This is a design for an arm for a Useless Machine. I was building one for my Dad for Christmas, and rather then cut out the arm in wood, I designed this to be printed on my new 3D printer.

Build a useless machine as described at http://makeprojects.com/Project/The+Most+Useless+Machine/91/1#.UNJnFOAZ-lg

Instead of cutting an arm out of wood, print this design. You may have to tweak it based on your other design choices. I have included my OpenSCAD source file if you need to make changes.