148 lines
6.9 KiB
Markdown
148 lines
6.9 KiB
Markdown
This document provides information on tuning the "pressure advance"
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configuration variable for a particular nozzle and filament. The
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pressure advance feature can be helpful in reducing ooze. For more
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information on how pressure advance is implemented see the
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[kinematics](Kinematics.md) document.
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Tuning pressure advance
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=======================
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Pressure advance does two useful things - it reduces ooze during
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non-extrude moves and it reduces blobbing during cornering. This guide
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uses the second feature (reducing blobbing during cornering) as a
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mechanism for tuning.
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In order to calibrate pressure advance the printer must be configured
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and operational as the tuning test involves printing and inspecting a
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test object. It is a good idea to read this document in full prior to
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running the test.
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Use a slicer to generate g-code for the large hollow square found in
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[docs/prints/square_tower.stl](prints/square_tower.stl). Use a high
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speed (eg, 100mm/s), zero infill, and a coarse layer height (the layer
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height should be around 75% of the nozzle diameter). Make sure any
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"dynamic acceleration control" is disabled in the slicer.
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Prepare for the test by issuing the following G-Code command:
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```
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SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500
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```
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This command makes the nozzle travel slower through corners to
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emphasize the effects of extruder pressure. Then for printers with a
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direct drive extruder run the command:
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```
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TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005
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```
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For long bowden extruders use:
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```
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TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.020
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```
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Then print the object. When fully printed the test print looks like:
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The above TUNING_TOWER command instructs Klipper to alter the
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pressure_advance setting on each layer of the print. Higher layers in
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the print will have a larger pressure advance value set. Layers below
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the ideal pressure_advance setting will have blobbing at the corners,
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and layers above the ideal setting can lead to rounded corners and
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poor extrusion leading up to the corner.
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One can cancel the print early if one observes that the corners are no
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longer printing well (and thus one can avoid printing layers that are
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known to be above the ideal pressure_advance value).
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Inspect the print and then use a digital calipers to find the height
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that has the best quality corners. When in doubt, prefer a lower
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height.
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The pressure_advance value can then be calculated as `pressure_advance
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= <start> + <measured_height> * <factor>`. (For example, `0 + 12.90 *
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.020` would be `.258`.)
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It is possible to choose custom settings for START and FACTOR if that
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helps identify the best pressure advance setting. When doing this, be
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sure to issue the TUNING_TOWER command at the start of each test
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print.
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Typical pressure advance values are between 0.050 and 1.000 (the high
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end usually only with bowden extruders). If there is no significant
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improvement with a pressure advance up to 1.000, then pressure advance
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is unlikely to improve the quality of prints. Return to a default
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configuration with pressure advance disabled.
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Although this tuning exercise directly improves the quality of
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corners, it's worth remembering that a good pressure advance
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configuration also reduces ooze throughout the print.
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At the completion of this test, set
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`pressure_advance = <calculated_value>` in the `[extruder]` section of
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the configuration file and issue a RESTART command. The RESTART
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command will clear the test state and return the acceleration and
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cornering speeds to their normal values.
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Important Notes
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===============
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* The pressure advance value is dependent on the extruder, the nozzle,
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and the filament. It is common for filament from different
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manufactures or with different pigments to require significantly
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different pressure advance values. Therefore, one should calibrate
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pressure advance on each printer and with each spool of filament.
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* Printing temperature and extrusion rates can impact pressure
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advance. Be sure to tune the
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[extruder rotation_distance](Rotation_Distance.md#calibrating-rotation_distance-on-extruders)
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and
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[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
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prior to tuning pressure advance.
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* The test print is designed to run with a high extruder flow rate,
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but otherwise "normal" slicer settings. A high flow rate is obtained
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by using a high printing speed (eg, 100mm/s) and a coarse layer
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height (typically around 75% of the nozzle diameter). Other slicer
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settings should be similar to their defaults (eg, perimeters of 2 or
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3 lines, normal retraction amount). It can be useful to set the
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external perimeter speed to be the same speed as the rest of the
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print, but it is not a requirement.
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* It is common for the test print to show different behavior on each
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corner. Often the slicer will arrange to change layers at one corner
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which can result in that corner being significantly different from
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the remaining three corners. If this occurs, then ignore that corner
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and tune pressure advance using the other three corners. It is also
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common for the remaining corners to vary slightly. (This can occur
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due to small differences in how the printer's frame reacts to
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cornering in certain directions.) Try to choose a value that works
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well for all the remaining corners. If in doubt, prefer a lower
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pressure advance value.
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* If a high pressure advance value (eg, over 0.200) is used then one
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may find that the extruder skips when returning to the printer's
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normal acceleration. The pressure advance system accounts for
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pressure by pushing in extra filament during acceleration and
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retracting that filament during deceleration. With a high
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acceleration and high pressure advance the extruder may not have
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enough torque to push the required filament. If this occurs, either
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use a lower acceleration value or disable pressure advance.
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* Once pressure advance is tuned in Klipper, it may still be useful to
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configure a small retract value in the slicer (eg, 0.75mm) and to
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utilize the slicer's "wipe on retract option" if available. These
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slicer settings may help counteract ooze caused by filament cohesion
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(filament pulled out of the nozzle due to the stickiness of the
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plastic). It is recommended to disable the slicer's "z-lift on
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retract" option.
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* The pressure advance system does not change the timing or path of
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the toolhead. A print with pressure advance enabled will take the
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same amount of time as a print without pressure advance. Pressure
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advance also does not change the total amount of filament extruded
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during a print. Pressure advance results in extra extruder movement
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during move acceleration and deceleration. A very high pressure
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advance setting will result in a very large amount of extruder
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movement during acceleration and deceleration, and no configuration
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setting places a limit on the amount of that movement.
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