## 3 Results and Discussion
### Surface Roughness  
Table 11 presents the results of the measurement of the basic surface roughness parameters of the inner and outer walls of the drawpiece, depending on the sheet hardening state. The average value of the Ra parameter for the EN AW-6082 alloy sheet before forming was 0.68 \(\upmu\)m, and the maximum height of profile was Rz 3.62 \(\upmu\)m. An increase in the values of the surface roughness parameters of the drawpieces was observed when compared to the undeformed sheet metal. In the case of the outer surface, lower values of the Ra and Rz parameters were obtained compared to the inner surface. Although the outer surface of the drawpiece is freely formed, due to the large deformations of the sheet metal in the circumferential and radial directions, a roughening effect is created. This effect is known as orange peel and consists in changing the orientation of neighbouring grains under the influence of the plastic deformations. The amount of free surface roughening is affected by the size of grains and the amount of plastic deformation [47].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Temper Designation**} & \multirow{2}{*}{**Surface of Drawpiece**} & \multicolumn{2}{c}{**Orientation of Measurement Relative to**} & \multirow{2}{*}{**Ra, \(\upmu\)m**} & \multirow{2}{*}{**Rz, \(\upmu\)m**} \\  & & & **the Axis of the Drawpiece Wall, \({}^{\circ}\)** & \\ \hline O & inner & 0 & 2.95 & 16.37 \\ \hline O & inner & 90 & 2.59 & 15.06 \\ \hline O & outer & 0 & 2.06 & 10.89 \\ \hline O & outer & 90 & 1.69 & 8.96 \\ \hline W & inner &  
Additionally, the surface roughness parameters measured in the direction parallel to the axis of the drawpiece wall was higher in all cases, which was related to the cyclic effect of the tool moving along a trajectory with a vertical step size of 1 mm. This resulted in the formation of the linear cyclic grooves on the inner surface of the drawpieces (Figure 18). The use of a smaller step size could contribute to the reduction in the surface roughness parameters, especially in the direction parallel to the axis of the drawpiece wall.  
During the motion of the tool over the sheet, friction plays a vital role in ensuring the proper surface roughness of the drawpieces. In SPIF, friction between tool and workpiece is very high, and thus it leads to galling, especially in the case of aluminium alloys, which are susceptible to abrasion and galling. The deterioration of the internal surface quality in SPIF is a well-known issue [48]. Through appropriate lubrication and the use of tools with a rotating tip, the influence of the tool on the deterioration of the surface finish can be minimized. The intensity of the tool impact also depends on the SPIF process parameters such as feed rate, tool diameter, step depth and wall angle [49]. The quality of the incrementally formed components is particularly limited by the step size. Reducing the step size improves the surface quality; however, reducing this factor makes the process time very long [50]. Long processing time is one of the disadvantages of the SPIF process, but, at the same time, this process allows for much larger limit deformations compared to conventional sheet metal forming techniques [51].