Once I had completed my MPCNC V2, 25mm version, F, (actually a rebuild of my first V1) I noticed the Z axis was off square by 1 to 2 deg. It was easily obvious to the eye. I have tried to adjust with the bolt bearing tightness but it has not cured it all. The instruction to just nip up the bolts was observed. I figure the X axis cross bar is setting too low, pulling the Z assembly down and producing the angular error.
I figure I could add a printed shim to raise the necessary cross bar on the rollers, or redesign and print new thicker spacer corners.
If you collectively tighten the top XYZ tension bolts and loosen the bottom ones that should move the Z stepper forward and the bit backwards. Or you can do the opposite. In the FAQ, I recommend taking the center apart and doing it again to get to the root of these kind of problems. Start loose, make sure the Z axis is good, then proceed. Last week there was someone with a twisted lower z mount, came to be a similar result.
I found when I first assembled the Z Axis, I had good contact on all the bearings and I was pleased with the way it had turned out. My first MPCNC V1 was true after adjusting the bolt tightness, but I found that some bearings were not contacting the tubes. It worked fine though for what I needed it for.
I have managed to adjust the z error, but now some bearings are not contacting.
But not to worry, as I had previously printed the 1" version for stainless steel, so I must put this one old one aside and will get on with the new improved (I think) one. I will pay careful attention, maybe assemble the z axis first and test for squareness as it develops.