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Revision to Pressure advance

Jason Znack

[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 less (try 0.05), and extruders with long Bowden tubes are likely to need more (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. If you have more than one extruder, you can specify the value for multiple extruders by seperating the values for D/S with a :
+
+[code]M572 D0:1 S0.3:0.1[/code]
+
+== How much pressure advance to use? ==
+
+The amount of pressure advance required will vary based on numerous factors that affect the elasticity of the extrusion system such as the distance between hotend and extruder gears, the type of material, the print speed and temperature, jerk and acceleration values. The predominant factor being the length of bowden tube between the extruder and hotend. A very long bowden tube (1m) may require S values approaching 1.0. Whereas a direct drive extruder like a Titan Aero has good results with a value of 0.05.
+
+A good starting point for tuning for PLA is as follows:
+
+* Direct Drive: S0.025 and up
+* Short Bowden (200mm): S0.1 and up
+* Medium Bowden (400mm): S0.3 and up
+* Long Bowden (600mm): S0.5 and up
+* Very long bowden (800mm+): S0.7 and up
+
+These are only starting points, your particular setup may require more or less. The only way to know for sure is an actual print test. Different materials may behave very differently.
+
+== Methods of finding the right amount of pressure advance ==
+
+The most simple method of tuning pressure advance is to slice a cylinder of 100mm diameter and at least 100mm tall, 1 bottom layer, 1 or 2 perimeters, and minimal or no infill. Ensure cooling is adequate for printing an object with very short layer times. Ensure the Z seam is aligned to a single point. Use a print speed close to your normal print speed. Ensure your extruder steps per mm has been calibrated. Now start the print with pressure advance disabled. After a few layers enable pressure advance by sending the M572 command with your desired starting amount in the gcode console. You may notice a brief pause in movement while the value changes. Continue the print for a few more layers. Try various amounts above or below the starting value and pay attention to the Z seam. Too little PA may result in a bulge at the end of the perimeter before the layer change. Too much PA may result in a gap at the seam.
+
+A more detailed test using a customizable scripted Gcode file is described here: https://forum.duet3d.com/topic/6698/pressure-advance-calibration
== Side effects of enabling pressure advance ==
If you use a large amount of pressure advance then the extruder may retract filament at the end of a move. If that move is followed by a travel move with retraction of the filament, then the total amount of retraction may be too great for your hot end. Therefore you should consider reducing retraction when you enable pressure advance.
-When the extruder acceleration changes, the extruder velocity has to change instantaneously. If the configured extruder jerk is too low to permit this, the extruder acceleration will be restricted so as to honour the maximum configured extruder jerk, and this may in turn limit the printing acceleration. So if enabling pressure advance slows down printing, this may be a sign that the allowed extruder jerk configured in the M566 command is too low.
+When the extruder acceleration changes, the extruder velocity has to change instantaneously. If the configured extruder jerk is too low to permit this, the extruder acceleration will be restricted so as to honour the maximum configured extruder jerk, and this may in turn limit the printing acceleration. So if enabling pressure advance slows down printing, this may be a sign that the allowed extruder jerk configured in the M566 command is too low. This may prove problematic for remote direct drive extruders like the Zesty Nimble which use a high gear ratio and low jerk values.
+
+You may notice that when using pressure advance there may be a gap between the perimeters and the solid infill lines. To combat this you may need to adjust the infill overlap value in the slicer.

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