Awesome pattern
I don't know if the top-left corner of the print corresponds to the front-right side of the printer where, if I'm not wrong, you found an issue on the mechanism (post #93), but could be related to it.
The pattern remain the same, no matter the printing direction.
And repeats regularly, apparently on both X and Y (looking like stationary waves). Given the particularities of UM, using parts of one axis mechanism for the other axis as well, this may imply that an issue on one part, may actually impact on the other axis movement as well.
By now, I can understand that, if something happens periodically on, let's say X, it will also periodically affect Y and that's how a "square" pattern appears.
The fact that the impact is more adverse should be related to the faulty corner. Logically should be the same, but I suspect is the opposite, which confuses me a bit).
The periodicity indicates that the the effect is given by... rotation.
Now, the frequency of the pattern. Looks like 3-4 cm or so This is pretty close to what I can estimate is the diametral pitch of the pulley.
Why "under-extrusion"? Since there is no reason to question the extruder feed rate, an alternative would be that the axis is travelling... faster and then is slowing down and repeat. This looks again like a consequence of something similar to the belt "climbing" on the pulley side hence "riding" a larger diameter.
I think the logic by now works with the given facts. However, why not on the first layer, this is something that does not fit yet my mind
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I did, here are the results:
It looks alright when it is on the bed, but a bit wonky when held like this (I rotated the image to match the printing orientation):
I then printed another one with the infill at 90 degree (to get the other axis):
The heat distribution is not even (it is worst on the top left corner).
I have printed with other colors or course and at higher temperatures. The strands not touching problem is never gone, just very much less visible with higher temperatures. And it always is worst on the upper right corner of a print. That's why I think that it is something mechanical.
I also printed the hollow pyramid but the stringing at 230° was not that pronounced compared to 210°.
(210 left, 230 right)
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