Karsten the EUCWolfOkay, so the problem with my delta is apparently the OrangePi Zero 3 somehow getting overloaded doing delta kinematics math that the 100mhz printer board could do fine if I put Marlin on it.
It is strangely only happening with specific directions of "diagonal" movement. I know as a delta, all directions (except straight line Z axis moves) should be pretty much the same in terms of kinematics/processor load, which is part of why this was confounding me for like a full week.
Specifically, it'll print a cube aligned with the Cartesian coordinate axes perfectly, but if I rotate it more than about 15 degrees it either loses its ability to hold the effector's Z position, or it starts drawing curvy lines while maintining Z.
It has similar problems if i try to print a cylinder, but much less pronounced.
With the current prices of Rasberry Pis, I'm really hoping I can get my hands on a used 4B since my single surviving full size RPi (i have a drawer full of zeros, but just the one RPi 5) which is currently doing triple duty as a print server, file server, and home automation host.
Orb 2069