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Corners on round shapes

Posted by Schild0r 
Corners on round shapes
July 15, 2018 08:00AM
Hey there,

I built a CoreXY after the design of Jand1122 (https://github.com/jand1122/RepRap-XY) into an Flsun Cube

But unfortunately when I print some shapes I get a strange behavior which I tried to point out in this picture
I am doing this with some filament I only use for calibration stuff and so on so excuse the color... You see that I get "corners" where it stitches the Perimeters Ends.
Also it seems to print a straight line there at the end where it should be round. This happens on the sections of the circlethat are parallel to the x-Axis but I can not measure any reverse play or backlash.

G-Code is all fine and round

Any tips how I could fix that I am becoming desperate on this...

Thanks for any help
Re: Corners on round shapes
July 15, 2018 08:57AM
What acceleration, jerk, speeds, motor currents are you using?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Corners on round shapes
July 15, 2018 09:27AM
Quote

What acceleration, jerk, speeds, motor currents are you using?

#define DEFAULT_ACCELERATION 1000
#define DEFAULT_RETRACT_ACCELERATION 2500
#define DEFAULT_TRAVEL_ACCELERATION 1500
#define DEFAULT_XJERK 9.0
#define DEFAULT_YJERK 9.0

Quite high motor current of 1.4A on MKS LV8729 drivers
Re: Corners on round shapes
July 15, 2018 01:34PM
You may be seeing the result of flexing in the guide rails, or play in the bearings or both.

Are you printing at high speeds?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Corners on round shapes
July 15, 2018 09:18PM
That looks like a mechanical issue along the x direction. Is there any play in the gantry? What diameter and length are you using for the y rods?
Re: Corners on round shapes
July 16, 2018 12:27AM
I see white belts. Are you using steel core with a too small pulley diameter?
Re: Corners on round shapes
July 16, 2018 07:37AM
Sorry for my late reply

Quote

Are you printing at high speeds?
-> No I am printing at about 50-60mm/s
Quote

What diameter and length are you using for the y rods?
-> 8mm Rods for both x(410mm length) and y(425mm length)
I would consider the x Axis more stable than y becaus of the two directly connected rods which are also a little shorter
Quote

I see white belts. Are you using steel core with a too small pulley diameter?
-> Yep Steel core GT2 but with 24teeth pulleys (~12mm Diameter) and fl624zz bearings (13mm Diameter)

As I said there is no play as far as I can tell. Belts are very tight but I don't think it is strong enough to bend these rods which I got directly from IGUS (so they should be high quality)

I also think this is a mechanical problem but I don't know what exactly causes this
Re: Corners on round shapes
July 16, 2018 08:22AM
The lead screws are constrained. Make sure they aren't bent.
Re: Corners on round shapes
July 16, 2018 08:50AM
The straight lines run parallel to the X axis. At that point when you're printing a circle, motion in the Y direction has slowed to a crawl. DRV8825s are known to skip usteps when the speed is low.
Does the machine use DRV8825 drivers?


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Corners on round shapes
July 16, 2018 03:44PM
Quote
boredom.is.me
The lead screws are constrained. Make sure they aren't bent.
The ones from the bed? No they are fine

Quote
the_digital_dentist
Does the machine use DRV8825 drivers
Nope as mentioned before I am using the LV8729 nevertheless that might be a point. I will try to snap on the standard polulus again and report back thanks for the hint.
Unfortunately I can not test this before wednesday evening because I am writing two examina til wednesday

Edited 1 time(s). Last edit at 07/16/2018 03:44PM by Schild0r.
Re: Corners on round shapes
July 18, 2018 03:49AM
Snapped on the A4988 drivers. Unfortunately this made no difference
Re: Corners on round shapes
July 18, 2018 03:58AM
Is a 24 teeth pulley big enough for steel core belts?
Re: Corners on round shapes
July 18, 2018 11:54AM
Quote
o_lampe
Is a 24 teeth pulley big enough for steel core belts?
is it? grinning smiley
Re: Corners on round shapes
July 18, 2018 04:23PM
The problem with steel core belts in the XY stage is that there are a bunch of pulleys and steel core belts take quite a lot of work just to bend around the pulleys. The ultimate problem with that is that the belts keep flexing back and forth, and if the radius of curvature is too small, the steel core breaks and the belt suddenly gets stretchy at the break site, which causes all sorts of weird behavior in a printer. I haven't found any recommendations on minimum bend radius for steel core belts, but I did see a rule of thumb in a steel cable use note saying that the radius of curvature should be at least 10x the diameter of the cable. The cables in steel core GT2 belt are about 0.5 mm diameter, so by the rule of thumb, a 5 mm bend radius should be OK, but the reality seems like it's too small just based on the effort it takes to bend the belt like that. I was going to try steel core GT2 in the XY stage in my printer, but aborted the install when I realized how much effort it took just to bend the belts around the 22 mm diameter pulleys.

I'm using glass core Gates LL2MR09 belts in my XY stage with 400 step/rev motors, and they perform just fine. My moving mass may be 2-3x your printer's moving mass, so glass core should work fine in your printer.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: Corners on round shapes
July 19, 2018 05:52AM
Thanks very much for that hint. I will definetly try this but I think I won't get these new belts very quickly...

I am not sure if this solves the problem (because it only appears parallel to the x axis ant not to the y axis)
but I will definetly change the belts. Maybe I will find the cause of this problem when putting on new belts.
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