Bizarre Underextrusion Issue Printer: SecKit Go w/ BMG/v6 and SKR v1.3/TMC2130 Drivers

Slicer: Simplify3d or Cura (problem is much worse in Cura?)

Long story short - after getting this printer running, I have noticed that it will underextrude ONLY on corners on an internal perimeter specifically once we're above bottom layers - and nowhere else. See below:



E-Steps have been calibrated, extrusion multiplier has been calibrated (single perimeters are 0.48&nbsp;mm), linear advance has been calibrated, but here it gets weird. Anything resembling a 'standard' value (suggested between 1-2) results in MASSIVE underextrusion after the speed change. I was able to get a good looking line using the linear advance test with a value of 0.15 (which seems very low).



Retraction (1&nbsp;mm @ 30&nbsp;mm/s), junction deviation (0.02) and all other settings are off the factory Simplify3d profile. Turning linear advance off makes the problem slightly better but doesn't fix it, going to a value of around 1 results in huge underextrusion after any change in direction.

I am absolutely lost as to what to try next.
From what I can find, the SecKit Go has a direct drive extruder, so 0.15 is a very reasonable linear advance K value. For comparison, I have 0.6 for PLA with a bowden. Values in the range 1-2 are way too high for direct drive except possibly for really soft TPU.

Geometrically localized underextrusion almost always means you have either


loss of material due to oozing elsewhere, or
filament slippage


usually due to whatever's being printed just before the part with the underextrusion. If it's oozing you may need to increase retraction or disable or limit combing in your slicer (to prevent unretracted moves; these can ooze a lot of material when crossing infill areas). If it's slippage it could be from trying to print too fast.