This driver pulley should go perfectly with my 608zz Idler Pulley and stand (http://www.thingiverse.com/thing:155018). This driver pulley would attach to the stepper motor at the bottom and align with the idler pulley at the top to offer nicely parallel strings top to bottom.
This was drawn in Sketchup (which is not known for being good at drawing spirals. Please let me know if it works at all.
The idea with this pulley is to wind string around it (about 0.8mm thick). For my Delta, I would need about 3 turns and 8 revolutions would give me full height movement with less than 10mm deflection from left to right.
Is it worth cutting spokes into the pulley ti save on material?
I redrew this pulley in OpenSCAD (My first attempt at using it). I made use of Karl Maximilian's Metric Screw Thread Library (http://www.thingiverse.com/thing:31363) for this one.
When I bought my stepper motor, I realised that not all Nema 17 stepper motors have the flat side to their shafts. I've added a version with a round shaft hole which then has three holding screws 120 degrees from each other as well. Included is a scad file with a parameter to turn the FlatShaft on or off which also toggles 1 or 3 holding screw holes. In the scad file I also tried out polyholes, but they are commented out as they didn't look right. (I would have to play with 3d printing the two options before I'll be satisfied that the polyhole option is better or not.)