Oddly shaped holes?
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Sorry mate, I'm lost here. You really need someone who actually has experience with coreXY machine. With the CNC this type of error is always backlash. No clue how backlash on each motor affect this at all. Normally you can tweak backlash compensation in the firmware of your CNC machine, but AFAIK there is no support for that in RRF (not sure any 3d printing firmware supports it, I know there was try to implement it in marlin but I think they gave up, never entered master branch).
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@arhi My theory here is that the igus bushings are binding up near Y max. This is causing 2 things:
- I lose microstepping, which causes a loss in resolution
- My belts stretch more (it's a pretty large printer, so the belts are pretty long) and that creates the "backlash" issue
I'll have the LM16UU bearings in this coming week. Those will get installed and hopefully this issue will be resolved.
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It is a good theory as any. I see even on large printers ppl go with 2mm belts which is weird to me, the only reason to go with 2mm pitch is tight bend radius, but if you are on big printer you can design so that you don't need tight bend radius and go with 5mm pitch where for much less $$ you can get higher quality belts .. but ..
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@arhi If I still have an issue, I'll re-design for 5mm pitch. I think the 2mm should be fine. I might need to upgrade to a non chinesium belt.
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@Surgikill even chinesium ones at 5mm work nice, the 2mm prc belts stretch, while a lot of ppl online try to say they don't my experience show different, also no proper reinforcement... 5mm you can chose if you want glass or steel, last a long time, only issue is if you go 180 degrees it has to go over large diameter (18 or 20 5mm pitch teeth is rather large circle) .. so the idea is to design motor to sit on the "straight" path of the belt with some idlers (3 point setup) rather then doing 180 around the motor forcing you to use large diameter driving pulley making 1 step of the motor move too many mm of belt
also, using 5-6mm wide belts on big printers... IMO 5mm is ok on something like 200x200mm print surface, you go over you need to step up to 10mm or more .. if you look at professional machines it's not hard to find belts that are in 10cm width class
... I remember some stratasys printer I was repairing back in the day motor was linked to extruder with belt that was 4-5cm wide, X and Y belts were both 6-7cm wide 
5mm pitch belts ... -
@Surgikill said in Oddly shaped holes?:
LM16UU
I think you’re on the right path regarding the igus bushings causing the oddly shaped circles. I bought into the hype of the polymer bushings but had print quality issues. In my case the sticktion could be felt when manually moving the axis with the belts removed. Either they were loose to prevent sticktion and had excessive play or they were tight and caused the oddly shaped circles like you are experiencing. Also found that the rods need to be completely dry and cleaned regularly. Not worth it, went back to LM16UU bearings and the issue went away.
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@mwolter Yup. I've had the same issues where they're either too loose or too tight. Hopefully the LM16UU fix that issue.
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Those IGUS bushings have pretty tight tolerances on how they are supposed to be mounted. There is a lot to read on their website.
I have tried them a few times, but as many of you say they either bind or have play.
They seem to be popular in the industry, "lubricated by dirt". -
Igus has plastic bushings that are encapsulated in a hard shell. This way you can clamp it without increasing the binding on the rod. E.g.
https://www.amazon.com/gp/product/B00QZ7YJ7W
Linear bearings are available in double length but I couldn't find double length igus bushings and installing two one after the other creates non co-linearity that increases the binding.
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@bondus Yes, the Igus bearings are built to quite tight tolerances, and to not like any mounting error or rod misalignment. Industrial equipment, which is what they are designed for, usually is toleranced more closely than 3d printers. I designed a bearing mount: https://github.com/mendenm/hemera_mount_plate which has a self-aligning lower bearing block which avoids this problem.
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@zapta Yea I was looking at those but I wasn't going to pay 20-30 dollars a bushing when I need 8 of them. I'll just stick with the regular bearings. Are there any recommendations on good bearings? I have all chinesium ones right now, but if I find out that it does end up fixing my problem, I'll most likely upgrade to better bearings.
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I used to use Thomson Super8 bearings when I was still using 1/2" round guide rails. I was buying them new on ebay for $5-8 each. They allow adjustment of preload and the races can tilt a little to compensate for slightly inaccurate mounting.
Make sure your rails are hardened- ball bearings will cut grooves into unhardened rails.
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@mrehorstdmd @zapta @mendenmh @mwolter @arhi I just replaced the Igus bushings with the LM16UU I ordered. No more issues. Now I have no idea what to do with all these Igus bushings I have.
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I'm a little late to the party I know but I started down the carbon fiber/igus route back in 2017 and although it "worked" it was way too finicky. The plain igus bearings MUST be press fit into a very specific hole diameter to be usable with the specified diameter diameter rods. Most of the carbon fiber rods I bought needed tuning with 1000-1500-2000 grit sandpaper to get them to have constant and reasonable friction in the bearings. Some could never be tuned properly because they were too narrow to begin with. Also, while most of the roll-wrapped ones were spot on for straightness, some were not which made trying to get matched sets almost impossible. In the end, I gave up.
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@Surgikill said in Oddly shaped holes?:
Are there any recommendations on good bearings?
When I need good quality bearings and rods I get them from Misumi. They have a large selection and they sell them also in small quantities, including but to length rods.
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@gtj0 said in Oddly shaped holes?:
I'm a little late to the party I know but I started down the carbon fiber/igus route back in 2017
The lure of light weight moving mass.
I tried bowden, igus bushings and carbon and aluminum rods but after a lot of trial and error realized that a solid design with stainless steal rods, real ball bearings and a direct drive give me the best print quality.
Time to adopt this term from the bicycling world
https://www.urbandictionary.com/define.php?term=weight weenie
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@Surgikill said in Oddly shaped holes?:
Now I have no idea what to do with all these Igus bushings I have.
ebay?
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@zapta You're correct of course but you can always learn something new by trying something no matter how it turns out.
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@gtj0 But many others have tried before with the same results.
How many times does a known failure have to be duplicated? It's one thing if you think you understand why something failed and have some new twist to apply that you think is going to fix the problem, but something else entirely to simply copy other failures and expect that it will magically work this time.
Of course, you don't always know that something is a failure. A lot of people post their designs on the web, but not many of them post their failures. Sometimes you have to read between the lines to recognize a problem that the designer doesn't realize or won't admit exists. Also, the web never forgets. Maybe you post a bad design, then learn later that it's a problem, but that bad design is still out there. It can be a lot of trouble to try to update or expunge old posts, especially if there are a lot of them.
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@mrehorstdmd Yeah isn't the definition of insanity "doing the same thing over and over again and expecting a different result"?
Still, as I've said in other posts, I learned a great deal in that exercise and would not trade it for anything. Some day someone will do something that "has been done 1000 times before" and discover something new.
