Lets eventualy start this CNC - MKI

[JAZZCNC]

On the front Z plate, the plate will be bolted to the square rail. Surely the 20mm rails will stiffen up the 10mm plate?

I don't want to put extra weight on it if I don't need to - but you guys know what works and what doesn't....

Yes to some degree they do but that means they are also under bending forces and linear rails don't tolerate much error and will bind. The Z axis is THE MOST IMPORTANT piece of the machine so if you skimp here then you will regret it later guaranteed.

The bearing plates you could probably get away with 10mm but why bother just buy one plate size for all key areas it will be cheaper or not much more money.