## 3 Results
### Validation of CAM-Based Patient-Specific Nuss Bar Fabrication Technology  
#### 3.2.1 Bending Test for Patient-Specific Nuss Bar Fabrication  
Before fabricating the patient-specific Nuss bar, a bending test was performed to verify the bending range and machining accuracy of the equipped CNC bending machine. * Drawing of Test Design: Central Arc and Transition Value of Nuss Bar  
A product drawing for machining test considering the maximum/minimum values of the arc and transition parts of the Nuss bar components was created. Figure 13 shows the components of the Nuss bar [18]. (1) Central arc design: This part lifts the depressed sternum and is divided into five steps by selecting 200-400 mm for the maximum/minimum lengths of the transverse diameter (50 mm intervals). The maximum/minimum AP distances are determined by using the HI [19]. Five drawings were created by using the circumference within 160\({}^{\circ}\) from the center of the ellipse shape. (2) Transition part design: The machining occurs predominantly in this part and the applied value varies by the degree of the chest wall deformation [18]. The maximum/minimum radius range of curvature of the transition part was set to 20-100 mm, and was divided into five steps (20 mm intervals). Five drawings were created with the representative angles of 30\({}^{\circ}\), 60\({}^{\circ}\), and 90\({}^{\circ}\) for each curvature. * Computer-Aided Manufacturing (CAM) Data and Fabrication of Test Designs  
The test products were fabricated using the biocompatible metal Titanium-6 Al-4V ELI (Ti-Gr5) and SUS 316 LVM (SUS) with a thickness of 3 mm and a width of 13 m. The bending method varied according to the product shape. The feeding method was applied when the curvature of an ellipse shape was low and machining over a wide range was required such as for the central arch. The multi-bending method was applied if different bending methods had to be performed for each narrow point such as for the transition type. The desired machining result could not be obtained from the initial CNC bending test where the design specifications were applied. In particular, Ti-Gr5 generated greater machining errors due to a strong springback phenomenon compared to SUS. The springback, which is a property that makes the material return to its original shape, is affected by the material properties (yield strength, modulus of elasticity) and thickness, machining angle, and bending radius [20; 21; 22]. Figure 13: Three components of the Nuss bar. Arc and transition were used to draw the test sample for the bending test. Therefore, the CAM data were established considering the springback phenomenon of materials to perform accurate bending. First, the initial springback factor, _Ks_, of each material was calculated as is shown in Figure 14. To calculate the initial _Ks_ value, the data for each factor in Table 7 were input into the Bending Springback Calculator (Figure 14) of CUSTOMPART.NET. The CAM data (machining angle and radius value) for the final angle and radius values were calculated reversely by applying the initial _Ks_ value to the springback factor (_Ks_) equation, Equation (3) [24], as follows:  
\[\textit{Ks}=\frac{\text{Initial angle }(^{\circ})}{\text{Final angle }(^{\circ})}\ =\ \left(\frac{2\times\textit{IR}}{\textit{Mt}}+1\right)\bigg{/}\left(\frac{2 \times\textit{FR}}{\textit{Mt}}+1\right), \tag{3}\]  
where _IR_ and _FR_ are the initial and final bend radii, respectively, and _Mt_ is the sheet thickness. The CAM data according to the material, angle, and bending radius were determined through multiple bending trials and errors. Different CAM values were required depending on the material even if a Nuss bar of the same shape was fabricated. The Ti-Gr5 product required additional bending between 2% to 20% as compared to the SUS product. The prototypes of the central arc and transition values were fabricated using the appropriate machining method and the derived CAM values.