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Question background:

I am building an extruder / hotend combination to be used with 2.85 mm filaments.

The extruder I chose is QR Extruder from Bondtech which comes with a stepper motor, with an integrated planetary gear system and a reduction of 5.18 to 1.

The motor's specification states that it has a backlash of less than 1 degree.

I have contacted the manufacturer but I could not get a more specific number.

Here is what I'd like to know (i.e., my question clarified) :

  • How does this < 1 degree backlash translate to pushing filament through the hot end?
  • Once the extruder steps are calibrated, can it cause artifacts on the prints?
  • Can linear advance (Marlin) or pressure advance (Klipper/Duet) make up for this < 1 degree backlash of the planetary gear system?
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    $\begingroup$ What is the diameter of the extruder cog? You need to know that in order to translate 1 degree into linear error. Unfortunately using 2.85 mm filament means you need ridiculous levels of positional accuracy at the extruder to get accurate fine extrusion at the nozzle (because of the 2.85²:0.4² or whatever ratio), but backlash might not actually matter since the accuracy is really only needed in the forward (extruding) direction, not for back-and-forth movement (retraction). $\endgroup$
    – R.. GitHub STOP HELPING ICE
    Commented Oct 28, 2020 at 14:54
  • $\begingroup$ Thank you for your message. 2.85 mm Bondtech gears have an inner diameter of 8 mm and an outer diameter of 22 mm. $\endgroup$
    – Phil
    Commented Oct 28, 2020 at 14:58
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    $\begingroup$ I've been down that road and chose to go for a belt reduction instead. $\endgroup$
    – 0scar ♦
    Commented Oct 28, 2020 at 15:09
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    $\begingroup$ Yes I completely understand that @0scar, but I would still like to understand ins and outs of this thing. I appreciate your suggestiong though. Thank you. $\endgroup$
    – Phil
    Commented Oct 28, 2020 at 15:22
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    $\begingroup$ I'm with @0scar, belt reduction will give you less backlash error. There is no way to compensate in modern 3d printer firmware. You will need a backlash mitigation strategy to do otherwise (which might be outside what you are willing to do). $\endgroup$
    – user77232
    Commented Oct 28, 2020 at 15:34

1 Answer 1

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During normal extrusion backlash has no effect.

During retraction you can perfectly compensate by increasing retraction length slightly.

Backlash cannot be taken into account for pressure advance, but unless it's a lot, it should not cause issues: pressure compensation is a second order effect and does not need to be tuned super accurately to produce results.

One degree does not seem to be enough to cause problems.

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