## 4 Conclusions  
The surface quality achieved by face milling for maple and oak using two types of end-mill tools (with straight and helical cutters) for three values of stepover (5, 7 and 9 mm) and two values of depth of cut (1 and 3 mm) was comparatively analyzed. The results were referenced to those of almost ideally smooth surfaces obtained by super surfacing (shaving).  
The reference surfaces obtained by shaving were very smooth and revealed with clarity and detail the species' anatomical structure, with negligible surface disturbance caused by processing. Maple and oak presented a similar Rk value around 6 \(\upmu\)m, characterizing the roughness caused by the shaving tool. Ra was strongly influenced by the wood species anatomy, particularly by the vessel deep voids of oak, which rendered Rk more reliable for the characterization of the machining quality.  
The comparative morphology of maple and oak face-milled surfaces evidenced a species-dependent machining quality, from which the inherent deep anatomical voids were excluded. In general, for both species, the machining surface roughness depicted by the core roughness Rk was significantly smaller for the straight-edged tool compared to the helical tool for both cutting depths and all stepover values, with the exception of the stepover of 9 mm, with which the surface quality worsened for both tools due to an increase in fuzziness and fiber detachment.  
For maple and oak, both milling tools enhanced the surface waviness, as well as the fuzziness, as the stepover value increased. An increase in the cutting depth generally increased the core surface roughness Rk, as well as the fuzziness (Rpk), and this trend increased as the value of the stepover increased.  
In the case of maple, the straight-edged tool showed a clearer quality trend with increases in the cutting depth and stepover than the helical tool, for which the variation in quality, was rather unpredictable. The maple surface processed with the helical end mill displayed significant fiber tearing, disrupted vessel outlines, and areas where bundles of fibers appeared fragmented, indicating an aggressive interaction between the tool and wood, while for oak, apart from material detachment and fuzziness, prominent tool marks appearing like regular ridges across the grain were noticed. The deepest valleys (Rv) observed in the CNC-processed surfaces were primarily related to the exposure of earlywood vessels cavities rather than to machining defects.  
Of the two tested tools, the straight-edged mill working with the smallest depth of cut (1 mm) and stepover (5 mm) approached the most the quality of the shaved surfaces, with values of Rk about 33-34% higher than that of the reference in the case of maple and 56% higher in the case of oak, whereas the surfaces processed by the helical tool were more than two times rougher. Further work would seek improvements in the selection of tool geometry and process parameters, tailored per species, to bring the quality of CNC-machined surfaces even nearer to the quality achieved through fine shaving.  
Conceptualization, L.G.; methodology L.G., A.-M.A. and M.I.; surface quality software A.-M.A.; microscopy, L.G. and A.-M.A.; data analysis, L.G. and A.-M.A.; validation, L.G. and M.I.; investigation, L.G. and A.-M.A.; resources, A.-M.A.; writing--original draft preparation, L.G., A.-M.A. and M.I.; editing, A.-M.A.; supervision, L.G. and M.I. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
We hereby acknowledge the structural funds project PRO-DD (POS-CCE, O.2.2.1., ID 123, SMIS 2637, No. 11/2009) for providing the infrastructure used in this work and the Contract No. 7/9.01.2014.  
The authors declare no conflicts of interest.