## 3 Results and Discussion  
The results, i.e., the arithmetic mean values of hardness, elastic modulus, and roughness measurements performed before and after each roller pressing, are provided in Tables 1-4. The results of hardness measurement (Tables 1 and 2, Figure 7) indicate an increase in hardness of all wood samples after roller pressing treatment. It is a direct consequence of the increase in wood density. The surface layer exhibits the most significant change in density, while the smallest changes occur at half the thickness of the processed samples. Figure 8 shows the microstructure of the maximum deformation zone, moderate deformation zone, and undeformed zone of the processed sample. Figure 5: Elastic modulus measurement. Figure 6: Surface roughness measurement. Regarding the increase in hardness, the presented method has better outcomes when used on certain types of wood: ash (175.9%), cherry (88.7%), oak (75.8%), walnut (63.2%), alder (60.5%) and beech (50.8%). The percentage increases in hardness, given in parentheses, were measured after the processing by applying the maximum normal force of 1570 N. The hardness of the cherry sample at the force of 1570 N is higher than the hardness of all \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Wood Type** & **HB** & **E (MPa)** & **Ra (\(\mu\)m)** \\ \hline Cherry & 2.007 & 7997.82 & 6.731 \\ Fir & 2.102 & 10,227.18 & 5.774 \\ Alder & 2.188 & 8797.24 & 9.064 \\ Linden & 2.36 & 13,752.09 & 5.936 \\ Beech & 3.107 & 11,823.74 & 7.399 \\ Walnut & 3.372 & 8594.25 & 7.214 \\ Oak & 3.078 & 13,386.74 & 9.344 \\ Ash & 2.788 & 10,779.82 & 11.086 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hardness, elastic modulus, and roughness of wooden workpieces before roller pressing. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Wood Type** & \multicolumn{4}{c}{**Pressing Force F (N)**} \\ \cline{2-5}  & **392** & **785** & **1177** & **1570** \\ \hline Cherry & 8319.65 & 9179.55 & 9633.2 & 9534.54 \\ Fir & 10,660.3 & 11,683.07 & 14,036.21 & 12,696.9 \\ Alder & 9014.75 & 9637.57 & 9942.77 & 9523.9 \\ Linden & 15,239.1 & 17,625.72 & 22,928.77 & 19,946.6 \\ Beech & 12,316 & 13,249.53 & 14,486.93 & 13,947.6 \\ Walnut & 8751.26 & 9249.76 & 9333.5 & 9569.54 \\ Oak & 14,018.2 & 15,821.92 & 15,999.43 & 16,020.3 \\ Ash & 10,785.1 & 11,362.02 & 11,485.04 & 11,250 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean arithmetic roughness of wooden workpieces after roller pressing. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Wood Type** & \multicolumn{4}{c}{**Pressing Force F (N)**} \\ \cline{2-5}  & **392** & **785** & **1177** & **1570** \\ \hline Cherry & 8319.65 & 9179.55 & 9633.2 & 9534.54 \\ Fir & 10,660.3 & 11,683.07 & 14,036.21 & 12,696.9 \\ Alder & 9014.75 & 9637.57 & 9942.77 & 9523.9 \\ Linden & 15,239.1 & 17,625.72 & 22,928.77 & 19,946.6 \\ Beech & 12,316 & 13,249.53 & 14,486.93 & 13,947.6 \\ Walnut & 8751.26 & 9249.76 & 9333.5 & 9569.54 \\ Oak & 14,018.2 & 15,821.92 & 15,999.43 & 16,020.3 \\ Ash & 10,785.1 & 11,362.02 & 11,485.04 & 11,250 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Elastic modulus of wooden workpieces after roller pressing. untreated wood samples, including hardwoods such as walnut, oak, beech, and ash. The hardness of alder after the processing is approximate to the hardness of beech, walnut, oak, and ash before processing. Therefore, it can be concluded that, after being processed by roller pressing, cherry and alder can replace oak, walnut, ash, and beech in most structures where hardness is the most important quality criterion. Bearing in mind that cherry and alder have a less frequent industrial application than beech, oak, and walnut, their price is significantly lower [53; 54; 55], so the proposed method is associated with economic benefits as well. Figure 9 shows the change in hardness of wood samples depending upon the measured thickness reduction due to the surface densification during roller pressing. The points on each curve denote the pressing force of 392 N, 785 N, 1177 N, and 1570 N, respectively.