# Surface Quality Evaluation in the Milling Process Using a Ball Nose End Mill
## 2 Materials and Methods/Research Methodology
### Methodology of the Shape Deviation
As shown in Figure 28, the lowest values of the surface roughness parameter Sa were obtained for the spiral circle strategy for all three heights evaluated, ranging from 0.7902 \(\upmu\)m to 0.9089 \(\upmu\)m. The highest values were shown for all three heights evaluated when using the spiral strategy in the range from 1.4644 \(\upmu\)m to 1.4918 \(\upmu\)m. Considering the surface topography, it can be concluded that this parameter decreased with possible increasing tool wear. This decreasing tendency was related to the deterioration of the surface quality and the wear of the tool, which caused \"wiping\" effects on the machined surface. The original single, well-visible toolpaths turned into dimples formed after the material was stripped off. Although the Sa parameter was reduced by using the spiral circle strategy, the machined surfaces showed a deterioration phenomenon due to tool wear. For the spiral circle strategy, a negative value of S\({}_{\text{sk}}\) = \(-\)0.0534 \(\upmu\)m was measured (Figure 29). It can be assumed that a greater number of valleys occur in the milled area with this strategy. Figure 6:8 Comparison of surface roughness Sa [\(\upmu\)m] for different heights with respect to the strategies. Figure 6:9 Comparison of surface roughness S\({}_{10}\)Z [\(\upmu\)m] for different heights with respect to the strategies. Figure 9.0 Comparison of surface roughness Ssk [\(\upmu\)m] for different heights with respect to the strategies. In contrast, the Ssk parameters for the Constant Z and spiral strategies showed positive values for all three heights. This indicates that the milled surfaces had many peaks, and the distribution of heights was skewed below the reference plane. The slope measurement results shown indicate that the slope factor of the profile is positive for the constant Z and spiral strategies. This indicates an increase in the coefficient of friction (due to the more rounded surface roughness). In the evaluation of the parameter S10z, the lowest values were measured using the spiral circle milling strategy for the two measured heights of 15 and 22.5 mm, as shown in Figure 9.1 The highest S10z values were measured with the Constant Z strategy. Average roughness values with the standard deviation are shown in Figure 9.2 The comparison of average parameter values S10z is shown in Figure 31, and the comparison of average parameter values Ssk is described in Figure 9.3 Finally, the comparison of average parameter values Sa is shown in Figure 9.4  
The graphical representation shows that the most optimal parameter values were obtained with the Constant Z strategy, which can be applied to the shape of machined surfaces of a similar character. In the case of the Ssk parameter, the values showed a uniform characteristic over the machined surface, but for the Sa parameter, the worst values were shown for the spiral circle strategy, which reached 1.8 times lower values compared with the spiral strategy. In terms of the statistical expression and display of standard deviations, which determine the variance of the values in the case of the roughness parameter Sa, it can be confirmed that in the case of the contact of the tool with the workpiece at a height of 7.5 mm, when a small effective diameter of the tool is used in the cutting process, there is a large variance, and in the case where the effective diameter of the tool increases in relation to the curvature of the surface, the variance decreases. In terms of the standard deviations of the evaluation of the average roughness from the total height of the machined surface for the parameters Sa and Ssk, it is clear that the lowest variance was obtained with the Constant Z strategy. This means that for this strategy used, the standard deviation varies the least within the three measured heights compared to the spiral and spiral circle strategies. Hence, for the Constant Z strategy, the change in roughness is the smallest over the entire machined sample area.