This is a small 200m test print of some threaded rod I made along with a matching nut. It is a double helix with 1 turn per 50mm. The math works out like this:

For a standard 200 step/ rev stepper motor 50mm / 200 step = .25mm per step add in a 2:1 gearing or half step motor controller and you have .125 mm/step add both and it's .0625 mm/step

From what i can tell, there is no perceptible backlash and the nut slides very freely.

I'm sure I'm going to incorporate this into my shuttle system or a printer somehow. Enjoy...

Edit: As per request, Here's a video of it:

http://www.youtube.com/watch?v=Hkjf8x__SrE The video shows that it's loose enough to slip down on it's own. I would recommend printing one to test tolerances for yourself.

Edit 6/18/2011 Uploaded the quad helical nut/thread. This should help with any wear issues, as well as addressing any concerns with backlash.

Print some threads, print a nut The center of the threaded rod is hollow, connect multiple pieces using a 5mm rod.

The nut might be a little tight at first. Once you work it on the threaded rod, the interior edges become very smooth.