## 2 Materials and Methods
### Processing by CNC  
The 24 prepared samples were machined using a CNC milling process along the grain direction, on the face surfaces, which were then analyzed to evaluate their quality. Although the used milling tools are capable of processing both the face and the edge surfaces, the study intentionally focused only on the quality of the face surface, as the edges involve radial orientations and additional factors that would necessitate a separate and more complex investigation beyond the scope of this work.  
All specimens were uniformly secured to the machine table to minimize movement and vibration during machining, as shown in Figure 1a. The milling process employed two types of Leitz end-mill cutters, each with a 10 mm diameter: one featuring two helical blades, with I.D. 042278 and active cutting length of 25 mm (Figure 1b), and the other with straight blades, with I.D. 038058 and active cutting length of 23 mm (Figure 1c). Both cutters were made of tungsten carbide; however, the helical-bladed tool (T1) was additionally coated with a Marathon layer to enhance performance and durability. Both tools were designed with a positive twist orientation, from the face side of the workpiece toward the bottom, facilitating effective chip evacuation and improved dust extraction. To ensure consistency in the cutting performance, both tools were new, and their blades were microscopically inspected to confirm the absence of any defects.  
Additionally, all milling operations were performed parallel to the wood grain, with each specimen carefully oriented so that cutting occurred in the direction of tree growth to minimize fiber damage and surface variability.  
Figure 1: Machining on CNC: (**a**) workpiece mounted; (**b**) spiral-end mill T1; (**c**) straight-end mill T2.  
The samples were cut using a three-axis CNC router, ISEL GFV model, manufactured in Germany, with machining parameter settings divided into constant and variable, as summarized in Table 1. The depths of cut were chosen to represent typical roughing and finishing operations: 3 mm for roughing, and 1 mm for finishing. These values were validated by previous studies focused on wood machining, including [13; 14; 15], which identified similar depths as effective in balancing material removal rate with surface quality. Separate sets of samples were prepared for each depth of cut to ensure a reliable and consistent evaluation. The feed speed was maintained at a constant 4 m/min throughout all experiments. This value was selected based on prior research findings [16; 17; 18; 19; 20], which demonstrated that this feed rate contributes to reduced surface roughness in hardwood machining while avoiding excessive tool wear or burning effects common at lower speeds.  
The stepover, defined as the lateral distance between adjacent toolpaths, was set relative to the tool diameter (10 mm) and varied to assess its influence on surface quality. Specifically, three stepover values were used, i.e., 5 mm, 7 mm, and 9 mm, corresponding to 50%, 70%, and 90% of the tool diameter, respectively. This range was achieved by adjusting the overlap between toolpaths from a minimum overlap of 1 mm (resulting in a 9 mm stepover) to a maximum overlap of 5 mm (resulting in a 5 mm stepover), with an intermediate setting of 3 mm (7 mm stepover) also included. This variation allowed for a comparative analysis of the effect of toolpath density on the resulting surface finish.