## 3 Results
### Validation of CAM-Based Patient-Specific Nuss Bar Fabrication Technology
#### 3.2.1 Bending Test for Patient-Specific Nuss Bar Fabrication
* Results of Bending Test  
The machining accuracies of 60 prototypes in total were evaluated through measurement of dimensions. For the dimension measurement test, the maximum width and height were specified as the major measurement indices. Figure 15 shows the dimension measuring points for machining accuracy. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & **Sheet Thickness** & \multirow{2}{*}{**K-Factor**} & **Yield Strength** & **Elastic Modulus** & **Initial Bend Radius** & **Initial Bend Angle** \\  & **(mm)** & & **(psi ’)** & **(psi)** * & **(mm)** & **(°)** \\ \hline Ti-Gr5 & 3 & 0.33 & 140,000 & 16,500 & \\ \hline SUS & 3 & 0.33 & 116,000 & 28,000 & Up to transition values \\ \hline \hline \end{tabular}  
* The material properties of Titanium-6 Al-4V ELI (Ti-Gr5) and SUS (SUS 316 LVM) were referenced from material property data (MetWeb.com).  
\end{table} 
Table 7: Data of springback factors. Figure 14: Bending Springback Calculator from CUSTOMPART.NET [23]; springback factor (_Ks_), final bend radius (_FR_), and final bend angle are changed by input values which are sheet thickness (_Mt_), K-factor, yield strength, elasticity, initial bend angle, and initial bend radius (_IR_). The dimension accuracy results of the five design prototypes considering the central arc were plotted as a graph. Figure 16 shows a graph for the measured width values of central arc designs 1-5 which were fabricated with SUS and Ti-Gr5. The SUS and Ti-Gr5 products have the dimension (width) errors of \(\pm\)0.41% and \(\pm\)0.53%, respectively. Figure 17 shows the height measurements of the central arc design products. SUS and Ti-Gr5 products have the dimension (height) errors of \(\pm\)2.61% and \(\pm\)1.48%, respectively. The major dimension measuring results of Ti-Gr5 and SUS products considering the central arc confirmed that the dimension accuracies of all the prototypes were within \(\pm\)5%. The dimension accuracy values of the five designs considering the transition values were plotted as a graph. Figure 18 shows the width of the transition Nuss bars 1-5 fabricated with SUS and Ti-Gr5. The width error is \(\pm\)1.19% for the SUS product and \(\pm\)0.57% for the Ti-Gr5 product. Figure 19 shows the measured height, and the height errors of the SUS and Ti-Gr5 products are \(\pm\)1.73% and \(\pm\)0.9%, respectively. This result confirms that the dimension accuracies of Ti-Gr5 and SUS products considering the transition value are within \(\pm\)5%. Figure 16: This histogram shows the dimensions of the Nuss bar. The maximum width of SUS and Ti-Gr5 central arc Nuss bars was measured and compared with original width. Figure 15: Designs of test sample: (**a**) central arc design with measuring point of maximum width and height; (**b**) transition Nuss bar design with measuring points. Figure 17: Results for the dimension accuracy of the central arc Nuss bar. This histogram shows the maximum height of the central arc Nuss bars (SUS and Ti-Gr5). Heights were measured and compared with original height. Figure 18: Results for the dimension accuracy of the SUS and Ti-Gr5 transition Nuss bar. This histogram shows the maximum width of the SUS and Ti-Gr5 transition Nuss bars. Widths were measured and compared with original width. All values within the central arc and transition values of the Nuss bar can be machined using the auto CNC bending machine, and the bending error range of the bending machine was found to be within \(\pm\)5%. In particular, it is expected that more accurate machining will be possible for products fabricated with the Ti-Gr5 material.