Square drill using a Reuleaux triangle rotor

In principle, a Reuleaux triangle -- a shape of constant width -- can be used to drill an almost square hole. I wanted a toy to illustrate the principle.

So when you put this together, and turn the crank, it shows how the triangle can move to create an almost square hole.

It's complicated by the fact that to drill a square hole, the center of the triangle must move, and the path it moves along is not circular. So I have joined the crank to the triangle with an Oldham coupler, which allows off-center rotation.

There are a couple of other Reuleaux triangles out there:
- http://www.thingiverse.com/thing:1082
- http://www.thingiverse.com/thing:1088
- http://www.thingiverse.com/thing:5456
- http://www.thingiverse.com/thing:1755
but they don't come with the crank :)