CAD revisited

http://www.thingiverse.com/thing:232578
Which 3D software should I learn for 3D printing?

https://www.youtube.com/watch?v=ltRL0qXgo2A
Solidworks - using variables and equations

http://www.rs-online.com/designspark/electronics/eng/page/mechanical
Designspark Mechanical (free CAD)

http://www.punchcad.com/c-12-consumer-cad.aspx
ViaCAD (cheap CAD)

http://www.openscad.org/
OpenSCAD (program watertight STL models)