Bearing and U bolt assembly
Wanted to ask others that have completed their builds of an MK3S. I'm in section 2 Step 27 - "Installing bearings on the Y carriage". So it's not 100% clear how tight the bearings and nuts should be after tightening "just until they're touching". At this point, how loose should the bearings be? Any give to them or should they not be moving? Is that last 90 deg turn even after they're nice and snug?
There is no "give" in the u-bolt mounting scheme. It needs to be just held in their seats and really no further. The u-bolts won't stretch. The bearings should not move, but don't need to be IMPOSSIBLE to move. Tightening further can only do one thing, deform the bearing housing.
How solidly do the bearings need to be attached? Not much. On one of my machines, the bearings are only held in place by thin beads of epoxy.
To re-iterate what guy-k2 said, err on the side of slightly wiggly bearings if you have any questions about how tight they should be. I initially over tightened mine. Once you figure it out, some amount of damage is done, even if you loosen them back up. It won't stop you from printing unless you really dented the housings, but it will mean that you will be doing a bearing and rod replacement a lot sooner than necessary.
The moment the bearing/u-bolt stops wiggling, stop tightening. No more turns, not even a quarter. All you really want is for the Y-axis frame to be stable. If there is no rotational play in it and no side to side movement then it's tight enough.
Oh, and clean and pack your bearings with grease first. You'll be happy you did.