The machine was commissioned early february, the mechanics work as expected. The firmware (see below) is now working perfectly with the controller.
If you download the current files, double-check in particular the distance between the servo mounts on the hub, they lack maybe 1mm. An improved model is in the works.
You will need to print:
2 x stepperbase
2 x plate1
1 x hub
2 x hubspacer
2 x hubring
1 x hubcap
You will need some form of T5 pulley, either printed or metallic (I went with metallic ones) and as in kritzlerbot, 6m of T5 belt. Alternatively, you substitute that with a small spool and nylon monofilament, for a much more affordable solution. You can also use ball chain instead. See http://www.thingiverse.com/tag:ball_chain
The 2xNEMA17 motors are totally overkill but I had them around. Each one is fastened with 3 m3x35 bolts. I added a m4 washer between pieces to gain in height, this will be added to next version.
The stepperbases and plates are symmetrical to simplify part multiplication and printing. You can optionally fit 3 small 624ZZ bearings on an m4 bolt to provide more contact between the belt and the pulley. A thin m4 washer between the plastic and the bearing may be of some use.
The hub or gondola's axis hosts: one spacer, two rings, one spacer, one cap fastened with one or two m4 bolts. Near the tip of the pen set three m4 nuts and bolts to fasten the pen.
I killed a boarduino by plugging simultenously two 5v power sources, so I settled for an on-the-test-board ATMega328 and two stepsticks.
Modified firmware using step/dir drivers: