We have had multiple m-20ia robots with this issue. While in JGFRM or USER coordinate system and moving in the linear X, Y, or Z directions the wrist (J6) twists or rolls. This makes teaching linear moves extremely difficult.
One robot started this after getting a new wiring harness. Remastering fixed this one. Another started acting up after replacing the J6 harmonic drive. Remastering worked here as well. A week later this same robot needed a new J6 motor and another new harmonic drive. It started doing the "roll" again. Remastering did not work this time. The 3rd robot started this with no prompting. Again remastering did not fix the issue.
I have checked the frames on all 3 robots and all are still at 0,0,0. The core programs also appear to be correct on all 3 (I have compared the data from these robots to a couple others without the problem in the plant).
This is driving me nuts! Please help!