Hey, sorry to bring up such an old thread.
I just had the time to try fixing this problem with the Y axis. As suggested I double checked that all pulleys were tight and then tried switching the X and Y steppers. The problem continued to be with the Y axis. I think it originated in the arduino because the stepper itself is actually turning the wrong way.
I'd be grateful for any ideas on how to fix this, I googled potentiometer but couldn't find anything on the board that looked like one. By the way I didn't see any black heatsinks on the stepper drivers.
What's strange is that sometimes it turns in the right direction and sometimes it doesn't, it also seems to get worse when the machine is on for longer but it's hard to verify that.
There's a direction pin that goes from the arduino to the stepper driver. There could be something wrong with that signal or with the stepper driver. I think it's pretty much impossible to be anything wrong with the stepper itself in this case.
But another possibility - is it at all possible that one of the pulleys is slipping - make sure the 6 (yes 6) set screws on the 6 pulleys are all super tight and mark each pulley and rod with a permanent marker to make sure they didn't slip. This is very very common on a newly assembled UM.
Back to the electronics. Could it be that this happens when something warms up? Consider swapping the X and Y stepper drivers. There are 4 of them for the 4 steppers. They each have a black heatsink. Pull them off very carefully and slowly without bending any pins - best to pry one side a little, then the other and back and forth. And reinsert very carefully. With power off of course.
If the problem moves to your X axis after that then you know for sure it is the stepper driver. You could try lowering the current. Try googleing how to do that - there is a tiny little potentiometer that you can turn to lower the current but make sure you find which model you have as some have higher current when clockwise and other stepper drivers are the other way.
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