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Revision to Pressure advance

David Crocker

* The filament in the Bowden tube behaves as if it is compressible, because its diameter is typically 0.25mm smaller than the inside diameter of the tube. When it is under tension, it will take the most direct path that the tube allows. When it is under compression, it will snake from side to side, so the length of filament in the tube will be greater.
* The filament itself is slightly compressible.
* In order to produce torque, the angle of the rotor of a stepper motor must lag the angle commanded by the current in the coils. The more the filament resists being fed, the greater the lag angle. So the motor itself is slightly "springy".
* In order to produce torque, the angle of the rotor of a stepper motor must lag the angle commanded by the current in the coils. The more the filament resists being fed, the greater the lag angle. So the motor itself is slightly "springy".
 
These factors cause under-extrusion whenever the extrusion rate is increasing, for example at the start of a straight line when the nozzle has to accelerate from zero or near-zero speed. This is because some of the filament fed at the start of the move is used to counter the elasticity and build up the pressure. Similarly, you get over-extrusion when the rate of extrusion decreases, for example when the nozzle slows down at the end of a straight line. This is because the pressure in the Bowden tube continues to push filament through the nozzle even after the extruder drive slows down or stops.
[code]M572 D0 S0.1[/code]
 
The D parameter is the extruder drive number, and the S parameter is the amount of pressure advance you want for that extruder drive. A value of 0.1 is a good starting point for typical printers with a Bowden tube of modest length (e.g. 400mm). Non-Bowden extruders will normally require lessless (try 0.05), and extruders with long Bowden tubes are likely to need moremore (try 0.2). You may also need more pressure advance if you are printing with flexible filament.
The D parameter is the extruder drive number, and the S parameter is the amount of pressure advance you want for that extruder drive. A value of 0.1 is a good starting point for typical printers with a Bowden tube of modest length (e.g. 400mm). Non-Bowden extruders will normally require lessless (try 0.05), and extruders with long Bowden tubes are likely to need moremore (try 0.2). You may also need more pressure advance if you are printing with flexible filament.
 
== Side effects of enabling pressure advance ==

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