These days it's possible to have tubes CNC laser cut, which opens up some neat options for semi-handmade bikes (tubes mitred by a machine, brazed/welded by a human). From a data point of view it would be handy if Bikecad could export tubes as individual STP files. The program obviously already calculates all the dimensions and the mitre contours so it doesn't seem like it would be an impossible thing to integrate but I don't really know what's involved!
Usually the machines 'cut to normal' ie at right angles to the tube, so the mitre would have to be cut along whichever of the inside or outside wall lines removes the most material. Some can make 3D cuts, ie with the head at an angle with the resulting cut being much like that done with a file or holesaw.
This process is only an option with straight, untapered tubes so accurate modelling of chain and seat stays (and other curved or shaped tubes) isn't a problem.
What do you think, Brent?