I hope this has not been discussed before: I just skimmed the thread.
On both my Solidoodles, and especially on my Trinity Labs Aluminatus, over-constraining the Z threaded rods ended up in wobble in the X and Y axes. Or at least that was alleged by various owners - I never personally had serious problems. At the moment, the Aluminatus has the steppers at the bottom, with the X carriage resting on the support nuts. No bearings at the bottom, and the top is free to flop about. This seems to produce better results than attemping to constrain the rod in three points (lower bearing, upper bearing, and moving nut).
I would advocate that the design not use the threaded rods for support, and allow for the possibility that the top of the rod is allowed to flop around. This might then require steel guide rods (which are present on both of my printers). More complex, but also more flexible should you encounter some X/Y variations.
On both my Solidoodles, and especially on my Trinity Labs Aluminatus, over-constraining the Z threaded rods ended up in wobble in the X and Y axes. Or at least that was alleged by various owners - I never personally had serious problems. At the moment, the Aluminatus has the steppers at the bottom, with the X carriage resting on the support nuts. No bearings at the bottom, and the top is free to flop about. This seems to produce better results than attemping to constrain the rod in three points (lower bearing, upper bearing, and moving nut).
I would advocate that the design not use the threaded rods for support, and allow for the possibility that the top of the rod is allowed to flop around. This might then require steel guide rods (which are present on both of my printers). More complex, but also more flexible should you encounter some X/Y variations.