Layer shifts - help me figure out the root cause
I have some layer shift issues on my LDO 2.4 that I cannot figure out. This is a "but it worked before!" situation, the shifts occurred suddenly. They happen in both A and B motor direction, so in the diagonals (I have not observed shifts in x or y direction).
So far I went through the mechanics quite extensively without disassembling the gantry (I think that my question is essentially if I should do that now). Belt paths are clear. No belt rubbing anywhere (the belts do have some discoloration in one location, both belts in the same position, must be an idler at the point where the x and y axes meet, but there is no dust or debris anywhere and the belts are not worn down).
When moving the toolhead by hand, I feel a slight "rattling" in both x and y axis. I think that this is in the mgn carriages, but I also remember that during building they were never 100% smooth, so for the moment I discard this problem. When moving the x axis back and forth, the toolhead sometimes does move a bit left and right, but this has been ever since upgrading to Stealthburner and Tap and went without issues for a long time.
I currently think that this might be a deeper gantry issue or two faulty motors at the same time. Before disassembling the gantry, I would like to ask the crowd if there is anything that I can do to narrow this further down. I measured resistance across the motor coils, all according to spec. I ran the Ellis speed test macro a few times with different speeds and accelerations and had skipped steps at all reasonable speeds (with before and after stepper step location):
Run current: 1A
TEST_SPEED SPEED=500 ACCEL=2500 ITERATIONS=5 BOUND=50
Max velocity: 500mm/s
stepper_x:98577 stepper_y:-19439
stepper_x:99800 stepper_y:12560 (bumped into the gantry)
Run current: 1.2A
TEST_SPEED SPEED=500 ACCEL=2500 ITERATIONS=5 BOUND=50
stepper_x:582 stepper_y:2
stepper_x:1458 stepper_y:1792 (bumped into the gantry)
TEST_SPEED SPEED=300 ACCEL=2500 ITERATIONS=5 BOUND=50
stepper_x:1554 stepper_y:1696
stepper_x:1572 stepper_y:1696 (seems ok at this speed)
TEST_SPEED SPEED=400 ACCEL=2500 ITERATIONS=5 BOUND=50
stepper_x:1990 stepper_y:1666
stepper_x:2353 stepper_y:1795
TEST_SPEED SPEED=300 ACCEL=3500 ITERATIONS=5 BOUND=50
stepper_x:2478 stepper_y:1666
stepper_x:2794 stepper_y:1888
Run current: 1.4A
TEST_SPEED SPEED=300 ACCEL=3500 ITERATIONS=5 BOUND=50
stepper_x:676 stepper_y:34
stepper_x:702 stepper_y:34
TEST_SPEED SPEED=400 ACCEL=3500 ITERATIONS=5 BOUND=50
stepper_x:1091 stepper_y:33
stepper_x:1056 stepper_y:34
TEST_SPEED SPEED=500 ACCEL=3500 ITERATIONS=5 BOUND=50
stepper_x:1438 stepper_y:96
stepper_x:1472 stepper_y:-12382 (strong skipping visible, sometimes stayed in one place with audible motors)
The motors are the stock LDO steppers, rated at 2.0A. Is there anything visible from these measurements? Should I do more measurements? Anything else that I can do to narrow this down before disassembling the gantry?