This is a slight redesign of iamburnys Parametric Double Helix Herringbone Z Gears. I found the gears worked great but the Z rod was not held securely enough and could wobble in the nut trap. I moved the nut trap down and inside the gear so that the top of the gear is clamped by the two nuts. This makes the rod fairly rigid.

I got the idea from Amsterdamman's design http://www.thingiverse.com/thing:465190 but I couldn't make his gears work, they seemed to have too much play.

This is a kludge of iamburnys openscad code, (included in the download) so the gears mesh extremely well.