RE: Gantry resonant mode and VFA artifacts
That's absolutely a risk I'm trying to avoid, too. Those bearings aren't cheap.
but in the same way, like both at 92 degrees. I think it's typical for parts from the same batch to look like that.
Absolutely - designed for symmetry - systematic error cancels out and everybody is happy (except the bearings...)
RE: Gantry resonant mode and VFA artifacts
After some searching: is it so that belt interaction with the drive pulleys is the main cause of VFAs? It would match the tooth spacing on the belt.
This would be easy to separate from dynamic effects such as resonance by simply slowing down the print. There is a 68-page thread to be read...
Not sure whether there is a consensus... My favorite hypothesis is that imperfect meshing of the belt teeth with the pulleys is exciting a vibration, but that it takes a resonance (somewhere else) to amplify the vibration at certain speeds to produce pronounced VFAs. Oscillations of the gantry/bracket/belt system, as suggested by @huggypanda, seem like a very plausible candidate for such a resonance -- especially since they would cause a transverse motion of the nozzle, causing those "wavy walls".
Although, as mentioned earlier, I am wondering why we don't see more pronounced VFAs towards the sides of the printable area then, and less near the center. That's what I would expect from a wobbling/rotating gantry movement.
I wondered about that, too. I think what's happening is rotating the gantry causes uneven stress on the belts and that in turn causes uneven stress on the print head, making it wobble. For what it's worth, I *do* see a little bit worse behavior near the edges but it's very subtle at best.
RE: Gantry resonant mode and VFA artifacts
I do keep coming back to the VFAs being at the belt tooth pitch and not being 100% sure why. Reading the belt manufacturer's fine print, the pulley geometry might not be absolutely optimal for the tooth design. I know people have used different pulleys, but I've never heard of any revelation in that department.
I also think that it's uneven travel related to the tooth pitch which excites the VFA-related motion. But it probably takes some mechanical resonance to amplify it at certain speeds, or to translate it into a print head deflection orthogonal to the main travel direction, to make the VFAs noticeable.
As you say, results obtained by switching to different pulley specimens, different pulley brands, different belt types etc. seem to be hit or miss. I guess those toothed belts were never meant for precision positioning at the level we want in 3D printers (and with dynamic loads on top of that). They are probably operated outside of their specification in this respect. So changing components is always a bit of a gamble. Maybe one just has to play often enough to finally hit a lucky component combination...
Yeah I feel like if possible the best choice is always to not excite a resonance in the first place. I think that discussion's been done to death on another thread: consensus seems to be the excitation has /something/ to do with the belt, maybe the pulley, maybe the tolerances in the teeth, maybe the belt pitch. I absolutely am going to try every one of those recommendations, too, but it's a lot.
RE: Gantry resonant mode and VFA artifacts
One more thought/question: If the underlying mechanism is a torsional oscillation as (very plausibly) suggested by @huggypanda -- wouldn't one expect to find stronger VFAs towards the left and right side of the print bed, and weak or even no VFAs right in the center? But I don't think that is what we observe on the Core One.
Of course one could also envision a lateral oscillation instead (i.e. a parallel displacement of the whole gantry). I don't think any of huggypanda's experimental findings rule that out so far. Intuitively I would say that the torsional motion, with a lower resonant frequency, "feels more likely" -- but can it explain the constant VFA amplitude across the whole X range?
Any thoughts?
The flimsy L brackets form a torsional spring and the belts act like another set of springs. The resonant frequency should be related to those spring constants and the moment of inertia of the bar about that axis.
I agree, not ruled out. A small accelerometer probe is the best way to figure that out, but I don't have one at home.
RE: Gantry resonant mode and VFA artifacts
I am currently trying to design and print a complete printed part.
I am merging the curved mounting bracket, the deflection pulley mount, and the linear bearing housing into one part.
Of course, plastic cannot replace aluminum or steel, but I think PPA-CF, PPS, PEEK, or PCCF come close.