Can 'pressure advance' cause negative filament feed ?
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@phaedrux said in Can 'pressure advance' cause negative filament feed ?:
@jens55 said in Can 'pressure advance' cause negative filament feed ?:
Do I need to do anything besides selecting relative extrusion in Cura ? Do I need to do anything in the Duet setup ?
Nope. All that happens is that Cura inserts M83 into the gcode. There's probably already an M83 in your config.g since it's the default for the Duet, but Cura still uses absolute by default so it inserts M82.
Ok, done and yes, my config.g already had an M83 in it.
Good luck. I'm interested to see if that helps at all.
Oh and I forgot to mention to disable wipe when using PA.
Wipe has already been set to 0
I'd also give Prusa Slicer a try if you have it up and running. Sometimes the way cura behaves for very small features can be rather problematic. Sometimes the thin wall detection in Prusa Slicer is much better, or at least you can disable it. You're already using a 0.2mm nozzle so that should help.
I have never tried Prusa Slicer ... not sure I want to learn yet another software package
Net outcome: Printer still doesn't print the teeth with pressure advance turned on, printer had issues printing the teeth with pressure advance turned off (not as much as with pressure advance on but more than before)
Deckingman, thanks for confirming the maximum feed rate at 256 microstepping - I was nowhere near there.
I was thinking about a geared extruder while still staying with a bowden setup but I believe that with the overall inaccuracy in a bowden setup, a geared extruder will probably not help me much (Comment ?)Since printing tiny stuff is not something I plan to do on a frequent basis (maybe my next printer ?), I will make do with what I have.
I also have many other issues I can use if I want to bash my head against a wall and feel the joy of pain
Thanks everybody for helping me understand what is going on here !!!! I learned a fair bit with all that messing about !
I have also discovered that if I print the model tilted at a 45 degree angle, I can get noticeably nicer teeth.
So, does anybody think that a 0.1 mm nozzle has any chance of working with a long bowden tube ?
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@jens55 This is just a theory but it might explain the symptoms you are seeing. Firstly, it might be best to start with an explanation of how pressure advance works just in case you aren't fully familiar with the mechanics involved. If you have ever used a sealant gun such as this, https://www.screwfix.com/p/p-c-cox-easiflow-gun/79714 then you would understand what I'm about to say.
It takes time to melt plastic and push it through a small hole. The molten plastic itself acts a bit like a sponge so when you start to push, you get a build up of pressure before anything much starts to come out of the nozzle. When you stop pushing, that pressure continues to force filament out of the nozzle as it slowly decays. So, at the start of a move, the print head accelerates up to speed but flow for the filament does not accelerate at the same rate. So you can get under extrusion at the start of a move. Conversely, at the end of a move the print head starts to decelerate but because of the pressure that has built up, the flow of filament out of the nozzle does not decelerate at the same rate. So you get over extrusion at the end of a move. Pressure advance works by accelerating the extruder more at the start of move to compensate for the "lag", and decelerating the extruder more at the end of a move to compensate for the pressure build up. For any given move, the extrusion amount should be the same as if no pressure advance had been applied.
That is a simplistic explanation which applies to a single, relatively long move. But where there are a series of very small segments, such as an arc or circle, each individual segment is too short for pressure to build up but because instantaneous speed change is applied between each small segment (so there is limited deceleration and acceleration) , then there will be a cumulative build up of pressure after a number of these small segments. So pressure advance has to operate on the principle that a series of small segments need to be treated as one longer move rather than trying to apply compensation to each individual move.
What might be happening (emphasis on "might") in your situation is that there could be some extruder slippage. This might be because the motor cannot cope with the sudden increase in extruder flow rate that pressure advance applies to the start of a move (or series of small segments). This is more likely to happen with a smaller nozzle because there is more restriction to flow. Also, an ungeared extruder has less torque available to push the filament than a geared extruder. So your small nozzle combined with an ungeared extruder might be the "perfect storm". Another possible reason could be the hobbed bolt grinding away the filament rather than pushing it.
So if the extruder accelerates faster than "normal" as required by pressure advance, but because of slippage this does not translate into actual filament extrusion then that might explain the problem that you are seeing.
If that is correct, then a larger nozzle would help rather than a smaller one because a larger nozzle causes less of a restriction than a smaller one. This also brings me back to the case for using a geared extruder which will have more torque.
Another thing to check would be that the Bowden tube is tight with no movement whatsoever in the couplings.
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As well as the spring effect of filament and motor there is also backlash if using gearing or a Bowden tube that has to be slightly bigger diameter than the filament. @dc42 does the pressure advance separate these two effects and compensate for them independently? I.e. to get over backlash you would want a very fast movement and then a slower accelerating speed once the filament is engaged.
