## 3 Results and Discussion  
The mean values and standard deviation for parameters evaluated in this analysis are contained in Table 4.  
The surface morphology was first studied by considering the largest wavelengths irregularities produced by milling on the surface, namely the surface waviness. This can  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & \\  & & &  
be observed directly by comparing primary profiles, which contain both roughness and waviness (Figure 5), selected for the four groups/combinations of rotation speeds and feed speeds. It can be seen from Figure 5 that an increase in the feed speed from \(\mathrm{v_{f1}}\) to \(\mathrm{v_{f2}}\) (3.6 times) has increased surface waviness, in the primary profiles, for both rotation speeds. The profiles are considerably wavier, containing large undulations, which can be spotted as patterns repeated at almost regular distance (approximately 6 mm), in Figure 5 and by a waviness profile overlapped on the ridged beech surface, in Figure 6. This observation is in agreement with the values of Wa parameter, which doubled in the case of both rotation speeds (Figure 7). This result was statistically significant for \(p<0.05\) (test ANOVA, followed by Dunnett T3 multiple comparisons) (Tables 5 and 6). An explanation may be that an increase in the feed speed increases tool vibration, which leaves deeper waves in the surface. When the rotation speed almost doubled from \(\mathrm{n_{1}}\) to \(\mathrm{n_{2}}\), a decrease of 20% in surface waviness was measured, but this reduction had no statistical support (Table 6). Figure 8 displays the roughness profiles measured for each pair rotation speed-feed speed. Isolated raised fibers are marked with red ellipses, while the yellow ellipses indicate isolated deep valleys. From definition, the core roughness of the roughness profiles does not contain the isolated deep valleys associated with the deep pores from earlywood or isolated detached wood material, nor the isolated raised fibers or surface fuzziness. Instead, it can be viewed as the density of data above and below the zero line. Therefore, it appears that an increase in the feed speed (respectively, in f\({}_{\rm z}\), for a constant n and z) is causing more dispersion above and below the zero line (Figure 6), which translated in an increase in Rk. These visual observations are confirmed by the measured Rk values, which increased, when the feed speed (respectively, f\({}_{\rm z}\)) increased 3.6 times, by 24% for n\({}_{\rm 1}\) and by a significant decrease in the feed speed. The reason is that the increased Rk values are more similar to the zero line, which is the reason for the increased Rk values. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Groups** & & **Mean Difference** & **Std. Error** & **Sig** \\ \hline n\({}_{\rm 1}\)\(-\)v\({}_{\rm f1}\) & n\({}_{\rm 1}\)\(-\)v\({}_{\rm F2}\) & \(-\)8.017 1 & 0.82 & 0.000 \\ \cline{2-5}  & n\({}_{\rm 2}\)\(-\)v\({}_{\rm F1}\) & 1.415 & 0.58 & 0.141 \\ \hline n\({}_{\rm 2}\)\(-\)v\({}_{\rm F2}\) & n\({}_{\rm 1}\)\(-\)v\({}_{\rm F2}\) & \(-\)2.98 & 1.07 & 0.063 \\ \cline{2-5}  & n\({}_{\rm 2}\)\(-\)v\({}_{\rm F1}\) & 6.45 1 & 0.90 & 0.000 \\ \hline \hline \end{tabular} 1  
\end{table} 
Table 6: Dependent variable Wa – Dunnet T3 Post Hoc Test for multiple comparisons between groups rotation speed–feed speed. Figure 6: Beech surface processed by n\({}_{\rm 1}\)\(-\)v\({}_{\rm F2}\) parameters combination with a waviness profile overlapped. Figure 7: The variation of waviness parameter, Wa, with combinations of the rotation speed with the feed speed. With blue arrows, pair means that differ statistically are marked. 57% for n\({}_{2}\) (from 10.19 \(\upmu\)m to 16.02 \(\upmu\)m) (Figure 9). The conclusion is that an increase in the feed speed (respectively, in f\({}_{\rm z}\), for a constant n and z) increases the core roughness Rk and is more pronounced when the rotation speed increases as well (Figure 9).