But isn't there a risk that you can get too tight on the belts? That was part of the basis about it being hard to get the right tension - that you want enough but not too much. And again, my belts are now tighter than ever, but I can't say that's made a difference.
Remember that the ringing effect is showing up when the printer has only moved a millimeter or less in the lateral axis. Acceleration and deceleration should have kept the speed for that small of a distance pretty low, at least in the meager bit I understand Marlin move planning. Again - I can understand ringing on major 90 degree corners, but not so easily on cases like shallow recesses for lettering or holes on thin walls.
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My four-sided ringing test print is complete. First, I realized that the case I printed yesterday had small radius rounded corners, so not having ringing there is no surprise. The test object had an identical notch and round hole on all four sides of a cube. Pretty equal ringing on all four sides. I'm not sure pictures will reveal details well, but I may try to take some in the morning.