OK, I have read everything that has been said so far!

All the machines that I have seen, watched, use this profile, on wheels, no rails, many belt driven and they knock out some nice stuff.

I know that you have all said they flex, so how are they managing it. Many make 1500mm x 1000mm machines from this stuff!

I thought by putting some HGR 20 linear rails on, that would introduce a lot more rigidity into the build

Most of the upgrades that people are doing is that, but often using 12mm rails and occasionally 15mm rails.

Most are still using 8mm Acme ball screws, I went for Rm1605, so twice the thickness.

Yes, I have seen the issues with long 8mm screws 1500mm long, I believe you call it whip or something else?

So I have exactly 40mm to play with between the rails, less the HGR20 carriages.

WoodKnot