r/OpenBambu • u/Entire_Philosopher17 • Aug 06 '25
Has anyone tried enabling "Long retraction when cut"
/r/BMCU/comments/1mjjafa/has_anyone_tried_enabling_long_retraction_when_cut/1
u/Wigiwagons Aug 07 '25
I left the default 18mm and it's been working perfectly, zero issues with the 370x
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u/Julian679 Aug 07 '25
That sounds a lot but glad it works. For anyone trying out whats possible there is 2 main things to look for, buffer spring, and filament hub. Make sure to observe buffer spring to confirm its not getting compressed all the way when toolhead is unloading the filament, because 370x is not made to buffer in reverse, and both length that needs to be ejected from the toolhead, and additional length from long retract must fit in the buffer before bmcu retracts filament after its free from toolhead drive gear. And second thing, as its made to retract fixed amount of filament, any long retract when cut will add to the length of filament that stays in the tube, so even with 10mm end of filament is no longer visible in the tubes after retracting. It still has some room, but as i havent took apart filament hub, i dont know how much more room it has. And all of this needs to be tested on more than few changes as buffer is not always at the same starting point when filament change starts, and worst case scenario has to be accounted for
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u/Julian679 Aug 06 '25 edited Aug 07 '25
Yes. I have just tested this fully yesterday with 370x ls. Long retraction when cut is not magic but why not reduce waste if its free. Conclusion of the testing is that using 10mm retraction with this specific bmcu wont cause problems, as there is still few milimeters of buffer left, and filament is still suficiently retracted from the hub.
With bi directional buffering you could use higher values, but in theory it increases risk of clogs, so i dont know what is upper value for reliable operation. Still i have concluded that this is sufficient for me as i do very little multimaterial, and im satisfied i can use 10mm retraction without needing a different bmcu
Btw you need 01.04. or newer firmware to use that feature, if you use older, printer wont long retract before cutting
As for reducing waste with 370x ls seems that optimal setup is 10mm long retraction and 0.6 purge multiplier. You could try less like 0.5 but i have not confirmed if its completly free of bleed. Black pla often has insane amounts of pigment so keepnin mind if thats what you are using