Calculating the required acceleration setting

I am working with PETG and it is important to keep the velocity constant. If I slow down the print speed the filament flows out to fast and I get blobbing. It becomes evident in circles where acceleration limitations govern the speed of the print head. Long lines are gorgeous and circles do not come out well.

Acceleration - Velocity^2/radius of circle

If I am printing a circle with a 3 mm hole and 60mm/sec print speed then.

Acceleration minimum = (60 * 60) / 1.5 = 2400

This is way higher than what Robo has. I am going to modify Marlin and run test prints to see if this is valid.

I will report the results.

Success: Printing with Florida Filament PETG my 3.3 mm hole with about 2mm walls prints identically to the straight lines. I put an acceleration max of 5000 into marlin and am running 3000. I am printing at 55 mm/sec.

Strangely III3Dmax PETG does not print well. It does not bridge well and I am probably printing it at too high a temp.

Acceleration is in the Configuration.h file in Marlin. Marlin and the working code are in these links:

https://drive.google.com/file/d/1-9G3DqEJBj6Ij6g693GBRFivEWTn7be-/view?usp=sharing, https://drive.google.com/file/d/1VJbKnYBNBaj2MGi5N9iI_ZXe1eGTvC6w/view?usp=sharing

 

Isn't this something that would be corrected with Pressure Advance/Linear Advance tuning?

 

I have no idea what that is. I use Cura and have not seen that option.

Just looked it up and it looks like a good tool that I did not know existed. I am playing with Marlins tool now: https://marlinfw.org/tools/lin_advance/k-factor.html

This does look like it may compensate for the printer not being able to maintain speed on small circles.

Cura does not support automatic. This is what is in Robo's version of Marlin

* Example: `M900 W0.4 H0.2 D1.75`, where:
* - W is the extrusion width in mm
* - H is the layer height in mm
* - D is the filament diameter in mm
*
* Example: `M900 R0.0458` to set the ratio directly.

 

Yeah, I believe Marlin incorporated the feature a year or so after the last Robo Marlin version. I remember it being one of the major reasons I wanted to ditch the Robo Marlin for vanilla Marling or (even better) Klipper.

In a nutshell, it compensates for the inconsistent flow that happens at the the beginning and end of an extrusion. Ie Without it when the printer starts a line, it moves the filament forward a set amount, but then it takes a period of time for the nozzle pressure to increase enough to actually extrude filament leading to underextrusion at the beginning. Conversely at the end of the line the filament is stopped being pushed into the nozzle, but it still takes some time for the nozzle pressure to reduce to the point that it stops extruding, leading to over extrusion.

Linear Advance (Marlin) and Pressure Advance (Klipper) compensate for this by attempting to start/stop the filament extrusion a bit early. How early that happens is controlled by a value called k. K is a tuned value will be different for different materials (even between brands as well) and temperatures (ie a hot filament requires less pressure to extrude than cold filament)

I believe there is a plugin for Cura that adds LA/PA support.

Edit:
There are a lot of tools to help tun LA/PA, this is the one I most recently found and have been relying on the most of the last few months: https://ellis3dp.com/Pressure_Linear_Advance_Tool/

 

I am posting the last version of Marlin Code Robo had which includes the M900 linear. I am also posting the version of Marlin I use to compile without bugs . I am also including a Hex editor. I search out the Robo name for my machine and the do a search and replace with a new name using the same number of characters. This allows me to have a reference for my changes. As long as the number of characters are the same it is okay to edit the hex code.

https://drive.google.com/file/d/1-9G3DqEJBj6Ij6g693GBRFivEWTn7be-/view?usp=sharing, https://drive.google.com/file/d/1VJbKnYBNBaj2MGi5N9iI_ZXe1eGTvC6w/view?usp=sharing, https://drive.google.com/file/d/1o6Y_jwGPM3XeRI1E-sb3Y2swW8ra8D9U/view?usp=sharing