I just took my whole x/z axis apart because of that noise and actual binding of the z axis.
It looks to me like there is an issue with the alignment of the carriages on the z axis. Specifically, the hole centerlines from left to right of the xaxis rods. When you push on the z carriages at the back where the x rods are, the carriage will want to twist and bind up on the leadscrew. It looks like the plastic right by the leadscrewnut touches the leadscrew and causes the binding.
As far as I can tell, the construction of everything seems to influence this. Since nothing is square, I get good motion on part of the z stroke, but not others. I think their trick is to assemble the entire machine somewhat loose…
The telltale sign was pulling the xrods out of the left side plastic holder. The rods want to be about 1/2" further back than the hole they are being shoved into. When you shove it all together, that tweaks the zcarriages and will cause the binding on the leadscrew.
I have pulled the frame diagonally and tightened the bolts back down. I have loosened the screws and pushed the leadnut over in the plastic to give clearance to the leadscrew. I have loosened and moved both the xbearing and the xmotor mount as far to one side as I could (opposite ways). You can do this by removing the 4 bolts on the aluminum bracket to get to the 3rd screw hidden behind the frame (nice). This still does not get the left/right holes to be on center, but it helped a lot. I think the design is just wrong, with the holes in the wrong place.
Next, you will have to play with the set screws on the xshafts, and the tension on the x belt. I was tempted to leave one side loose, but the belt tension would not like that. Pry the carriages apart or together, then tighten the set screws and tension the belt. Then see what happens. It will bind, of course, so iterate several times until you get it the best you can. Make sure the left/right carriages are level, without one higher than the other (normal calibration step…).
I hope that helps. The real fix does not look too easy. Either move the location of the zxaxis assembly on one of the sides (which means new motor bracket and upper bearing bracket - which can be printed, minus the brass heat inserts…) or redesign the aluminum plate on one side to move the one of the xrod mounts. I guess you could slot the aluminum - but that means the bolt holes and the rod holes.
That was a good 4 hours, so be prepared…
Well, looking at it again, I see everything “Should” line up (per the design), but doesn’t in real life. So there must be an issue with the hole perpendicularity thru the xbearing block/x motor block. I could open them up, but I think that would be a bad idea. I think I may be able to SHIM one side to angle the rods back to centerline. I’ll give that a try next. That may be the way to go…
Anyone else having this issue???