I know some of you in out there do CAD/CAM development. I'm starting up a project where I would need to make some molds of an organically shaped fuselage. Until now I have stayed away from curved surface modeling because my favorite modeling tool, Sketchup, doesn't property model curves.
Sketchup can model line segments, triangles and things made up out of triangles (meshes or poly meshes). Meshes can be derived from lines but once these meshes are created they cant be easily or accurately modified. Also the resolution of a triangular mesh impacts what you can do with it. Imagine taking the cross section of an SR-71 fuselage to make bulkheads. If the mesh that makes up the fuse skin is composed of flat triangles the bulkhead edges will have flat spots. That might be OK if the flat spots are smaller than the resolution of your CNC equipment but on a large model (say 2M) that wont be the case.
So the answer is to use a CAD program that models curved surfaces using a mathematical representation called
NURBS. With NURBS you can compute exactly the x,y,z location of any point on a surface. So when you slice a bulkhead through a curvy part you get a curve of infinite resolution no matter how large you scale up the part. The same model could be used for the full scale airplane with no loss in resolution.
NURBS programs tend to be 2 things: Difficult to use and Expensive. The cheapest one that's often pointed to is
Rhino which will set you back a cool $1K. I played with Rhino all
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