## 3 Results
### Quality Evaluation of the CNC Milled Surfaces
#### 3.2.2 Surface Roughness and Stereomicroscopy Analysis of CNC-Processed Maple  
This section presents an in-depth evaluation of the surface quality of CNC-processed maple, combining quantitative roughness measurements with qualitative observations obtained through the roughness profiles and stereomicroscopy. The analysis focused on the surface morphology resulting from different tool geometries and stepover values, under controlled machining parameters. Table 3 presents the mean values and standard deviations of the surface morphology parameters for maple, corresponding to machining with tools featuring straight (T2) and helical cutting edges (T1), at depths of 1 mm and 3 mm, and stepover values of 5, 7, and 9 mm. The results obtained from the super sufficer are also included, representing the values for a high-quality reference surface (R) against which the machined samples were comparatively assessed. In a study on beech edge-glued panels [35], using a 6 mm diameter straight-end router bit with two cutting edges, geometrically similar to the T2 tool used in the present study, the Ra value at a 4 mm stepover was 4.463 \(\upmu\)m, while the Ra values for depths of cut of 2 mm and 4 mm were 4.216 \(\upmu\)m and 4.381 \(\upmu\)m, respectively. The values were close to the Ra  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Roughness** & **Cutting** & & & **T1 (5)** & **T1 (7)** & **T1 (9)** & **T2 (5)** & **T2 (7)** & **T2 (9)** & **R** \\ \hline \multirow{4}{*}{Ra} & \multirow{2}{*}{1} & mean & 4.66 & 6.99 & 5.84 & 4.68 & 4.50  
values for maple in our study, and the minimum values were recorded for T2 and were around 4.5-4.6 \(\upmu\)m for a depth of cut of 1 mm and 4.03 \(\upmu\)m for a depth of cut of 3 mm at a stepover of 5 mm. Although Ra is the most used parameter in the literature, here Rk was considered more reliable to differentiate the quality with different processing variables. The parameter Rk most effectively captures the central concentration of data points, as it is a robust parameter that excludes the influence of isolated surface features such as protruding fibers (surface fuzziness) or deep irregularities (pores, cracks). The evolution of the Rk parameter in relation to the stepover, the tool type, and the cutting depth can be seen in Table 3 and Figure 6. The surface obtained through super surfacing is also represented (R), marked by a gold line, serving as a reference for surface quality. In general, for both cutting depths, the machining surface roughness depicted by the core roughness Rk was significantly smaller for T2 compared to T1 and for all stepover values, with the exception of the stepover of 9 mm at 3 mm depth, with which the surface quality worsened for both tools, leading to no statistical differences between corresponding surfaces. Compared to the reference shaved surface (Rk = 6.16 \(\upmu\)m), the Rk values for T2 were the closest (Rk = approximately 9 \(\upmu\)m), 33-34% higher for the milled surface for a cutting depth of 1 mm, whereas the surfaces processed by T1 were more than two times rougher. In Table 4 is an example of a statistical analysis, in which, at a stepover of 5 mm, two-way ANOVA without replication indicates statistically significant differences among the four combinations of tool type (T1 and T2) and cutting depth (1 mm and 3 mm). The very low \(p\)-value (\(p\) = 6.00 \(\times\) 10\({}^{-5}\)) confirmed that the variations in the Rk values were not due to random factors but were instead influenced by the combined effect of tool geometry and depth of cut. Conversely, the high \(p\)-value for the replicates (\(p\) = 0.9818) demonstrated the consistency and reproducibility of the measurements. A similar trend was observed at the 7 mm stepover, at which the influence of the processing parameters remained statistically significant, while the variation among the replicates remained low. Figure 6: Comparative analysis of the core roughness (Rk) mean values for maple CNC-machined with T1 and T2, referenced to the values for the shaved surface (R).