Really strange.
Do you use a Mac or a Windows PC?
And would you be willing to share your machine settings ($$ in the console)?
To me the original problem looks like the laser assembly is not secured to the gantry and is physically wobbling during direction change. Or possibly a loose lens. See if you can wobble the lens or laser assembly.
I can almost guarantee that PC or Mac is irrelevant in this case.
I am going to check the whole mechanic again tomorrow.
But what is strange to me is the fact, that the ādamagedā spots of the circles (where the two lines are getting closer) are always on the same places (always in the top right and bottom left ācornersā).
So this looks for me as if the laser is going a different circle path when going counter clockwise as it does when going clockwise.
Did you do something special for the linear rails? I only put some machine grease on it. They are running ok and mostly smoothly. But you can still feel some small stutters when moving the laser head by hand.
Donāt know if that can cause the problems though.
Not really. I think I put some āLithium SprĆ¼hfettā on it some time ago, but I doubt that changed much. I just checked, the belt of the x-axis is slightly less tight than the y-axis belts. I can also feel some minor stutters when moving the head.
Thanks for checking this.
What Iām still not understanding if it is a mechanical problem:
Where is the difference between a circle that is scanned clockwise and a circle that is scanned counter clockwise. Even in a single circle there are direction changes for both axes (x-axis changes direction at 3 and 9 oāclock, y-axis at 12 and 6 oāclock).
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I can imagine that itās āplayā or better to say backlash that is influencing this. So, the change in direction is in both cases, but the difference is in which direction it starts. So after doing the first circle in one direction, the laser stops and does the same circle but the other way around. In this particular moment, the motor does a few steps without moving the laser, because the the belts might be to loose or the pulleys are not completely fixed. This results in a different actual starting position for the second circle.
Is the laser head possibly loose on the mount?
As in, when it changes direction it flops to one side or the other? Or front to back.
OK, thanks. This is an explanation I am understanding.
Iām pretty sure, that the pulleys are tightened, I checked them just yesterday. I will have a look at the belts tomorrow though. There has to be reason for this and I want to find it 
I am fixing the head by tightening the 2 screws on the back of the mount. I am tightening these by hand. So the head is fixed, but yes it can be slightly moved on the mount by pushing a little bit harder against it. But I think all S10 models do have this āproblemā or behaviour.
Ok, just thought I would toss that out there.
Not really on-topicā¦ but a very big thank-you for posting that link to mechanical adjustments and maintenance Wiki page. There is a huge dump of very helpful information in that link.
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OK, so I checked everything again.
I readjusted the frame, made sure that everything is perpendicular and all corners have right angles. The Frame sits tight on the desk, there is nothing that is wiggling.
I readjusted the wheels, the eccentric nut, and all the belts.
I readjusted the gantry, so that it touches the frame (at the back) left and right at the exact same position. To make sure the gantry is in a right angle to the frame.
I checked the calibration of both axles. Then I cut out a rectangle (100 x 100 mm) and measured it: x = 99.65, y = 99,75). I also checked that the rectangle has 4 right angles, which it does.
Everything works fine, except of the circles.
So I am really out of ideasā¦
Maybe there is something, that I am doing wrong with both Lasers when putting them together and adjusting them. But I have really no glue what it could be, because I think there is nothing left, what could be checked.
OK, hereās an update:
I found out that I can reduce this problem by loosen the lower wheels of the gantry (the ones with the eccentric nut). When I loose them until I can turn the wheel with my finger (which in my opinion is way too loose), the circles are almost ok. You can see a tiny bit of distraction, but not much.
But: I think that the lower wheels should not be that loose. At least the gantry should not be loose due to this I thinkā¦
When I set up my 3d printer that was the way I was told to set the eccentric wheels. So you can turn them, but not without a bit of friction. Very little.
I did the same with my laser and I have not seen any problems in doing so.
And the gantry does not wobble with me setting them that way.
Ok, maybe Iām going in the right direction for the first time.
As I said, the circles do look better, but the problem is not gone completely though.
If you havenāt, try to get all the eccentric wheels to the same or as close as you can.
At least it is a step towards the fix.
Not sure if relevant, but can you swap the eccentric wheels side for side?
Your problem certainly appears to be mechanical
Do the erratic circles happen in only one direction, e.g. clockwise or counterclockwise? If so does the laser produce a perfect single circle when going either way?
I managed to reduce this problem to a very low level by loosening the lower wheels to the point where you are able to turn it slightly with a finger. Also I loosened the belts. So now the laser moves very smoothly in both axles.
Single circles are perfect. The problem was only visible when making two circles which were very close to each other (0.5 mm distance) and when the the circles were done in different direction (one clockwise one counter clockwise).
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