Marlin: [FR] Documentation on how to tune Junction Deviation

[Itox001]

Hello! Just updated to the new 2.0 bugfix and so far looking good, movement seems smoother with s curve acceleration, adaptive step smoothing and all the misc fixes and improvements done to the planner!

One of the big features I wanted to try is junction deviation, but as soon as I was going to enable I noticed that I don't really know how to tune it. I've actually read through all discussions involving it here on the Marlin's git but practically I'm still not sure how to go about it.

From what I understood this are the key points:

*Max acceleration is used in the calculation (so default acceleration no longer has any effect?)
*The junction deviation value relates this Accel to the effects old jerk had

But how do I start? I guess I could use the defaults and go from there, but I'm not sure if I should lower Junction deviation value or acceleration if I find too much ringing, or even a starting point for my Max Accel. For example, I'm trying this on a tevo tornado, I use 500 printing Accel and 1000 travel Accel, 7 jerk for x and y. What values would be sensible to start with? How do I tune travel and printing Accel if there is only one single max Accel? I saw the formula relating max Accel and junction deviation value to old jerk, so I could do that to get a starting point with the default 0.02 junction deviation, but how do I go from there?

Edit: This is the most relevant comment for tuning this parameter I think: https://github.com/MarlinFirmware/Marlin/issues/9917#issuecomment-401517930
But after reading all I still can't help but feel like this guy: https://github.com/MarlinFirmware/Marlin/issues/9917#issuecomment-401703840

Thanks for the awesome work Marlin team, you're doing the world a great service!

[mylife4aiurr]

I would always appreciate a how to tune junction deviation.... i leave it @ default because idk how to tune

[marcio-ao]

I second this request :)

[boelle]

i dont know where i stand on this one, but i have not figured how to tune it either

[ArtemKuchin]

I kinda understand how to tune it. Basically there are two parameters: acceleration and deviation. The higher the acceleration the higher will be speed at corners, also, the higher the deviation also, the higher speed at corners. Deviation applies only for corners, acceleration for everything. So, set needed acceleration, print, check your corners, if okay then okay, if not, reduce either acceleration or deviation.

However, what i do not understand now is how it behaves on just straight lines.
Let the axis rest.

How it was before.
ACC=1000; jerk=10. I say G1 X100 F6000
Right after that ut start with jerk speed of 10 mm/sec and the accelerates to set speed using ACC=1000mm/s*s

How does it work now? It start to accelerate to set speed using acc=1000 right from 0mm/s or
from MINIMUM_PLANNER_SPEED ?

[joaomamede]

Still no clear guide? :)
There a calibration tower in thingiverse by the way

[The-slunk]

According to This guide its measured by .4 x jerk x jerk /printing acceleration so for @Itox001 it would be .4 x 7 x 7/500 to give you a JD value of .04

Edit: formatting

[ArtemKuchin]

Well, the question still remains:
How does it work now for streight lines when starting movement from rest.
Does it start to accelerate to set speed using acc=1000 right from 0mm/s or
from MINIMUM_PLANNER_SPEED ?