So, I’ve got a freshly built MPCNC Primo. I can move it up and down in the Z axis. I can also move it in the X axis cleanly. However, the Y axis will vibrate and shudder when I try to move it. I can move it fine by hand. I have taken a video showing what’s going on when I attempt to move it. I will drop a link here. How should I go about troubleshooting and resolving this problem so that I can start a Crown test?
I had a very similar issue. I did a complete rebuild of the gantry, trucks, and core, as the first time I built everything was pretty rushed and I only loosely followed the instructions. It was kind of a pain if I’m being honest, however I think spending an hour or two once is much better than having constant issues in the future. Really pay attention the the tightness of the bolts that hold the bearings in, the belt tension, and especially the 4 plastic pieces that hold the core to each gantry rail, those were the biggest issue for me.
From the video, it appears that one of the motors is not doing its job. It could be unpowered, or have a bad connection so that only one coil is working, or even be running backwards. If it is running backwards, just flip one of the connections.
Or the grub screw is loose. 
That was my first thought as well. So I unplugged and replugged the connections in as well as flipping one. There wasn’t really an improvement. I haven’t checked the grub screw yet.
Swap 2 motors, see if the problem follows the stepper.
Agree with the swapping, or looking closer at the stepper shaft to see if both are moving, only one, backwards, etc, while moving it like in the video. (Might need a second person for the assist)
Another option is to remove the two steppers from the machine, so they can be free of the machine, and rotate without the machine binding them, because…
There is (assuming you did extensions or modified wiring) a very small possibility that the wiring is off, and the “order” or pairs have been accidentally modified.
Aka: 3 things could be going on, both are powered and moving opposite directions (flip plug at the controller), one is powered and the other isn’t powered (one bad wire/connection), both are powered, one rotating relatively quiet, the other making a “grinding” like noise and not rotating. That would be the “order” of the wires wrong (and the least likely problem)
But removing them both from the machine, might make it easier to troubleshoot, however more time for disassembly and re-assembly once the correct problem is addressed.
So I have swapped two motors. Now, both axes work…and I’m not really sure why. I tried varying the belt tension as well as ensuring that my grub screws were tight with no success. I am now going to swap them back into their original positions and see if I’m back to square one or if this is a case closed scenario.
Edit: I have since swapped the motors back and it’s still working good!
Loose or incorrectly terminated wire connection, most likely.