My new 6040 improvement plan.

[Zeeflyboy]

The main gain was in Y-axis bearings.

They are adjusters. When they are tight, there is no noticeable flex/movement on the gantry. I tighten til they stop, then quarter turn to nip up. Gantry feels rock hard.

When I loosen the grub screws, the gantry flexes as if it's on rubber mounts. Difference is remarkable.

But that simply isn't how these bearings work, there is no pre-load adjustment... they are just a few rows of bearings inside a tube. You are achieving a reduction in slop most likely by forcing them out of alignment and thus introducing an increased load on one side. It achieves the goal but the problem is that the load isn't evenly distributed and wear will likely be accelerated.

Whatever works for you, but they are retainer screws not adjustment screws.

I'm just left with the flexing of the 20mm unsupported x-axis shafts. There's a youtube video where the guy replaces them with supported rails, but improvement was MINIMAL at best. Waste of time.

I have a better, cheaper and easier solution. Just sketching out the plan and I'll post it.

Supported rail doth not equal profile linear rail. My current machine is effectively a 6040 with proper 20mm profile rails and the difference is very obvious compared to the old machine... I don't need a guy on youtube, I've extensively experienced both. Of course even with proper rails you are still left with the general somewhat lightweight design but it's still a significant improvement over the previous model I had with unsupported rails like yours.

I'm not belittling you, you are obviously keen to get stuck in and learn - just be aware that despite what you may think at the moment, those unsupported rails (and to a lesser extent the supported rail on the bottom) are a limiting factor in this machine which you will need to learn to work around and will limit the level of tolerance and finish you can ultimately achieve in harder materials.