## 3 Results
### Quality Evaluation of the CNC Milled Surfaces
#### 3.2.3 Surface Roughness and Stereomicroscopy Analysis for CNC-Processed Oak  
Oak samples were evaluated under the same CNC machining conditions applied to map. Table 5 provides a summary of the mean values and standard deviations for the oak samples, covering all tool and parameter combinations, with the super surfaced reference included for comparison. The value of Ra in the case of T2, depth of cut 1 mm, and stepover 5 mm was apparently higher than that of Ra for the shaved surface. This only proved, once more, that Ra is not reliable for species with deep pores like oak, because it is clearly influenced by wood local anatomical voids. The shaved surface not only provided an exceptional processing smoothness but also uncovered the wood anatomy almost like a microtome-processed surface. Open pores directly affect Ra, which cannot express the real machining quality of oak. Therefore, Rk was preferred in this research, as recommended in [26], although the vast majority of researchers base their reports on Ra. For example, in [36] the value of Ra for _Quercus alba_ samples processed by surface planning, using a three-blade cutter head with a 76 mm diameter, a rotation speed of 4500 rot/min, and a feed rate of 6 m/min, was 11.27 \(\upmu\)m on tangential surfaces. In comparison, the CNC milling tests conducted in the present study using a straight-edge geometry tool with a cutting depth of 1 mm, a stepover of 5 mm, and a feed rate of 4 m/min resulted in an Ra value of 8.50 \(\upmu\)m. This indicates that milling with a straight-edged tool can produce a smoother surface on oak than conventional planing. Similarly, in [37], Ra after the planar milling of oak with a 130 mm spindle cutter was 8.33 \(\upmu\)m, very close to the value observed in the present CNC milling tests with the T2 tool. The evolution of Rk with different stepover values, tool types, and cutting depths is illustrated in Figure 11, alongside the value for the super surfaced reference (R), marked with a gold line. with the reference surface quality, the Rk for both CNC tools exceeded by more than twice the Rk after super surfacing. The tool T2 performed best for the stepover of 5 mm at the 1 mm cutting depth, leading to an Rk (10.1 \(\upmu\)m) that was approximately 1.5 times the Rk obtained after shaving (6.46 \(\upmu\)m). An increase in the stepover value increased the Rk for both tools and depths of cut (Table 5 and Figure 11), but this trend was found significant by ANOVA only in the case of T2. The roughness profiles in Figure 12, as well as the micrographs in Figure 13, seem to confirm a surface deterioration with an increase in the stepover value for both tools. Figure 12: Comparative roughness profiles for oak processed by the tools T1 and T2 at the cutting depth of 1 mm and reference profile: red—shaved surface, navy blue—stepover 5 mm, magenta—stepover 7 mm, and light blue—stepover 9 mm. For visualization clarity, each roughness profile is vertically offset relative to its own baseline; the vertical positioning does not reflect the absolute height differences between conditions. The core roughness Rk increased with an increase in the cutting depth for both tools (Table 5, Figure 11), and this was found significant by ANOVA, especially for values of 5 and 7 mm stepover in the case of T2 and 7 and 9 mm stepover in the case of T1. The increasing roughness with the cutting depth is also visible in Figures 14 and 15. The two-factor ANOVA (without replication) presented in Table 6 for oak milling at a 5 mm stepover indicates that both tool type and cutting depth significantly influenced the surface roughness (\(p\) = 0.016). Comparing the two tools, T2 generated lower roughness values, particularly at the 1 mm depth, confirming a better performance under these conditions.