Concentric top/bottom fill pattern closure issue

Slic3r 1.1.7 on Mac OSX Yosemite but I've tested on Windows 8 with same result...

I've experienced this problem for at least a year with Slic3r and each time I give up using it. Now I feel compelled to get to the bottom of the issue. First a photo:


These parts are to a fly fishing reel that I designed. The outer diameter is 75mm and the inner diameter of the ring is 60mm. The solid disk prints the concentric bottom layer perfectly, flawless in fact. The ring has a gap in the middle. It's a little tricky to see. On the bottom, there are 9 traces of filament on each side of the gap and the gap is at least 2 traces wide. I print with .4mm nozzle and width. Doing the math:

(75mm-60mm)/2 = 7.5mm

7.5mm/.4mm = 18.75 - which is 18 rounding down, what Slicer is creating. This is fine except the gap is much wider than 3/4 of a trace, it is almost 2 complete traces wide. The top side is even worse. The gap on the top side is over 1 mm but there are 9 traces on either side of it too! There are pocket features on the top side so it is not printed in a continuous series of concentric circles.



Finally, I have spent a lot of time tweaking the design to address this situation. I actually started out with a part that was 75mm OD and 59mm ID so the difference was 16mm/2=8mm wide. This is an even multiple of my .4mm extrusion width (20 traces exactly). This left a hideously large gap, nearly 1.5 mm wide on both the top and bottom! I then designed for an odd number of traces (19) and that also left a very wide gap. This version with 18.75 traces yields the smallest gap on the bottom and a wider on the top for some reason. Perplexing. I'd like to use concentric shells for this part for aesthetic reasons.

Has anyone else run into similar concentric fill issues on a circular part?

Cheers,
Michael

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With Slic3r, the diameter of a hole (inner diameter) is about 1 (one) extrusion width smaller than the designed diameter.
So to say, if you want to have a 60 mm inner diameter, with a 0.5 mm perimeter width, you should design the part with 60.5 inner diameter.
Then, you may want to set the relevant extrusion widths to a value that will produce an integer number of traces within the given area between the inner and the outer edges.

On the other hand, how to you fill? Are you using perimeters or concentric infill? Are any of the extrusion widths set to 0 (automatic)? If you have a 0.4 mm nozzle, and you do not set the extrusion widths in Advanced section, then Slic3r will compute an extrusion width that is slightly higher than 0.4 mm (open the gcode file with a plain text editor, and you'll see the actual values). Might be that this is your problem.

This would be a nice enhancement - extrusions always on the inside of the part - or maybe "CAD mode". I do a lot of prints that have interconnecting parts - I'd rather not design a part for every different slicer. At the moment, I usually finish the holes with a drill or reamer.