[Clive Loynes]

I use NanoCAD to draw what I want to make.(see gearbox on avatar) 3D would be nice but not always necessary and to make a chassis it would be overkill.

NanoCAD is just about identical to AutoCAD and can export in .dxf format. It is free for home use.

For complex shapes I draw and export cutter paths as .dxf files and then import them into Gsimple, also free, to generate G-Code for my CNC mill.

I would have thought that the biggest challenge would be measuring the original parts rather than drawing up your design. Although I've never had the patience to produce a drawing in the manner that Fybrelite want it done!