Max Instantaneous Speed Change Relating to Extruder



  • I can understand the other settings such as max speed and acceleration. I guess what I'm confused on is the purpose of Max Instantaneous Speed Change as it relates to the Extruder.

    For an axis move I can understand there's a ramp up, constant speed, and a ramp down portion to the move. I'm having an issue visualizing the same for the extruder.

    I suppose the case I'd have to think about is a corner where the nozzle was at constant speed, had to slow down and then change direction and speed up again.

    I know at the end of it's move it would slow down to the value set for M566 "jerk", make the direction change and then speed up again.

    What does the extruder do in that case? I'd assume it need to do the same.


  • administrators

    @blt3dp said in Max Instantaneous Speed Change Relating to Extruder:

    I can understand the other settings such as max speed and acceleration. I guess what I'm confused on is the purpose of Max Instantaneous Speed Change as it relates to the Extruder.

    If you command a stepper motor to change speed instantaneously by too much, it will skip steps. That's why there is an extruder jerk limit. It's especially relevant when using highly geared extruders (e.g. remote direct drive extruders) or when using large amounts of pressure advance.



  • Would it be correct to say that it's the limit to which it will immediately jump to from either no movement or any movement under it's value?

    Say for example I have this set at 20 mm/sec, I request a move from standstill of 300mm distance at 120mm/s. It will immediately jump to 20mm/s and then accelerate to 120mm/s This is of course providing that limit of 20mm/s isn't too high and causes skipped steps?

    In terms of an already moving nozzle, lets say a 90 degree corner. At the end of a move it would decelerate to this value turn the corner and then ramp back up? Without this, it would essentially decelerate to zero and then accelerate again the other direction

    I think I get it in terms of nozzle movement, but extrusion I still don't. I guess what I'm looking for is a more scientific approach to tuning extruder acceleration and jerk values other than "just try it and tweak it until it looks good"



  • @blt3dp said in Max Instantaneous Speed Change Relating to Extruder:

    I know at the end of it's move it would slow down to the value set for M566 "jerk", make the direction change and then speed up again.

    What does the extruder do in that case? I'd assume it need to do the same.

    That is my understanding too. The way I think of it is that the extruder is just like another axis. So any print move will slow down to the lowest of X, Y or E.

    It is certainly that case that if extruder "jerk" is too low, then all print moves will slow down more, which would indicate that deceleration continues beyond the X and Y axis "jerk" settings until the extruder "jerk" speed is reached.

    What I'm not sure about is what happens with extruder only moves - retract and un-retract. If these are treated as moves from standstill, then "jerk" will not be applied. In which case, one might as well set extruder "jerk"really high so that it cannot affect print speed. If on the other hand, retract and unretract moves are treated as changes of direction, then "jerk" would be applied at the start and end of those moves, in which case setting it too high could be detrimental. I would imagine that retract and unretract are treated as moves from standstill because that is often what happens. i.e. retract, move the print head while the extruder is stationary, then un-retract. In which case, extruder "jerk" will only ever be applied during print moves and then only if it is set lower than the equivalent "jerk" speed of the print head, so we could set it silly high and be done with it (I think, but I stand to be corrected).



  • I think you've clarified most of my question, but the questions remain as far as retract and un-retract.


  • administrators

    Jerk is not applied to retraction and un-retraction moves. The cases where extruder jerk is needed are:

    1. When the GCode generated by the slicer commands a change in extrusion rate between two extruding moves.

    2. When using pressure advance. Extruder jerk has to be applied at the start and end of each acceleration segment.



  • In that case, are we saying to set extruder jerk high as to not affect print speed then?



  • @blt3dp said in Max Instantaneous Speed Change Relating to Extruder:

    In that case, are we saying to set extruder jerk high as to not affect print speed then?

    All I can say is, that's what I do. On my machine, the carriages (X and Y) are set to 1200 mm/min and the extruders are set to 3600. I use 0.5 pressure advance and it works well. Oh, and I can print at up to 300mm/sec. https://somei3deas.wordpress.com/2018/10/14/real-3d-printing-at-high-speeds-and-even-higher-melt-rates-with-a-large-nozzle/



  • @deckingman said in Max Instantaneous Speed Change Relating to Extruder:

    All I can say is, that's what I do.

    Same here.


  • administrators

    @blt3dp said in Max Instantaneous Speed Change Relating to Extruder:

    In that case, are we saying to set extruder jerk high as to not affect print speed then?

    Yes, as long as the extruder doesn't skip steps.



  • @deckingman said in Max Instantaneous Speed Change Relating to Extruder:

    Oh, and I can print at up to 300mm/sec. https://somei3deas.wordpress.com/2018/10/14/real-3d-printing-at-high-speeds-and-even-higher-melt-rates-with-a-large-nozzle/

    I did see that article previously, that you were using Bondtech BMGs. If I recall, you were using E3D Titans at one point. Similar results as far as speed and settings?



  • @blt3dp said in Max Instantaneous Speed Change Relating to Extruder:

    I did see that article previously, that you were using Bondtech BMGs. If I recall, you were using E3D Titans at one point. Similar results as far as speed and settings?

    Yes. Both use the same gearing so the steps per mm are almost the identical. In my particular usage case with mixing hot ends, where I often retract and un-retract the same piece of filament, the Bondtech is less prone to grinding down the filament.


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