## 4 Mock-up Fabrication
### Aluminum Mock-up Panel Production  
Constructing free-form panels in general requires 2D fabrication drawings. Figure 9 shows the 2D fabrication drawings of the double-curved mock-up panel of the study, which were extracted from the 3D BIM modeling data. The single-curved surface had one curvature value so that the 2D drawings could be easily defined from the 3D BIM data, but, as for the double-curved surface, points and lines in the drawings should be approximately  
Figure 8: Modeling of the mock-up panels.  
drawn with a certain range of error and close attention needed to be paid during the actual production of the panel so as to keep the error within that range [5].  
As stated earlier, carving, molding to pour, and hardening material are not appropriate for metal panels. Roller and press bending are techniques commonly used for curved metal surfaces. In the case of roller bending, the radius of a curved surface must be more than 200-300 mm [7]. If the radius is less than 200 mm, then press bending is used. The mock-up aluminum panels of the study were produced by both roller and press cold bending techniques. Metals are bent either at room temperature or with application of heat. Cold bending forcefully bends and fixes flat, thick, sheet metal pieces along the curved surface on site at room temperature [18].  
During roller bending, the aluminum sheet was placed between a surface controlling roller and a fixed roller set. Both roller bending and press bending are typically used to produce a uniformly bent profile, and are known that those are not suitable for creating a bent shape with varying curvatures or double curvature panels. When producing the planar and single-curved panels in the study, the sub-frames were in the first place cut with a CNC machine and the four sides of a panel were assembled using bolts before fixing a sheet on top. Figure 10 shows the single-curved panel fabrication process performed.  
Unlike the planar or single-curved panel, the double-curved panels were likely to return flat by tensile force so that an intermediate sub-frame seemed to be necessary just in case in the mock-up production. With the engineers engaged in forming mock-up panels, the double-curved panels were formed with the combination of press bending with hammering by means of their panel-forming proficiency following the procedures shown in Figure 10. During the forming operation, it was found that the double-curved panel with concave curvature (the fourth panel in the upper row of Figure 8a), which was the double-curved panel with two coordinate curvatures, could be fastened to the mainframe by its edge with small deformation (see Figure 11b).  
Figure 10: Single-curved panel fabrication process.  
Figure 9: 2D panel assembly drawings for the double-curved mock-up panel.  
But in case of the convex curvature, i.e., the double-curved panel with three coordinate curvatures (the third panel in the upper row of Figure 8a), the panel got destroyed shortly after fabrication and the sub-frame was necessary in addition to the mainframe to maintain its curved shape. Figure 11a shows the destroyed double-curved panel. While more specific test and analysis would be required, it was thought that the panel could not bear bending stress imposed to form the convex-shaped double curve with three coordinate curvatures.