Clarify Home Before Filament Change comments

[thisiskeithb]

Requirements

HOME_BEFORE_FILAMENT_CHANGE

Description

I've had to correct users about HOME_BEFORE_FILAMENT_CHANGE many times because they assume it meant home before a filament change no matter what. This is not the case and will only home if homing is needed like when you disable steppers or first boot up your printer.

I debated renaming the feature for clarity, but updating the comment was less intrusive.

Benefits

Clearer intent of the feature.

Related Issues

None, but it originally lead me to look into the code behind HOME_BEFORE_FILAMENT_CHANGE. This lead to the fix in #17681, since the feature would only home a single time after bootup and never again, even if homing was needed.

[Tannoo]

I do believe the HOME_BEFORE_FILAMENT_CHANGE executes a G28 regardless if it is needed or not.

[thisiskeithb]

I do believe the HOME_BEFORE_FILAMENT_CHANGE executes a G28 regardless if it is needed or not.

This is precisely why I’m putting in the PR. 😀

It will only home all if XYZ positions are unknown:

Marlin/Marlin/src/gcode/feature/pause/M600.cpp

Lines 99 to 102 in 104d020

	#if ENABLED(HOME_BEFORE_FILAMENT_CHANGE)
	// Don't allow filament change without homing first
	if (!all_axes_known()) home_all_axes();
	#endif

[wikolii]

I do believe HOME_BEFORE_FILAMENT_CHANGE is still broken in the latest bugfix.
It triggers home Z. I've had that happen mid-print, causing a mess.
1st of all, it homes Z, if you're lucky no mid-printed parts crash
2nd, Z position was never recovered after filament change.

So why is Z homing before parking the toolhead?

I think i was the one who started this whole thing a few months ago i lost track now, but i can say that nothing has improved since. I was looking for a very simple feature: "No motion for filament change if positions are unknown, except for the extruder, of course"

Thanks

[thisiskeithb]

I do believe HOME_BEFORE_FILAMENT_CHANGE is still broken in the latest bugfix.

It won't home all unless your steppers time out (aka - not all axes are known):

Marlin/Marlin/src/gcode/feature/pause/M600.cpp

Lines 103 to 106 in abf1222

	#if ENABLED(HOME_BEFORE_FILAMENT_CHANGE)
	// If needed, home before parking for filament change
	if (!all_axes_known()) home_all_axes();
	#endif

I use this feature on my printers that are all on bugfix-2.0.x and the printer will not home all unless the steppers time out.

I was looking for a very simple feature: "No motion for filament change if positions are unknown, except for the extruder, of course"

Then disable HOME_BEFORE_FILAMENT_CHANGE.

[wikolii]

Disabling HOME_BEFORE_FILAMENT_CHANGE still causes crashing against the frame after steppers time out

But the previous behavior where Z would home during mid-print filament change is no longer happening

Thanks

[thisiskeithb]

Disabling HOME_BEFORE_FILAMENT_CHANGE still causes crashing against the frame after steppers time out

That’s to be expected because Marlin doesn’t know where your bed/nozzle is located before attempting to park. If you don’t want to enable HOME_BEFORE_FILAMENT_CHANGE And still want to prevent possible crashing, enable HOME_AFTER_DEACTIVATE to ensure Marlin is homed/knows all axis when your steppers timeout or are disabled.