Odd artifacts on perimeters
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Just a quick update: found that my x gantry screws had begun to come loose from vibration and threadlocked them down properly. This reduced the wobble, but its still there. I'm chalking this one up to mechanical issues for the time being and try to smooth out my printer motion.
Thanks for the help!
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There was a post recently about idlers wobbling on shouldered bolts. May be worth a few searches.
I had to use 5mm shaft with drive pulleys because they didn't do bearing idlers with 16t. Maybe I escaped a common gotcha there. The printed housing for the pulley just had two press fit bearings, and I used grub screwed collets to lock it all up.
Another option that I'm sure I saw mentioned on this forum was precision machined shouldered bolts that should offer a far closer fit to the bearing.
With close fits it maybe worth while keepimg a windscreen wiper puller handy to pull bearings and pulleys from close fit shafts!
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I am using machined bolts, and I've nearly got all of them fully supported. I think once the remainder have been fully supported this issue should disappear. Having 9mm belts significantly increases the axial torque the shoulder bolts handle, so that could also be a source of the issue.
And nice idea on that wiper puller!
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Bit late to the party, but I have always been concerned about the common use of threaded screws as bearing shafts on corexy machines. It's simply not the right way to do it, even if there are spacers pulled in tight against the center hub of the bearing.
Then you see sometimes a precision should bolt is used, but only supported on one side. Also not ideal.
Even using shoulder bolts supported on both sides, but mounted in a printed part may still give movement on a micro scale that will leave artifacts and patterns. Metal mounts will always be better, but I have heard that PC filament is a good option if printed hot enough.
I'm going to give this issue some serious engineering in my next printer.
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@Corexy I agree with those points you made, and after switching my plates to aluminum ones for all shoulder bolts, and adding the Jubilee delrin support plates to the top, its definitely decreased the issues I'd been facing and made the printer quieter.
But even then, I can't still explain the odd artifacts I'm seeing on the external perimeters (on the Y-axis photo of the pink part above) that just seem to always persist.
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@H2B said in Odd artifacts on perimeters:
@Corexy I agree with those points you made, and after switching my plates to aluminum ones for all shoulder bolts, and adding the Jubilee delrin support plates to the top, its definitely decreased the issues I'd been facing and made the printer quieter.
But even then, I can't still explain the odd artifacts I'm seeing on the external perimeters (on the Y-axis photo of the pink part above) that just seem to always persist.
Looks to be some good old fashioned ringing? Combined with some blobbing leaving those little bits and pieces everywhere.
Possibly you could revisit the mechanical bits, look for bearing smoothness and wheel tension. Then you should look at your extrusion steps/mm (feed rate), temperatures and almost certainly your jerk and acceleration settings.
3D printing is very fickle...one fault can be exacerbated down the line with other factors, and often disguised by them. In laymans terms, it's a total pain in the arse sometimes.
- Mechanical intergrity. Must be square, tight and rolling freely without binding.
- Feedrate and temp settings.
- Jerk/acceleration/retraction settings.
- Extrusion multiplier, filament density and things like that in the slicer.
That's what I'd be looking for, but I don't claim to be the guru so please take it as you will. Your mileage may vary.
Hope that helps and good luck.
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@H2B I'm glad this thread got re-ignited otherwise I would've missed it.
I'm seeing identical artefacts on my BLVMGN cube, slightly modified to be running 9mm Gates belts and a Hemera. I always put it down to using cheap toothed idlers (which I do plan on replacing) but it's interesting that you have the same with genuine Gates hardware. I have 30t toothed idlers and the BLV design means they are fully supported but they are on threads.
Again, they are more visible on slower/hotter areas with the glossy finish but you can't feel them.I'd be interested to see the result on a different tool to the Hemera, perhaps a bowden and see if it's any different. I like the idea of adding tool changing to mine but that's some ways off yet.
Just to add I'm also using Duet2 Wifi and this is with Moons motors.
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So here's a shot of a brim perimeter under harsh lighting that shows the ripples I'm talking about. They're uniformly the same at any speed or in any orientation which to me just points to the extruder. Their period is also on the order of magnitude of the direct drive gears which further backs this up. From what I've been reading this is due to the constrained filament path between the gears and the hotend and is inevitable, since there's nothing damping this movement as a Bowden tube would do. I'm tempted to run a Bowden Hemera test when they're back in stock and thoroughly test out this hypothesis.
In all cases its a great extruder and there are always tradeoffs with any design..
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What you are seeing are the physical products of the electro-mechanical properties of stepper motors.
Because steppers are synchronous induction motors, with square cut teeth, they will tend not to move smoothly. Instead tending to snap from half-step to half-step (because torque is a function of both phase vs rotor angle as well as distance from tooth center).
The combination of winding characteristics and rotor will produce an optimum speed where this is least pronounced (and the motion the most smooth).Using larger steppers (so you are using less of the available torque) helps a lot, and testing will help you find the optimum speed. But calculating this is something that even motor manufacturers don't do very often (they mostly just do physical tests).
The fact that the 4 axis nature of FDM can produce this effect in lots of ways makes it hard to nail it down, but its something I've worked on, off and on, for several years.
Your last photo might just be the pinchwheel... they are inherently not actually circular, so you see the variation in output as the filament moves from tooth to tooth (larger pinchwheels with resulting increases in number of teeth engaged would in theory help with that).There are lots of ways to try and damp it out, but its an inherent characteristic of stepper drive. Its part of the reason why the CNC guys (at least back in the day) would say that steppers are only accurate to a half-step.
On my printer at home, getting the speed down to about 15mm/s produces vastly better surface finish. And printing the outer perimeter at such low speeds is a trade-off I'm happy to make.
The only guaranteed cure is going to brushless servo drive. Or at least, that's probably the cure (closed loop steppers might be enough).
When I finally get around to putting the servo's I have into a printer we will see. -
That's a really interesting observation and I'm going to run a few tests this week to see if I can find that sweet spot. Multiple speeds and perhaps even changing the chop register on the TMC might yield some improvement. Thank you!
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Sorry to revive an old thread again but I have an additional theory on this.
Is it possible that these tiny waves on the surface of the extruded material are a result of the deformation of the filament due to the hobbed gear?
If the teeth of the extruder bite into the filament enough to deform it, then the volumetric flow of filament will pulse with the amplitude of the volume difference between a thin section and thick section.
I see this surface affect when using softer (more deformation) and glossier (more visible) filaments.
Might be nonsense but its just a thought.
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Sounds like it may be a valid scenario. Not a bad idea at all! I've learned to live with it
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It could be the resolution of the E axis itself is causing the pulsing.
Do some math. See what the correlation is between x/y mm and E mm. Ie find out how many e steps are generated per mm of toolpath travel.
My money is on those pulses lining up exactly with step pulses.
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So I took a sliced file and randomly chose 3 different points to calculate the steps per mm. The ratio is between 7.3 to 7.5 XY steps to E steps.. Don't think they're lining up too well in terms of pulses. Still have my doubts on what's causing this, as it seems to be the result of several things lining up to create it..
