CoreXY assembly | Linear rail seems skewed
Hi everyone,
I'm assembling my Core One and am on step 5.35 ( https://help.prusa3d.com/guide/5-corexy-assembly_835522#845609). I think I may be running into the issue that Juergen is alluding to in the comments. When I tighten the 6x M3x8rT screws, the rail no longer sits flush at the top or bottom of the rails. It has a skew. I can apply a little pressure with my finger to get the rail to lay flat, but it won't do so on its own.
I attached photos showing this below. The angle meter reads 0.3 degree skew. That doesn't seem like much, considering how visible it is. There is a ~4mm difference in the resting position at the bottom of the rail. This is the same but swapped left-right at the top of the rail.
I've tried relaxing all of the screws and repeating the rail attachment with the frame vertically oriented, so that the rail might sit level. But no luck, it still has some kind of warp. This doesn't seem right, so I wanted to ask you all how I should proceed (or backtrack). Yes, I still have my safety pins. Thanks for your advice.
RE: CoreXY assembly | Linear rail seems skewed
I wanted to clarify but the time window to edit my post had expired. I don't have any reason to think the linear rail itself is to blame; it's seems to be some other part of the assembly that isn't lined up quite right that's causing the issue. My guess is one of the two Linear Holder parts or something connected to them...
RE:
I you detatch the rail and place the gauge onto the "linear-rail-holders", maybe it shows, that both/ore one of the holders are bent slightly ?
RE: CoreXY assembly | Linear rail seems skewed
I agree with @ckobar. It's likely that the two angular brackets ("linear rail holders" in Prusa's terminology) are not properly bent to 90°.
The brackets are made of soft steel; you can easily bend them by hand. I checked them with a machinist square before installing them, clamped the weaker bracket arm in a vise (to ensure it stays straight), and bent the other arm with finger force to obtain an exact 90° angle. But others have reported that adjusting the angle after installation has worked well for them: Just detach the bracket on one side, then bend the opposite bracket using the X rail as a lever; repeat on the other side.
I you detatch the rail and place the gauge onto the "linear-rail-holders", maybe it shows, that both/ore one of the holders are bent slightly ?
RE: CoreXY assembly | Linear rail seems skewed
(i'm still waiting for my kit) Maybe there could be a fix to make those brackets stiffer?
But i suppose that there is almost no space left for some kind of reinforcement.
RE: CoreXY assembly | Linear rail seems skewed
(i'm still waiting for my kit) Maybe there could be a fix to make those brackets stiffer?
No, as discussed in the long thread on this topic, I don't think the brackets need to be stiffer. They are clearly strong enough to impart their incorrect angles onto the X gantry, even after the belts are installed and tensioned. (Unless the belts are deliberately tensioned unevenly to mask the problem.) And there are no reports of the brackets bending out of shape during operation of the printer if they were correctly made and installed initially.
The brackets just need to have the exact 90° angle upon installation. It's Prusa's job to produce and install them correctly, but fortunately it's also easily fixed by the user if Prusa didn't get it right.
RE: CoreXY assembly | Linear rail seems skewed
I'm starting to think, that the engineer who's responsible for the brackets made them that soft deliberately.
The belts, in most cases, easily bend them in place (my belts are at 81/86 Hz).
If they'd be stiffer, the belts wouldn't be able to force them in position?
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Yeah, those Linear Holder parts seem to be to blame. Just clamping one to the other, you can tell they are not bent to the same angle.
I am going to have the guys at work make a fixture to bend it much closer to 90 degrees. I am glad this is an easy fix, but at the same time, I'm disappointed. This is like a Creality-level build quality failure. I paid more for a Prusa (for the first time!) to avoid the likelihood that very kind of QA fail. I'm sure I'll be happy with the final product, but I'd suggest to Prusa avoiding the contractor that made those parts. Otherwise everything has been great and seems high-quality so far.
RE: CoreXY assembly | Linear rail seems skewed
I'm starting to think, that the engineer who's responsible for the brackets made them that soft deliberately.
The belts, in most cases, easily bend them in place (my belts are at 81/86 Hz).
If they'd be stiffer, the belts wouldn't be able to force them in position?
Thst my theory. I think the trick with the included spanner and bending the frame square was meant to be part of the build process but got dropped in favour of the belts being tuned to different frequencies
RE:
I am going to have the guys at work make a fixture to bend it much closer to 90 degrees.
No need for a fixture. You can easily bend these by hand. I recommend clamping the weaker (narrower) arm in a vise to ensure it stays straight, then give the other arm a nudge; finger force is sufficient.
All you need is a machinist's square or some other reference to check whether you have obtained a proper 90° angle. Rinse and repeat if necessary.
RE: CoreXY assembly | Linear rail seems skewed
It's probably even best to bend the brackets while they're connected to the gantry. That way, you can simply measure until they are exactly equal distance (touching) to the end stops. In the end, that's the only measurement that's important and bending the brackets away from the gantry does not guarantee this, although it will probably be very close.
RE: CoreXY assembly | Linear rail seems skewed
It's probably even best to bend the brackets while they're connected to the gantry.
Yes, you can do that, and it may still be needed for the final touch even after correcting the brackets before installation. The required force is so low that I don't think the linear bearings will suffer any damage.
But you should do it one side at a time, detaching the opposite bracket while you correct the angle of one of them. Otherwise you may end up with both brackets having the same deviation from a 90° angle, "fighting" each other. That would still give you a squared gantry, but with permanent strain on the Y bearings.
(And of course loosen the belts if they are already installed, so they do not influence the gantry angle.)