Yes, you have backlash - caused by the belts being too loose - most likely the short belts. Also, an infill overlap of 15% should be plenty - maybe even too much. Even at 15%, with certain geometries, the infill can start to protrude through the wall.
You make convincing points. It *could* be backlash/play. Or it might be something else.
play: So are all the belts tight enough to get at least a very low audible pitch? Like the bass string on a string Bass (or higher - higher pitch is fine also) and also it might be play in the head or nozzle. With the nozzle cold (or your fingers protected) push the nozzle around and see if there is any play before the rods start moving. The head could be loose or the bearings that attach the head to the rods could be loose.
Assuming not play:
Inner diameter circles are *OFTEN* smaller than desired. I think as you lay down the layer, the PLA pulls a bit like a rubber band and the circles are "pulled" in towards the center. Also if your model only has say 10 line segments on a circle - well that decagon is effectively now a smaller circle as each line segment cuts into the circle. Plus PLA shrinks. Combine those 3 properties and you get smaller circles. You can fix by increasing your temperature - a lot - try 240C but only for the first layer. Once the first layer is almost done put it back to what you had before.
Also you can try raising your bed height by a tiny bit (loosen the 3 thumb screws by a half turn maybe). This will squish the bottom layer a bit more making it closer to the .4mm wide "strings" that Cura intended. Although I might not do this last thing as the infill bottom layer looks quite good.
Third point:
25% overlap is for wimps. Go for 50%, 100%, 200%, 300%, lol. 
Personally I use 15% and it usually works fine for me. At least on blue tape it works fine - haven't printed a bottom layer hole on glass yet.
I've got the same problem. It is especially noticable with the last version of Cura, with previous versions this problem occurred less. Which PLA do you use? Another cause could be that the first perimeter on the the bed does not stick enough.
Hi all,
First of all, thank you very much for taking time to look at this issue, I really appreciate it!
My thought were into backlash as well, I made belt tensions to make sure the pitch is quite high. I took this one this morning:
The short belts are less tight, so I've tighten them (a lot actually). Things improved a bit, but I'm still not satisfied. But why is the infill at 1, 2, 3 and 4 perfect (so in all directions), and infill on inner wall not.
@gr5, the smaller circles you suggest is actually happening. I've found Cura is not retracting on the first layer, it follows barriers. While moving from one circle to the other, retraction stops, then starts again running a circle. However, since PLA oozed out during the move from one circle to the other, there is not enough material to create a proper circle touching the first circle already there.
I also wonder if changing the settings would work. I used this link to build Marlin: http://marlinbuilder.robotfuzz.com/
The "Acceleration" rate for X and Y is 9000. I wonder what it is in your machines. I need to go off to work now, but I'll try to lower it to like 3000 and see what the difference is.
@Maurice has a point about the Cura version. 13.10 is fine for me, but a friend tried 13.11 and had some similar issues to your before switching back to 13.06.5 .
9000 is probably too high for acceleration. I use 5000... others have 3000.
That is *much* tighter than my belts. Probably twice the force. Maybe 3X the force (my belts really aren't that tight and I get great results). Although that is about the same as the tension on my UM2. As far as acceleration is concerned I use 5000mm/sec on my UM original. I don't think I set that myself but that's what it's been for many months.
At this point I think your hardware is fine.
The next thing I would try is raising your print bed so that the first layer lines are squished more. I would go up about .05 to .1mm. Try turning each of the 4 screws counter clockwise by about 1/8 of a turn. Looking at your photo the plastic should be squished more so that it stays where it's supposed to go and doesn't pull inwards to the center of the circles.
If the PLA is squished a little flatter into the blue tape and the contact outside is still bad then I would just blame Cura for not compensating well enough on bottom layers and just increase the infill overlap from 25% to 75%.
But I have to tell you - the UM can do this. I printed a cell phone case with a UM Original back in August and it came out great. However because of the complicated nature of what I was doing (changing filaments) I was printing .4mm off the bed but Cura thought it was .2mm off the bed and I had flow at 200%:
https://www.youmagine.com/designs/galaxy-s4-ballistic-case#!design-comments?data-id=208
Actually in the above photo it was at 150%. I did it again with 200% and it came out better.
@gr5 - that print look excellent! Wauw!
@illuminarty - I agree, guess 9000 was to high.
@AnthonyT - did you also switch to other firmware while switching Cura versions? I've also seen this on NetFabb sliced prints, so it isn't just Cura.
Great news to anyone facing the same issue. On the Ulticontroller I changed "Accel" to 1000, and "Amax x" and "Amax y" to 3000 (I'll try 5000 later). Now the infill touches the border *excellent* and I'm confident I can go back to 15% overlap. I think I do not need to change the bed, since the print comes out shiny flat, at 220 degrees PLA.
Thank you all very much for thinking together with me! I'm very happy now and print the full case tomorrow. Cheers!
Just to clarify: The main acceleration setting wins provided it's less than the axis-specific maximums, so you're running at 1000 with that setting.
ArunC posted a topic in UltiMaker Cura,
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Dustin posted a topic in Firmware,
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anthonyt 0
Are you sure it is not loose belts? Check out this posting:
http://umforum.ultimaker.com/index.php?/topic/1872-some-calibration-photographs/?p=14474
Also note that the back motor (X-axis) cannot normally be moved down low enough to tighten the short belt and the holes in the wood may need filing at least 1mm more downwards.
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