Build log at last

[routercnc]

I have 15mm rails on the Z and wouldn't do it again - more like 20mm. Much stiffer and give better spacing to fit all the parts in.

I've always gone with 600mm max width for single ballscrew, above that twin ballscrews. One of my earlier machines was a single ballscrew at around 600mm wide and you could push on one leg of the gantry and move it a mm or 2 without much effort. In practice this was still surprisingly effective even in hardwoods cutting off to one side, but I'm sure it contributed to errors cutting aluminium. So it's close, and Clive is right in that it depends on what you want to cut, but I would say 2 ballscrews at 700mm for robustness personally.

HGH20CA etc usually refer to the carriage style - wide, long etc. so google it to be sure. Some people prefer the wide version for for the X bearings (which the gantry then sits on), where you often want a large area to connect into. Whereas on the Y and Z you need every mm so go for the standard/narrow ones.