so, I just built this today.. it's kindof crappy but it works like it should.. I just used scrap pieces of wood and stole a spring and hinge off a door at my shop

.. took me about a hour to build and I was kind of designing it as I was going along. I made it as simple as possible.
I'm going to use it for small 1 or 2 color signs where lining up both colors isn't a big deal and once I get everything figured out I will build a bigger one out of steel with tighter tolerances and a rotating center piece and registration pins for the arms and all that fun stuff. This one was basically just for testing purposes, and if it works (which it will) I'll use it.
anyways, here are the pics.
