How to fix ZX / ZY skew ?
Hi all,
My (pre-assembled) Core One has a significant skew (to me, I only print functional parts and a ~0.35mm error over 100mm is annoying) on the ZX / ZY planes:
ZY Skew: -0.195 degrees
ZX Skew: 0.107 degrees.
How can I find, and fix, the root cause ?
Thanks!
RE: How to fix ZX / ZY skew ?
Maybe there is a standard terminology, but I am uncertain. Could you confirm where the skew occurs please? I assume you mean the the Z axis is not orthogonal to X, but 0.107 degrees off, and Z is not orthogonal to Y but 0.195 degrees off -- right? So if you print a cube, it is tilted sideways and towards the back a bit?
This would have to stem from the smooth Z guide rods being not-orthogonal to the CoreXY assembly, I assume. Several users have reported that their CoreXY assembly is not parallel with the printer's base, with deviations of up to 2 mm. (Mostly front-to-back, but sometimes also left-to-right.)
Having built the kit, I can't figure out where such large deviations would come from: The vertical frame profiles will define the distance of the base and the CoreXY planes at the four corners of the printer, via screw holes with pretty small tolerances. But if the CoreXY plane is indeed tilted relative to the printer's frame and base, it would also be tilted vs. the Z guide rods, causing skewed prints.
RE:
As far as I understand it you have it perfectly right: the XY gantry and the bed aren't parrallel.
From my understanding, the bed has to top right corner slightly lifted.
As you have built the Core One from scratch do you see how it could happen and any way I can solve it? I've tried going through the documentation and I'm not entirely sure.
RE: How to fix ZX / ZY skew ?
As mentioned, I don't actually understand where the reported large deviations in base > CoreXY distance (and hence bed > CoreXY distance) can stem from. The holes in the vertical frame profiles of the enclosure, which define the distances, only have a few 1/10 mm of play, hence the distances should be relatively well-defined. But deviations of 1 to 2 mm have been reported here for some printers.
You could start by checking the distances from base plate to CoreXY assembly with a ruler on all four corners, to see whether there actually are large deviations. If so, the screws which attach the base and the CoreXY assembly to the vertical frame profiles are all accessible from the outside of the printer. You could loosen them all a bit and see whether you can level the CoreXY assembly vs. the base a bit better. (Plus there is one screw in the rear center which holds the CoreXY to the back wall. That one is hidden by the cable cover above the power supply and electronics box; remove that cover via the two screws inside the printer, just left and right from the Z lead screw in the back.)
If this sounds too invasive: Some users have just added shims above the base plate, at the base of one or two of the Z lead screws, to define the end points for the Z axis levelling (calibration).
Caveat: Even if the CoreXY assembly and the heatbed are perfectly parallel, the heatbed could still travel downwards on an axis which is not orthogonal to the CoreXY plane. This would still result in skewed prints. One would also need to ensure that the two vertical smooth rods which guide the heatbed are orthogonal to the print plane. Ultimately this is defined by the printer's frame geometry, which is quite exact in my experience, but probably not perfect. You could check for right angles between the base and the frame edges, and between the base and the smooth Z rods, with a machinist's square.
RE: How to fix ZX / ZY skew ?
Can you connect your printer to Octoprint? There is a plugin to measure the print bed.
You can then manipulate it at the lowest points so that the print bed is parallel to the gantry.
For further fine-tuning, there is also the option of optimising the software, as here.
Vector3D also offers tools for this.
RE: How to fix ZX / ZY skew ?
I don't have octoprint but apparently I can just get the bed mesh from the USB C serial, I'll try thanks.
And I did see goskew but I'd prefer to solve that mechanically, especially since goskew can't work with G2/G3 arcs or G5 paths.
Btw I think my problem is related to these ones:
https://forum.prusa3d.com/forum/prusa-core-one-hardware-firmware-and-software-help/front-z-steppers-1-2mm-of-missaling-impossible-to-recover/
Can you connect your printer to Octoprint? There is a plugin to measure the print bed.
You can then manipulate it at the lowest points so that the print bed is parallel to the gantry.
For further fine-tuning, there is also the option of optimising the software, as here.
Vector3D also offers tools for this.