The problem of the Bambulab printer is that it is so cute that it is hard to dare to disassemble it 
Anyway are you 100% sure it will resolve the problem ? would you be ready to bet 1000$ on this ?
Well done MadGoat! Good warning on your printables page as well.
I was hoping this would be the golden ticket. The logic was sound.
Now what?
Since the thread is a bit long, I didn’t read it all, so excuse me if it already was suggested or tried.
You could try to manually change the belt tension, increasing them a bit by running the retensioning cycle and adding some manual push against the tensioner black backplate.
It might change the frequency range at which it occures or lessen the effect at least, but if you overdo it, it might also reduce the lifetime of the pulleys a bit.
Other then that it might be that some part of your printer is concentric and not turning perfectly round.
I’m fairly certain I have proven it is not the “Smooth Idler Pulley” situation. I’m sure Bambu had tested that along with many other things in the gantry before release. At least allowing the disbelievers the ability to see and test for themselves. Moving forward, I think we should turn our attention to the motor resonance of these machines as the ridged nature of the machine could exacerbate any motor vibrations that would then in turn manifest in a print. This would also explain the seemingly random nature of this problem since it would simply come down to luck of the draw with the sourced stepper motors used in manufacturing of these machines.
Hmm, interesting tooth shape… May want to actually match manufacturer specs.
Did you print those pulleys or are they just purple?
I haven’t had a chance yet to swap out my toothed belts for the non cogged ones I have but one thing I did notice is if you watch the belt position on the pulleys they will move up and down depending on where the xy axis is
I’m starting to wonder if the belts are contacting the shoulders on the pulleys and causing motor or belt resonance which may explain the somewhat random nature of the artifacts and why it is more prominent on larger prints 
Since we have devolved in our forum technology as to not be able to multi quote anymore:
Gza32: Feel free to modify the tooth shape and or the pulley itself and test further. However, in my testing of even this admittedly quick and crude prototype there was no measurable change in either direction. Literally what can be considered the same result on multiple prints.
Jrock: Yes these are printed pulley’s (Link to the printables project in the very same post) I would have to further disagree with your possible assessment. During normal operation the geared part of the GT2 belt has plenty of room to ride and does so in the middle of the idler pulley with plenty of room before contact with the flanged side.
I’m not convinced the belt system in these machines have any appreciable change on the print quality. and (again) instead I would be interest in the steppers themselves. I would imagine a firmware update could smooth the motor steps and frequency at X speed. Furthermore, this could be applied dynamically throughout a print as the motor steps through the gcode. Whether these machines can be adapted in the manner only Bambu knows, but at the very least a combination of firmware and software (slicer , Bambu Studio / Orca) could (and I’m positive will) smooth out the VFA we are seeing in the future.
Copy that
Appreciate your insight and the degree of work you’ve done so we can disregard the belts/pulleys
Interesting facebook post that showed sigificant VFA after following the recommended belt tensioning process. Not sure if this link to a facebook post will work though:
(1) Bambu Lab Official User Group | Facebook
Post is by Patrick Cruse from 12.24pm yesterday (4/9).
Fix was to perform a modified re-tension to ensure things were kept square. The difference was significant and could explain why we see such variability in VFA even after people have performed the recommended belt tensioning.
I can’t seem to get that page to open on my current browser (probably because I haven’t used facebook in years), I’ll try again later but it might be helpful if you could possibly copy/paste the “modified re-tension procedure” here so people can try it and report back on it here too?
I don’t currently have the issue, but it’s one of a couple of issues I’m keeping an eye on cos they seem common enough that I’ll have to deal with them eventually. Hoping there are solutions in place by that point!
Sorry, that’s exactly what I meant - I may have done noninvasive modifications to a machine and it made zero change to the ringing at all. It was something very similar as MadGoat did. As I said, the smooth idler is a red herring.
I’m throwing in some of my own opinions here, but having looked at a lot of the design decisions inside the printer is designed, the Bambu labs team is quite…capable…and I really doubt the problem would be as simple as a single idler change or they would have announced it long ago.
It really looks like some much more subtle resonance/motor control issue.
While running the VFA tests at low speed, you can absolutely hear that something is going on. Again, they’re using a more advanced digital motor control scheme and a surprising amount of the driver control is actually firmware upgradeable, unlike most of the hobby drivers (TMCxxxx)
By the way, best workaround for anyone who is unhappy with this artifact and urgently needs to print better looking parts on the outer wall - slice and make sure your outer shells are printed at high speed. If you need better z-adhesion you can rely on slower printed inner perimeters. If you’re feeling lucky you can disable slowing down for fast layers - the safer strategy is to add a dummy object (I think pausing can cause dribbling issues, which is why it’s not typically done) and make sure the layer times are beyond minimum.
His details about what he did were spread across
many comment responses. That’s why I was hoping the link would work. You can find his post if you search that group though. Although it may be a closed group now due to spam.
Yeh the group is private now, so I can’t get in there or I would copy out all the relevant comments and paste them here.
This is a copy/paste from a Facebook thread by someone else:
“ I have been running my X1-C fairly hard since new so I decided to give it a good birthday. I may have inadvertently found a reason behind the prominent VFA’s that some people are experiencing. The right part is after following BL wiki guidelines, the left print is after I worked through what could have changed since before carrying out maintenance. I am running a few more tests and working on a idea for a jig that could make adjustments simpler for the majority of users. I will release more details when I have that sorted.”
“Brian Dodds I follow the Bl instructions then check the X gantry for square to the Y guide rods. I then loosened one adjuster and added a little more preload until the gantry was square to the guide rods. It didn’t take much.”
“Just a brief walkthrough, I followed the Bl instructions then checked the X gantry for square to the Y guide rods, it was off by 2-3mm. I then loosened one adjuster and added a little more preload until the gantry was square to the guide rods. It didn’t take much.
The belts must be slightly different lengths or haven’t stretched equally because the belt adjusters are in slightly different positions, I would assume this would give different load from the springs which could be causing the issue. If you want to easily see the result of belt tension on the gantry, gently pull or push one belt and you will see one end of the X gantry move.”
Interesting. Would explain why a lot of people have good prints, while some have bad, if those people have ‘followed the instructions’ and ended up making their belts assymettrical or something.
Will be nice if/when someone comes up with an easy solution to belt tightening which works perfectly every time.
I did the Bambu recommended belt retensioning and it didn’t obviously make my VFA noticeably better or worse. But i predominantly print with silk so speeding up isn’t an option if I want to try to maintain shiny. I tried printed extra hot and that just made the shine reach to higher speeds and still showed VFA. I haven’t tried the suggestion from that post yet.
Interesting catch. Never occurred to me. I’ll take a look at mine (along with a couple other items on other threads I meant to catch up on).
Not so concerned with tensioner distance as squareness of the head and if it stays once set.
Belt circumference being identical is a ■■■■ shoot. You would have to get sister belts cut from the same mother, which unless stamped with lot number is luck of the draw.
Please keep us posted. Especially if you do design some sort of jig. The more tools in the box the better😀