Available DIY plan sets show it both ways. If no bearing is at the bottom end of the lead screw there won't be any lead nut binding that would result from imperfect alignment of a lower bearing relative to the top bearing. When you don't have machinery to drill or bore holes for the bearings accurately, you have to be creative when making upper and lower bearing mount plates that align perfectly. My first machine was built from Solsylva plans that had no lower bearing. The two machines I built afterwards both have bearings at each end of the Z axis lead screws. Backlash due to side loads on the cutter is usually limited by the bearings on the guide rails and is usually not transferred to the lead screw. Only the up and down backlash due to plunges would need the thrust bearing and collars to keep it from damaging the router or spindle.
I put thrust bearings above and below the bearing mounts with a collar on each side to insure that no thrust loads get to the router in either direction. I've seen machines that work fine with no thrust bearings at all.
CarveOne
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