docs: Update Pressure_Advance.md to use tuning tower

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
This commit is contained in:
Kevin O'Connor 2019-09-21 14:16:02 -04:00
parent b8a6160646
commit c794e11c67
3 changed files with 57 additions and 55 deletions

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@ -13,58 +13,64 @@ uses the second feature (reducing blobbing during cornering) as a
mechanism for tuning.
In order to calibrate pressure advance the printer must be configured
and operational. The tuning test involves printing objects and
inspecting the differences between objects. It is a good idea to read
this document in full prior to running the test.
and operational as the tuning test involves printing and inspecting a
test object. It is a good idea to read this document in full prior to
running the test.
Use a slicer to generate g-code for the large hollow square found in
[docs/prints/square.stl](prints/square.stl). Use a high speed (eg,
100mm/s) and a coarse layer height (the layer height should be around
75% of the nozzle diameter). It is fine to use a low infill (eg, 10%).
[docs/prints/square_tower.stl](prints/square_tower.stl). Use a high
speed (eg, 100mm/s), zero infill, and a coarse layer height (the layer
height should be around 75% of the nozzle diameter).
Prepare for the test by issuing the following G-Code commands:
`SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500` and
`SET_PRESSURE_ADVANCE ADVANCE_LOOKAHEAD_TIME=0`. These commands make
the nozzle travel slower through corners and they emphasize the
effects of extruder pressure.
Prepare for the test by issuing the following G-Code command:
```
SET_VELOCITY_LIMIT SQUARE_CORNER_VELOCITY=1 ACCEL=500
```
This command makes the nozzle travel slower through corners and to
emphasize the effects of extruder pressure. Then for printers with a
direct drive extruder run the command:
```
TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.005
```
For long bowden extruders use:
```
TUNING_TOWER COMMAND=SET_PRESSURE_ADVANCE PARAMETER=ADVANCE START=0 FACTOR=.020
```
Then print the object. When fully printed the test print looks like:
For the first print use a pressure advance of zero by running
`SET_PRESSURE_ADVANCE ADVANCE=0.000`. Then print at least 10 layers of
the test object. While the object is printing, make a note of which
direction the head is moving during external perimeters. What many
people see here is blobbing occurring at the corners - extra filament
at the corner in the direction the head travels followed by a possible
lack of filament on the side immediately after that corner:
![tuning_tower](img/tuning_tower.jpg)
![corner-blob](img/corner-blob.jpg)
The above TUNING_TOWER command instructs Klipper to alter the
pressure_advance setting on each layer of the print. Higher layers in
the print will have a larger pressure advance value set. Layers below
the ideal pressure_advance setting will have blobbing at the corners,
and layers above the ideal setting can lead to rounded corners and
poor extrusion leading up to the corner.
This blobbing is the result of pressure in the extruder being released
as a blob when the head slows down to corner.
One can cancel the print early if one observes that the corners are no
longer printing well (and thus one can avoid printing layers that are
known to be above the ideal pressure_advance value).
The next step is to increase pressure advance (start with
`SET_PRESSURE_ADVANCE ADVANCE=0.050`) and reprint the test object.
With pressure advance, the extruder will retract when the head slows
down, thus countering the pressure buildup and ideally eliminate the
blobbing.
Inspect the print and then use a digital calipers to find the height
that has the best quality corners. When in doubt, prefer a lower
height.
If a test run is done with a pressure advance setting that is too
high, one typically sees a dimple in the corner followed by possible
blobbing after the corner (too much filament is retracted during slow
down and then too much filament is extruded during the following speed
up after cornering):
![tune_pa](img/tune_pa.jpg)
![corner-dimple](img/corner-dimple.jpg)
The pressure_advance value can then be calculated as `pressure_advance
= <start> + <measured_height> * <factor>`. (For example, `0 + 12.90 *
.020` would be `.258`.)
The goal is to find the smallest pressure advance value that results
in good quality corners:
![corner-good](img/corner-good.jpg)
It is possible to choose custom settings for START and FACTOR if that
helps identify the best pressure advance setting. When doing this, be
sure to issue the TUNING_TOWER command at the start of each test
print.
Typical pressure advance values are between 0.050 and 1.000 (the high
end usually only with bowden extruders). If there is no significant
improvement after gradually increasing pressure advance to 1.000, then
pressure advance is unlikely to improve the quality of prints. Return
to a default configuration with pressure advance disabled.
improvement with a pressure advance up to 1.000, then pressure advance
is unlikely to improve the quality of prints. Return to a default
configuration with pressure advance disabled.
Although this tuning exercise directly improves the quality of
corners, it's worth remembering that a good pressure advance
@ -72,8 +78,8 @@ configuration also reduces ooze throughout the print.
At the completion of this test, update the extruder's pressure_advance
setting in the configuration file and issue a RESTART command. The
RESTART command will also return the acceleration, cornering speeds,
and look-ahead times to their normal values.
RESTART command will clear the test state and return the acceleration
and cornering speeds to their normal values.
Important Notes
===============
@ -91,20 +97,16 @@ Important Notes
[nozzle temperature](http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide#Nozzle_Temperature)
prior to tuning pressure advance.
* It is not unusual for one corner of the test print to be
consistently different than the other three corners. This typically
occurs when the slicer arranges to always change Z height at that
corner. If this occurs, then ignore that corner and tune pressure
advance using the other three corners.
* Check for warping at the corners during the test prints (the corners
detaching from the bed and rising a small distance upwards during
the print). If one corner appears warped then ignore that corner
when tuning. If significant warping is seen throughout the test then
typical solutions are to reduce the slicer's first layer speed,
adjust the bed temperature, and/or to use the slicer's brim feature.
Pressure advance itself is unlikely to impact warping, but this
tuning test is sensitive to it.
* It is common for the test print to show different behavior on each
corner. Often the slicer will arrange to change layers at one corner
which can result in that corner being significantly different from
the remaining three corners. If this occurs, then ignore that corner
and tune pressure advance using the other three corners. It is also
common for the remaining corners to vary slightly. (This can occur
due to small differences in how the printer's frame reacts to
cornering in certain directions.) Try to choose a value that works
well for all the remaining corners. If in doubt, prefer a lower
pressure advance value.
* If a high pressure advance value (eg, over 0.200) is used then one
may find that the extruder skips when returning to the printer's

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