The threadless ball screw system which was introduced by Mark Sollack is very useful concept on RepRap's z-axis structure. Normally, the RepRap has two rod on the each side of Z-axis, the one is smooth rod and the other is threaded rod. But it need just one rod on each side when the threadless ball screw system implemented. It makes possible to guide and move the x-ends simultaneously with just one smooth rod at the each side of z-axis. I think the threadless ball screw system can make the RepRap structure so simple. I designed the X-end motor holder and the 4.55 deg angle threadless ball screw nut for 2 mm lead length. But it's not tested yet. 20140104 - Added the XZ-idler. This assembly need following parts.