Created a new 3D printable Gondola that only requires two bearings, and two M3 metal screws. No brass tube!

The two M3 metal screws can thread themselves in the holes printed, or you can use the included recess to use a pair of hex M3 nuts to secure them.

This allows two point holding of the pen. The central tube opening works with pens up to the diameter of standard Sharpie fine points. It includes a bracket for attaching a pen lift servo!

The design is on thingiverse; http://www.thingiverse.com/thing:372244

For your second question, yes, the pen clamp has an interior diameter of 17mm, so if you drill a couple of clearance holes (3-5 - 5mm) in your brass tube (use the plastic piece to mark where), you can just use the pen clamp on the original design.

Sandy, what perimeter are you thinking can be thinned down?

Thanks for the suggestion. I'll see how this works and if I need another version, I'll make that change.

I've used them before on a Kite Aerial Photography rig and they worked great there. So far, I have been having trouble getting the software side of my polargraph to work. I can run test Examples to verify the wiring on the steppers and the servo are good to go, but getting the software to do anything looking reasonable... Well I am still working on it.