## 1 Introduction  
The assessment of the surface quality following wood processing is very important in order to establish the impact of various processing parameters combinations [1; 2; 3]. The optimization of milling operations by considering the surface quality will reduce the amount of further sanding and can contribute to a reduction in the finishing costs. Therefore, it is important to find a combination of processing parameters suitable for a minimum surface roughness. Furthermore, the surface roughness of wood is very complex and represents a superposition of morphological features, such as waviness, raised fibers, tool marks, wood anatomical voids, as well as possible processing surface defects, such as material detached from the surface [4; 5; 6; 7; 8; 9]. In this respect, the most used roughness parameters such as Ra or Rz are not enough to give a complete picture of the origin of the different irregularities present on a processed surface. According to the studies performed by [PERSON] [10], [PERSON] [11], and [PERSON] et al. [12], the cutting parameters affect the surface roughness of beech wood. The study performed by [PERSON] et al. [13] on _Pinus sylvestris_ after CNC milling showed that the surface roughness decreased with increasing rotation speed and decreased with decreasing the feed speed for all machining parameters. The evaluation of surface roughness for different depths of cut revealed that the maximum surface roughness value was registered for the highest depth of cut. [PERSON] et al. [14] have found that the cutting speed has not significantly influenced the surface roughness of walnut processed by straight-tooth milling, but the depth of cut was significant. [PERSON] and [PERSON] (2014) stated that the quality of beech, when routed on a CNC machine, improves with an increase in the cutting speed. [PERSON] et al. (2016) concluded that an increase in the feed speed and the chip thickness increases the surface roughness of beech milled by CNC, while the cutting speed had no effect on surface roughness, but just on the cutting power. Most researchers have used Ra and Rz (2014); Rz and Rz (2014); Rz (2015); Rz and Rz (2016); Rz (2017) to assess the quality of wood after milling, but these parameters do not consider the nature of wood irregularities and include the inherent wood anatomical gaps in the evaluation. The roughness parameters Ra and Rz decreased with a lower feed rate and a higher spindle speed in a study by [PERSON] et al. (2012) on beech (_Fagus orientalis_), Ayous (_Triplochiton Scleroxylon_), and MDF processed by CNC. Similarly, in another study of [PERSON] and [PERSON] (2017), the surface roughness evaluated by Ra decreased with increasing the spindle speed and decreasing the feed rate as well as the depth of cut when beech wood was milled on a CNC machine. However, these two parameters employed by the researchers are not sufficient to correctly interpret the quality of wood and wood-based surfaces, which means that they must be evaluated together with other roughness parameters ([PERSON] et al., 2016). [PERSON] et al. (2016) evaluated the effect of milling on the surface roughness of beech and aspen wood. On the basis of statistical analysis, they found no significant difference between the surface roughness determined on the tangential and radial surfaces. They observed that a number of roughness parameters, such as Ra, Rz, Rk, Rpk, and Rvk roughness are useful, to be used together, to evaluate the surface quality of wood. [PERSON] (2016) studied the influence of the wear of the cutting edges on the surface roughness for several species (oak, larch, beech, and pine wood) processed on CNC milling machines. The experimental results showed that the Rk parameter, which measures the core roughness and is the closest approximation of the tool marks, may be an important indicator of the tool edge wear. In the case of beech wood, the Rk value after milling with a blunt tool was approximately twice the value of milling with a sharp tool. [PERSON] et al.