Circles progressing to spiraling ovals in tall print.

My new tall format printer has a strange problem when printing large circles with Z over 256 mm
The circles progressively become ovals which rotate as the print progresses making a spiral pattern.
See attached.
Any ideas what it could be?

... maybe the elasticity of the part?

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Viktor
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256 (mm) is a suspicious number in the digital world. I hope it's not a software problem of the 8bit CPU.
How does the part look in a 3D viewer?

Interesting idea, the problem has not occurred with structured prints or with thick walled vases which matches the idea, I can test that by placing a light pointer against the side of the forming vase and see if it gets pushed away.

I was thinking its a software bug but searches don't find anyone else with the problem.

Thanks! Unfortunately it will be a while till I can test the idea, I don't get to the shed that often.

Looks ok in the preview, sliced in cura15.04.6 with G0 codes removed after the base is formed to prevent the mark on the side of the spiral print.
Previewing the Gcode in Simplify3D no wobbles are seen.

Edited 1 time(s). Last edit at 09/19/2017 06:25AM by Leenderm.

If you print small perimeters (like screw holes, maybe 5 mm diameter) at high speed with a corexy they tend to come out elliptical which is probably due to the larger momentum of the Y axis motion compared to X axis motion. The answer there is usually to slow down small perimeters so the system is better able to control the motion. It may also be a function of the jerk/junction deviation settings in the firmware.

In your case, when the bed is near the extruder nozzle, shaking of the frame translates to very tiny motion of the bed relative to the extruder nozzle. As the print gets taller, the bed moves downward and the shaking of the frame results in greater motion of the bed/print relative to the extruder nozzle. I believe that if you print slower, the frame and print will shake less and the print quality will improve. Bracing the printer's frame may help, too.

I think it may be possible to use custom gcode to make the print speed a function of the Z position. That way you could print faster when the bed is close to the nozzle and slow down as the print gets taller. You'd have to scale the print speed as an inverse multiple of the Z height at the start of each layer.

What is the print envelope of that machine?

Edited 1 time(s). Last edit at 09/19/2017 08:53AM by the_digital_dentist.

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Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]

The print envelope currently is 230x277x910 it will be by 300 once I have the cooling blower better situated.

I do still need to stabilise the top of the printer, I think the short vibrations at the top of the vase are due to the frame vibrating.
I don't think the large wobbles in the middle of the print are due to frame wobble, primarily because I have seen the same on
my previous corexy large format printer envelope of 410x425x600 which has a very ridged frame. See pic attached.

Yep, your correct it was the part deflecting from the extruded, tried one printed thinner at 0.4mm thick and the ovals were bigger, pic attached is one printed at 1.1mm thickness and the wobble is gone. Thanks all for your help!