How to fix offset on CoreXY
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reading this https://forum.duet3d.com/topic/12503/reprapfirmware-2-04rc4-released/48 ikt seems the promega has some serious flaws.
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@Veti said in How to fix offset on CoreXY:
https://forum.duet3d.com/topic/12503/reprapfirmware-2-04rc4-released/48
Yes I'm aware of that and I'm also disappointed of the product. But I was able to fix most of the problems with the leadscrews and the Bondtech extruder. Now it is a decent printer. Not as good as my Prusa, but also not far away. And it was an interesting project so far and I learned a lot. So I'm hoping I can also find a solution for this final problem.
I guess the Z moves in the logs come from the mesh bed leveling. Maybe the mesh bed correction do make such small prints worse for some reason. I will try printing without mesh bed leveling as a next step.
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looking at the picture my guess would be small friction on the z axis.
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@Veti I've no printed without mesh bed compensation but that makes no difference. I'm using leadscrews with 4mm pitch and the pattern on the walls also repeat every 4mm. Also the walls are alternately thicker, then again thinner on the outside and on the inside of the walls.
The problem could be that one turn of the screw matches 4mm, but that a quarter turn of the screw doesn't always exactly match 1mm and because of this some layers get a bit more plastic than others. Could this be the cause of this issue? And if so what could be done to improve this? I'm using those couplers and those anti backlash nuts. With the backlash nuts I don't know if there is a specific way they have to be installed. I just tried to press them together as much as possible when inserting the leadscrew.
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Hi,
I stopped using couplers of any sort and opted for connecting the stepper to the lead screw via belts and pulleys.
In any case my CoreXY works fine - though I did have a minor problem initially caused by unequal belt tension.
Frederick
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Looking at the video seems like there's backlash in the XY mechanism somewhere. I'd start by trying to wiggle the hot-end, then the extruder carriage, then the X axis, then the pulley mounts on the X axis, then the motor mounts and corner pulley mounts. Are you using round guide rails or linear guides? Can you post some photos of the machine?
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@TheNetStriker said in How to fix offset on CoreXY:
Also the walls are alternately thicker, then again thinner on the outside and on the inside of the walls.
i was not talking about the leadscrews. i am talking about the guides that holds the entire bed assembly in place.
if that motion without the leadscrews is not super smooth and you get minimal binding, then for example it would not move as much due to friction, then overcome the friction and finaly move to the requested position. this would result in thicker and thinner layers. -
@mrehorstdmd You can find a lot of pictures of the CoreXY system of the machine on Google:
The Y axis is done with two linear rails, but the X axis has round guide rails.
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Did you put lead screws in the Z axis? How many and where?
What were the prints like before you did that modification? -
@mrehorstdmd I followed this guide on Thingiverse: https://www.thingiverse.com/thing:3601545
There are 3 leadscrews and 3 linear rails on the z axis.
Before the modification the quality was very bad. Also the printer was not reliable. The print bed was originally driven by 4 small belts and one motor. It was very complicated to install and tension the belts and when I put a little force on the bed the belts skipped over the gears. Also the positioning of the bed was really bad. On the first print I watched the nozzle melt into the print a few millimeters. This is now all much better compared to before, but I guess there is still room for improvement.
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I've now also zip tied the coupler tops and bottoms together to be sure the don't get pulled out when driving down the bed, but still no difference in the print. I've also tested vase mode and printing without z hop, but also no difference.
On the vase print I have noticed that one outside wall of the print is almost perfect, but all other three walls show the pattern. I guess this could be related to the problem with the CoreXY position, but I have no clue why this happens.
I also compared the photos from the microscope and If I look only at every other picture the position is perfect. So when driving back it lands on the exact same position. Only when the movement changes from clockwise to counter clockwise it ends up in a slight different location.
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Just for the fun of it, if it's not already, try rotating the test part so the walls are -45deg, +45deg instead of 0 and 90 and see what happens.
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@gtj0 I also tried that but there is also no difference. I also tried printing without the heated bed, but the pattern is still there. The only idea I have at the moment is that I could try different stepper values per motor. Maybe one motor moves the head a bit more than the other for some reason.
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@TheNetStriker said in How to fix offset on CoreXY:
With the backlash nuts I don't know if there is a specific way they have to be installed. I just tried to press them together as much as possible when inserting the leadscrew.
Perhaps you've put too much compression on the springs and it's causing excess friction and binding. Honestly with a corexy and the bed moving down, I don't think you'll need anti backlash nuts anyway.
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@Phaedrux I've just tried that, but without the backlash nuts I still have the exact same result. I will try to adjust the belt tension next and if that not helps I will try with different stepper steps for each of the CoreXY motors.
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@Phaedrux @gtj0 @Veti I've now improved my microscope test script so that it creates a video from every point the head moves to. I then tried if I can correct the positioning by tensioning the belts and also by adjusting the stepper steps, but with both methods I was not able to get better positioning.
I now found out that the problem already shows if I move the print head on just one axis with three points. My videos show that the start and endpoint are always at the exact same location, but the point in the middle is always offset when the print head changed direction. It also doesn't matter which axis is used, I tried x, y and also directional in both ways that only one motor is working and the offset shows every time on the point in the middle. The Duet logging shows that for each movement between the points the exact same amount of steps is used. I also tried to tension the belts as much as possible so they cannot flex anymore, but that also doesn't help. I'm out of ideas at the moment.
Do any of you have any other ideas what could cause such a problem?
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@TheNetStriker said in How to fix offset on CoreXY:
but the point in the middle is always offset when the print head changed direction.
overcoming friction in the rails?
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Belts can be too tight as well.
What perhaps matters most is they are the same.
I had problems something like yours but after matching the belt tension as best I could they problem went away.
Frederick
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If you remove the belts and move the carriage by hand does it move freely throughout the entire range of movement?
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If the mechanism is built so that without the belts the X axis is perpendicular to the Y axis (and that's how it should be built), when you tension the first belt, the X axis will get pulled out of square. When you tension the second belt, you apply just enough tension to square the X axis again. When you increase the second belt's tension,
the tension in the first belt will increase, so when you tension the first belt, leave it just a little loose.After tensioning, if the X and Y axes are perpendicular, the belt tensions should be pretty closely matched, but the most important thing is that the axes are perpendicular. If the axes aren't perpendicular all your prints will be skewed.
