## 3 Results and Discussions
### Surface Quality Measurements
#### 3.1.1 Surface Quality Resulting from CNC Milling of Wood
As expected, Rv (Figure 8b), which measures the deepest single valley within the measured profile, confirmed the same observation: the occurrence of pull-out material is increasing with an increase in the processing angle in the range 0\({}^{\circ}\)-60\({}^{\circ}\), with the highest magnitude for 60\({}^{\circ}\). The occurrence of pull-out material was visible also by naked eye. \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{3-6}  & & & **2 m/min** & **2.5 m/min** & **3 m/min** & **3.5 m/min** & **4 m/min** \\ \hline \multirow{3}{*}{75} & Mean Rk & 26.62 & 31.63 & 35.46 & 35.94 & 40.73 \\  & (STDEV) & (4.08) & (6.48) & (5.74) & (3.82) & (8.90) \\ \cline{1-1}  & Significance & A,B & D,E & B.C & B & B \\ \hline \multirow{3}{*}{90} & Mean Rk & 29.59 & 27.99 & 29.05 & 33.68 & 40.13 \\  & (STDEV) & (13.81) & (7.12) & (3.35) & (6.03) & (10.45) \\ \cline{1-1}  & Significance & A,D & E & B & B & B \\ \hline \hline \end{tabular} \({}^{1}\) Standard deviation.  
\end{table} 
Table 1: _Cont._  
Figure 6: The variation of the reduced peak height parameter, Rpk, for various cutting angles, with the feed speed. The ratio Rpk/Rvk confirms that fuzziness was greater in magnitude than isolated gaps for a cutting angle of 15\({}^{\circ}\) and for all feed speeds. This particular cutting angle generates accentuated fuzzy surfaces. The ratio Rvk/Rpk confirms that the surface pull-out was greater in magnitude than fuzzy grain, for mostly all cutting angles, with the exception of 15\({}^{\circ}\) and sometimes 30\({}^{\circ}\), where fuzziness was predominant. The findings are in agreement with those of other researchers [28], who found, when CNC milling Douglas Fir, with a feed speed of 5 m/min, that for small angles (10\({}^{\circ}\)-20\({}^{\circ}\)) the occurrence of pull-out material is limited, but fuzzy grain is formed, while for greater angles, the pull-out phenomenon becomes more important. An explanation comes from the researchers [29] who analyzed the wood surface defects occurred by milling under different angles. When the wood reaction against splitting is higher than the strength of material, surface failure occurs and this appears when cutting takes place under an angle different than 0\({}^{\circ}\) with respect to the grain. Figure 9 presents the variation of Ra, the most used parameter in the literature and its values for various feed speeds with the cutting angles. Judged in separation from other roughness parameters, it is hard to be interpreted. However, in view of the previous discussion, it appears that Ra cumulates the effect of processing, with the high fuzziness for a 15\({}^{\circ}\) cutting angle and with the effect of occasional surface destruction by material  
Figure 8: The variation of the roughness parameters Rvk and Rv for various cutting angles with the feed speed: (**a**) Rvk; (**b**) Rv. Figure 7: The variation of the roughness parameters Rsk and Rp for various cutting angles with the feed speed: (**a**) Rsk; (**b**) Rp. pull-out, for a 60\({}^{\circ}\) cutting angle with respect to the grain. Ra appears maximum at these two cutting angles. However, wood is a heterogeneous material, contains areas with different structure and density within the growth ring. As result of the interaction with the cutting tool, the surface presents a variation in the elastic response, so that waviness is born, which differs in wavelength from roughness. Waviness may be also caused by the kinematic interaction between wood and the tool or by any vibration occurring during cutting. In general, waviness increased with the feed speed and this was clearly the case for the cutting angles of 0\({}^{\circ}\), 45\({}^{\circ}\) 75\({}^{\circ}\) and 90\({}^{\circ}\). The least wavy surfaces were obtained for angles of 0\({}^{\circ}\), 75\({}^{\circ}\) and 90\({}^{\circ}\).