circ}\)**} & \multirow{2}{*}{**Data**} & \multicolumn{6}{c}{**Rk, in \(\mu\)m, at Various Feed Speeds**} \\ \cline{3-8}  & & **2 m/min** & **2.5 m/min** & **3 m/min** & **3.5 m/min** & **4 m/min** \\ \hline \multirow{4}{*}{0} & Mean Rk & 14.63 & 17.03 & 16.59 & 17.90 & 17.55 \\  & (STDEV) 1 & (5.47) & (5.57) & (4.64) & (4.39) & (3.68) \\  & Significance & A & A & A & A & A \\ \hline \multirow{4}{*}{15} & Mean Rk & 58.05 & 51.19 & 47.19 & 45.42 & 40.12 \\  & (STDEV) & (12.83) & (5.74) & (6.66) & (5.02) & (4.13) \\  & Significance & C & B & D & C & B \\ \hline \multirow{4}{*}{30} & Mean Rk & 43.11 & 45.46 & 48.03 & 47.26 & 55.85 \\  & (STDEV) & (11.97) & (8.49) & (6.42) & (5.97) & (11.71) \\  & Significance & D & B,C & D & C & C \\ \hline \multirow{4}{*}{45} & Mean Rk & 32.90 & 38.25 & 42.39 & 46.69 & 47.75 \\  & (STDEV) & (3.96) & (5.07) & (2.12) & (7.35) & (5.09) \\ \cline{1-1}  & Significance & B,D & C,D & C,D & C & B,C \\ \hline \multirow{4}{*}{60} & Mean Rk & 27.90 & 33.53 & 37.30 & 39.72 & 39.69 \\  & (STDEV) & (3.34) & (4.97) & (7.61) & (3.95) & (5.98) \\ \cline{1-1}  & Significance & A,B & D,E & C & B,C & B \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Checking the significance on Rk of varying the cutting angle with respect to the grain for various feed speeds. Figure 5: The variation of the core roughness parameter, Rk: (**a**) for various cutting angles, with the feed speed; (**b**) for various feed speeds with the cutting angles. The roughest surfaces were obtained for angles of 15\({}^{\circ}\) and 30\({}^{\circ}\) (Figure 5b). The best processing roughness, Rk, was always obtained for a processing along the grain, followed nearly double by 90\({}^{\circ}\) and then 75\({}^{\circ}\). Considering the influence on Rk of the cutting angle and the feed speed, the cutting angle was the most significant parameter after statistical analysis with ANOVA and the Duncan multiple range test (_p_ < 0.05). Further, there was the question why the angles of 15\({}^{\circ}\) and 30\({}^{\circ}\) are causing a rough surface and whether this phenomenon is extending beyond the core roughness. By analyzing the parameter Rpk, which is sensitive to isolated peaks, it is confirmed that the special angles of 15\({}^{\circ}\) and 30\({}^{\circ}\) give rise to accentuated fuzziness, 7-8 times stronger in comparison with 0\({}^{\circ}\) (Figure 6). In support to this observation comes Rsk, which was positive for all feed speeds for a processing angle of 15\({}^{\circ}\) (Figure 7a), but also Rp (Figure 7b), which measures the highest single peak within the measured profile. A high magnitude Rsk in the negative direction is an indication of isolated surface gaps, caused by material pull-out. This was observed especially for a cutting angle of 60\({}^{\circ}\). In order to check the state of the surface below the core roughness, Rvk can be of help, because it is sensitive to the presence of isolated valleys (Figure 8a). Indeed, the processing with a cutting angle of 60\({}^{\circ}\) increased the occurrence of pull-out material and Rvk increased 5-7 times as compared with processing along the grain.