Y axis off 1/8 of an inch sometimes

Sometimes like today I start my Shapeoko 4 XL with a bit setter and it will cut off 1/8 of an inch to the left. My bit setter was set perfectly in the center of the button before this happen. I noticed that the bit would be off to the right and sometime in the center. I reset the bit setter again to the dead center to the button to see if that would solve the problem. Again, if I turn on the machine to load a bit it will center on the button and sometimes to the right as before. The weird thing is that when it is to the right it cuts exactly to the software measurement for the test sample which is 5/8 from the left of the board. If the bit hit the button right on center, it cuts to the left 1/8 and measures 1/2". The photo is a test board with 4" lines to show the difference with the bit in the center and to the right on the bit setter.

Check the set screws on the pulley(s) for that axis. If they are slightly loose, then the motor can move without moving the carriage.

Two screws, both tight, one over the flat of the motor shaft. You could start with a witness mark and see if anything is moving:


Thank you, I will check that out and all of the other set screws too. This month is the one-year anniversary for my XL and loving every minute it runs. It only happens when it loses connection then turning it on to start-up again. I noticed that the home position changes on the y and x to different locations for some reason. Could be like you said the pulley. I checked the belt tension and seemed to be fine.

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