Jonathan,

this thread makes me wish I had space to set up my RF45 clone and finish the conversion... I too did a huge part of the machining on a round coloumn mill. Will post photos when I find them

Just wanted to chime in on the bearings front. I would suggest you go with angular contact bearings and ones specific for ballscrews or ones specifically made with medium preload as standard.

Angular contact bearings have lower thrust rating than taper rollers but you will not need to add any spacers to get the preload to them right and also have lower friction. Additionally they take up less axial space. There is also a lower limit to the ID of taper rollers which from memory is 14.989mm (imperial).

Did you mention what size/grade ballscrews you are using?