I want to DIY.. some questions please!

[Jonathan]

You could use shimming steel to do this, a bench grinder can also be used to take the nut down a little.

I'd be very wary of grinding a ballnut since any dust from grinding in the ballnut is a recipy for disaster. You could try and cover it up but I don't think its worthe the risk.

CAN NOT STRESS ENOUGH. . . . . .DONT DO IT. . . . ABSOLUTLY DONT DO IT.

+1

When I said earlier that 16mm rail would be fine 'if done right' I was assuming supported rails. You'll be much better off getting some SBR20 (supported) rails.

The 6mm single flute cutters are brilliant...get lots! Although they are much better than most at plunging you shouldn't need to plunge - use spiral toolpaths or ramping. Router cutters with a 1/2Q shank are much much stronger than the 1/4" ones, so well worth the extra.

Very nice mounts. If I use your super rotating ballnut systems on the Y and X then I only need a nut-bracket for the Z right?

Yes... but how long is the Y-axis screw going to be? It's a bit sledgehammer to crack a nut using a rotating nut on the gantry, althought it would work well.