I should know this, but I never quite understood the proper tightness of the tension bolts A, B, and C. I had them pretty loose, simply looking by eye for the plastic parts to be touching, but it seemed like it was still too loose.
I observed when I pull back and forth on the Z axis there was a very slight ‘tick’ noise as the bearings touched and lifted off the rails. At the same time I found some bearings could easily turn by finger since they were not touching the rails.
So I started tightening bolts until all the bearings were touching. For the bearings in the XYZ part that hold the Z axis, I tightened tension bolts C just until they touched the Z rails. For the bearings on the gantry rails near the front of the center assembly, I tightened tension bolts A until they were all touching. And then for the ones in the back I tightened tension bolt B.
Now all bearings are touching their respective rails and there is no noise when I push the Z axis back and forth. I suppose it feels stiffer but I don’t have a measurement and I don’t trust my memory.
Is this a good way to know if the bolts are tight enough? I am not concerned about squareness because it is close enough and I have a method to easily force it square. So I don’t need it to be square when the motors are relaxed.
Is there anything else I should be watching out for? Is it possible (or likely) for it to be too tight even when the bearings are not touching the rails?
Ryan has thought about this more. But here are my thoughts on it. Maybe it will spark some new thoughts about it.
It is over constrained (in a geometry sense). Each triplet of bearing makes a point. Two points make a line, but the two Z rails have a relationship (they are bolted together, close to but not exactly parallel), so they are over constrained. The X and Y rails also have a relationship (they are held by the steppers), so they are over constrained.
So if you had perfectly rigid material, and some errors in the shape, there would always be some bearings that don’t touch. It is easiest to think about the Z axis. If the tubes were not parallel, but the center piece was square, some of the bearings couldn’t touch.
Also at play is the fact that the pipes aren’t perfect lines, so they will change diameter and surface texture will vary.
Given all of that. It would be easy to tighten too much. The test is, can you still move it freely (in the way it is supposed to travel)? If a bearing isn’t touching, but it isn’t loose. That is ok too. If you have all the bearings touching, and it moves smoothly, then your Z and gantry are printed pretty square, which is great, and it is probably a good place to stop. If you tighten it a little more, then there will be a bit of spring to account for irregularities in the tubing. As long as it doesn’t start to bind.
Besides binding, tightening can bring it out of square, and also make the tool not perpendicular to the xy plane. You said you don’t have a problem with that, but others might.
Is that method something else than just pushing the gantry against the frame on one side before you enable the steppers?
My center assembly is not exactly square, but when I try to force it more square by hand I need a reasonable amount of force. If it’s too far out of square, I’m not sure the steppers can hold it square when encountering cutting forces.
I use skew correction in Marlin to correct the non-squareness of my center assembly, seems to work well.
My machine doesn’t feel particularly rigid in the squareness of the gantry rails (compared to what I don’t know). I see this as a good thing because then I can force it square and it’s not straining against the motors much.
I should also mention that I took the same approach with the rollers on the side rails. I noticed that if I lift up, there is a barely perceptible movement and I can hear a slight tick sound of one of the bearings coming into contact with the rail. And one of the bearings would spin freely because it wasnt touching.
I tightened the bolts just slightly until the roller no longer had that play, but no tighter than that. As far as I can tell it still rolls just as smoothly.
For the rollers it shouldn’t matter because the forces on the tool shouldn’t be high enough to lift the rollers (plus the belt tension). But if there is no penalty to rolling smoothly then I figure I might as well eliminate the slack.
I think so. Don’t over think it. For me, usually, When I put them together the Z axis is in already. And adding each gantry rail all bearings are usually touching already when I add the rails. The 2.5" Z axis bolts (I really do not know the labels I put on them) are usually the only ones I need to adjust a bit tighter until they all touch.
I do kinda rough it up a bit and wiggle it and then check again. Too loose is probably better than too tight.
Dang it, I am close to a new center to try out. It is set up in a way that is much easier to visualize and set each triplet. I will keep plugging away, almost there…One part left with the rollers and I will get back to the center.