Help me understand the Acceleration settings

[tsteever]

There are two different spots in Marlin that I see for acceleration.

#define DEFAULT_MAX_ACCELERATION
#define DEFAULT_ACCELERATION

What is the difference? What does each do/control?

I'd like to have my Y a little slower since it is a bit more massive than the X axis.

[printbus]

That starts to be obvious by comparing the parameters for the two. DEFAULT_MAX_ACCELERATION has parameters for each axis (X, Y, Z, extruder). DEFAULT_ACCELERATION only has a single term.

For a given axis, the acceleration value Marlin will apply is whatever is the lowest value between the two settings. DEFAULT_ACCELERATION will have no effect unless its value is less than one of the axis settings in DEFAULT_MAX_ACCELERATION.

Why are there two? IDK, but note that there are two gcode commands related to this. M201 can be used to set the four terms associated with DEFAULT_MAX_ACCELERATION. M204 can be used to set the value associated with DEFAULT_ACCELERATION. I could see DEFAULT_ACCELERATION being used more on a delta printer where there'd be less of a reason for x, y, and z acceleration values to differ. In your case, you'd set the Y value in DEFAULT_MAX_ACCELERATION to something lower than the X value.

[tsteever]

So it will always default to the lowest value? So if my DEFAULT_ACCELERATION is less than whatever I put in for the four value for the DEFAULT_MAX_ACCELERATION it will use the lowest one?

[printbus]

So it will always default to the lowest value? So if my DEFAULT_ACCELERATION is less than whatever I put in for the four value for the DEFAULT_MAX_ACCELERATION it will use the lowest one?

Yep. If you want to experiment, I'll offer the gcode snippets in the LINEAR CONCENTRIC SQUARES TEST post in the Marlin Motion Related Configuration.h Settings for MakerFarm i3v thread. They provide non-extrusion movements that can be used to observe the printer over a range of distances at both printing and movement speeds. I provide gcode there for both the 2014-era MakerFarm defaults and the tweaked values I settled on via testing. Use a text editor or an integrated gcode editor like that in Repetier Host to tailor values in the gcode commands as you wish.

I found the concentric squares approach very instrumental in learning how different acceleration settings affects the movement speeds achieved over various distances.