Help with machine flex..

Your gantry is too tall. It's creating a fair amount of leverage.

 

If it's gantry flex, your cutting forces are too high for the height of your gantry (ie. the leverage of your z-axis), and it isn't internally braced sufficiently to deal with it.

1) Epoxy or cyanoacrylate together your x-axis beams, if this holds through the vibration it would noticably increase torsional rigidity.

2) Make some 40mm wide aluminum plates, probably 3-6mm, with like 20 or so pairs of holes drilled along the length of them, and screw them to the top and bottom of the gantry. This will have a similar effect as gluing the center.

3) Turn the speeds and feeds up- faster RPM for a smaller chipload per tooth, dial everything up to the point of pretty much scorching the wood, then pull it back 10%. The less your machine is cutting off per revolution of the cutter, the less force it needs to push through the material, and the less resistance it's going to see.

Looking at the pictures, though, I'm not entirely sure that's purely flex; it's in multiple directions, perhaps depending on where the initial plunge was, it looks like the belt's jumping/rebounding as the router sees different cutting forces. What happens if you do a helical toolpath? Cut the circle with a shallow ramp, maybe really shallow like 0.5mm per pass, with a final 0.5mm finishing pocket. What does it do then?

If it was purely flex in the gantry, you'd see weird variations, elongations, distortions in the Y direction, and here you have a clear jump that's parallel with the x-axis.

 

"...Let's kill the people who point out the socioeconomic biases, inequities and harms endemic in society wrought through lack of forethought or carelessness- or outright malice- toward other people as an authoritarian defence of our privileges under the status quo."

Certainly an interesting solution. Not very useful for fixing gantry flex or belt stretch, though, I suspect.

 

This escalated quickly. Go on. *grabs popcorn*