How does the firmware handle max extruder speed relative to feed rate?



  • Say, I have an extruder that can only move at X mm/s, and I have set the max speed of the extruder in firmware to X mm/s, but I have commanded a feedrate and layer height/line width combo that commands a speed greater than the extruder can handle. Will the firmware slow the toolhead's movement to accommodate the X mm/s limit on the extruder? Will the feedrate continue as commanded, and the extruder will be extruding less filament than required, or delayed?


  • administrators

    The move will be slowed if necessary so that all axis and extruder speed limits are honoured. In practice extruder speed limits are normally much higher than required during printing, so they will not slow down printing.



  • @dc42:

    The move will be slowed if necessary so that all axis and extruder speed limits are honoured. In practice extruder speed limits are normally much higher than required during printing, so they will not slow down printing.

    Actually that got me thinking. When I was doing silly things messing about with print speeds, I discovered that the maximum I can go before I reach the point where my hot end can't melt filament fast enough and the extruders starts to chew the filament is around 120 to 130 mm/sec. So, one would think that this is the absolute maximum and that is that. However, I also discovered a bug/feature of Slic3R where, when it does small gap fill it does so by increasing the extruder speed by 50%. So, hypothetically speaking, one could print really fast at say 90mm/sec (not that I would normally want to) then run into this bug/feature which puts the extruder speed up to the equivalent of printing at 135 mm/sec. So, having the XY movement speed limited by the extruder speed might not be a bad idea (in extreme circumstances such as this illustration).

    I'm not clever enough to work out what the extruder speed would be at a print speed of say 120mm/sec using a 0.5mm nozzle but I'm sure some of you clever people could work that out. It might be a valid way of limiting print speeds to what is practically possible for a particular hot end? On the other hand, I guess putting a limit on maximum extruder speed is going to bugger up retraction which needs a fast speed.



  • Great to know how the firmware handles it, thanks.


  • administrators

    Whatever you limit your max extruder speed, acceleration to in the firmware will be the limit, no matter what your slicer asks the printer to do.

    Same is true of other axis so, on those with lead screw driven Z axis for example, if you do a combined move (for returning to a set point at the end of a print or on pause for example) that includes a long Z move, all the axis will be slowed down to the max speed the slowest axis allows. That means if you wanted to retract during that move as well (for example on pause) the retract would be slow, so worth retracting on a separate G1 command.


 

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