## 4 Comparison of Simulation and Experiment Results
The outer end of the panel to the left of the reference line was defined as \"growing\", with positive values indicating growth in that direction, as shown in Figure 12. Simulation and experimental results demonstrate that the panel thickness, \(H\), except for Figure 13b, exhibits an initial increase followed by a decrease, with the maximum thickness occurring at the midpoint in both the straight-edge and curved-edge sections, as depicted in Figure 13. In the experiment, we measured the outer profile three times at every marked position. Additionally, in both the straight-edge and curved-edge sections, simulation result of the door panel's cross-sectional shape revealed that the simulation consistently depicted the panel's thickness after bending to be approximately 0.4 mm and 0.2 mm, respectively, larger than the experimental results. The reference line represents the initial flanging position of the panel before any bending deformation occurred, as shown in Figure 14. Therefore, it was essential to minimize both of these values (growing and creepage) to obtain optimal results. Cross-sectional observations were made on the door panel, focusing on the outer profile of the outer panel, as shown in Figure 2. In the straight-edge section of the experiment, marked from P9 to P14 in Figure 8, points P10, P12, and P14 were selected, along with the straight-edge section of the simulation, marked as W1, W2, and W3, for cross-sectional  
Figure 11: Comparison of the mean values of external forces (tangential force represents the force in the x direction, while normal force represents the force in the y direction, as depicted in Figure 10, Load Measurement Tool Model) in roller hemming. Figure 12: Displacement on the roller hemming reference line showing creepage and growing. analysis. In the curved-edge section of the experiment, marked from P7 to P8 in Figure 8b, and in the curved-edge section of the simulation, points S1, S2, and S3 were selected for the cross-sectional analysis. More particularly, the experimental findings revealed that greater creepage was observed in the curved-edge part compared to the simulation results. This discrepancy in the geometric relationship between the finite element modeling of the curved part and the actual roller may lead to deviations from the simulation preset trajectory during travel. However, in both the curved- and straight-edge sections, the simulations indicated initial growth in the straight-edge and creepage in the curved-edge, whereas the experimental results exhibited creepage in both sections. This discrepancy can be attributed to the presence of a free edge in the initial position of both curved and straight edges during roller hemming in simulation operations. This discrepancy is further summarized in Table 7. \begin{table}
\begin{tabular}{c c c c c c} \hline  & \multicolumn{2}{c}{**Measuring Position**} & \multicolumn{2}{c}{**Crepage (–)/Growing Value (+) [mm]**} & \multirow{2}{*}{**Error [mm]**} \\ \hline \multicolumn{2}{c}{**Experiment Position**} & \multicolumn{2}{c}{**Simulation Position**} & \multicolumn{2}{c}{**Experiment Value**} & \multicolumn{2}{c}{**Simulation Value**} \\ \hline \multirow{3}{*}{Curved-edge part} & S1 & S1 & \(-1.14\) & \(-0.2\) & \(0.94\) \\  & S2 & S2 & \(-0.94\) & \(-0.31\) & \(0.64\) \\  & S3 & S3 & \(-1.18\) & \(-0.48\) & \(0.7\) \\ \hline \multirow{3}{*}{Straight-edge part} & P10 & W1 & \(-0.44\) & \(+1.44\) & \(1.88\) \\  & P12 & W2 & \(-0.45\) & \(+0.12\) & \(0.57\) \\ \cline{1-1}  & P14 & W3 & \(-0.21\) & \(+0.12\) & \(0.33\) \\ \hline \end{tabular}
\end{table} 
Table 7: Comparison of creepage and growth values in the roller hemming process between our experiment and simulation. Figure 13: The panel thickness results obtained from both simulation and experimental roller hemming processes. In the straight-edge section, data were collected from positions P10, P12, and P14 during experimentation, and from positions S1, S2, and S3 during simulation. Similarly, in the curved-edge flat surface section, data were gathered from positions S1 to S3.