## 3 Results and Discussion
Figure 11: The change and percentage increase in elastic modulus of wooden workpieces after roller pressing. Figure 12: Change in the elastic modulus of wooden workpieces after roller pressing, depending upon the penetration depth, i.e., the thickness reduction. The obtained experimental results related to the elastic modulus prove that softer, more lightweight, and cheaper types of wood (linden and fir) have a significantly higher relative increase in stiffness after being processed using the proposed method. Therefore, they can be used as construction materials instead of heavier and more expensive wood types (oak, beech, ash, walnut) if the primary quality criterion is the elastic modulus, i.e., stiffness. Furthermore, bearing in mind that linden and fir grow faster than beech, oak, walnut, and ash [7], the proposed method is associated with economic benefits as well. The analysis of the results related to the quality of the treated surface (Tables 1 and 4, Figure 13) leads us to conclude that the proposed processing method has positive effects in reducing the surface roughness. The achieved relative reduction of the arithmetic mean deviation of roughness profile Ra ranges from 11.57% on the fir sample to 54.75% on the alder sample. As the diagrams in Figure 13 show, different types of wood require different pressing forces to achieve the minimum surface roughness. On fir, the minimum surface roughness was measured after applying the force of 397 N. Larger values of pressing force have only led to the deterioration of the surface quality due to the damaged surface structure, as discussed before. The best results in surface roughness on linden, oak, and cherry samples were recorded when the force of 785 N was applied. Achieving the minimum surface roughness on beech, ash, and alder samples required the force of 1177 N. The largest relative decrease in surface roughness of 53.6% was measured on the walnut sample at 1570 N.  
Figure 13: Change and percentage increase/decrease in the arithmetic mean deviation of roughness profile after roller pressing. Figure 14 indicates how the rollers' penetration depth affects the surface roughness in different types of wood. The optimum penetration depth, corresponding to the minimum surface roughness, can be determined for each type of wood by analysing the diagrams in Figure 14. This information is valuable in practice when the processing is based on the specified penetration depth instead of the force of penetration (pressing force). It is important to note that, in the case of linden samples, the force values that are larger than the optimal 785 N corresponding to the minimum value of Ra have almost no influence on the surface roughness (Figures 13 and 14). That leads to the conclusion that linden has an excellent ability for plastic deformation without structure destruction, which results in a large relative increase in the density and elastic modulus. The calculated correlation coefficients between the pressing force as an input variable and the elastic modulus, hardness, and roughness as output variables, are given in Table 5.  
For all tested types of wood, high values of correlation coefficients (>0.9) indicate a strict dependency of both the elastic modulus and hardness on the pressing force, which confirms the effects of roller pressing on the improvement of mechanical properties of wood. As the values of the correlation coefficients between force and roughness indicate, cherry, linden, beech, walnut, and ash exhibit a strict negative dependence. Therefore, roller pressing is certainly a recommended method for reducing the surface roughness of these types of wood. The remaining wood types show high negative dependence, except for fir. Roller pressing cannot reduce the roughness of fir, so it is not a recommended method for that type of wood. Figure 7:0 Change in the arithmetic mean deviation of roughness profile after roller pressing, depending upon the penetration depth, i.e., thickness reduction.