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Slic3r Generates Wrong Dimensions?

Posted by coelestinian 
Slic3r Generates Wrong Dimensions?
July 16, 2013 02:28PM
I am trying to get accurately sized prints, including inside dimensions.

Right now, I'm using a 20x5mm cylinder with a 10mm square inside hole as a calibration piece (file attached).
I priny a piece 19.6mm in diameter, 4.55mm high, and 9.69mm on the inside.
Looking at the gcode, the nozzle moves 10.482mm with a .52mm extrusion width, making a 9.962mm hole.

At the top, the gcode file includes the line:
"; A 10mm square inside hole measures 9.42mm (~ the same as an extra perimeter)."

Does this always happen? Is there any way to make a 10mm square inside hole measure exactly 10mm, with a tendency to be too large rather than too small?

I am using Slic3r 0.9.10b with a .35mm nozzle, blue Protoparadigm ABS.

Edited 2 time(s). Last edit at 07/16/2013 02:29PM by coelestinian.
open | download - Ring.stl (21.1 KB)
open | download - Ring.gcode (142.7 KB)
Re: Slic3r Generates Wrong Dimensions?
July 17, 2013 10:02AM
I believe this is a fundamental problem of the printing process. If you print a circle or any other curved perimeter the material shrinks in direction of the center, like a rubber band.

I'm having similar problems, this is the reason many people use polygon holes this reduces the shrinking, but I'm not satiated with this solution especially if you use like me a profession CAD program the generates always round holes.

I'm fighting this for a long time. But every time I calibrate my printer to the rectangular shapes, I mess up the round ones and vice verse.

The worst thing on this is the perimeter loses contact to the infill.

To compensate this I think you need a factor depending of the curvature of the line.

In the hydraRaptor blog was a nice post about the problem.
I although believe the problem increases with a smaller nozzle size, after long trying I got good results with a 0.5mm nozzle, my new printer has 0.35 nozzles and I still got no satisfying results.

Re: Slic3r Generates Wrong Dimensions?
July 17, 2013 10:04PM
That sounds reasonable, and may account for part of the problem. However, this hole is square, not round.

I might try out some different gcode generators instead... does Cura do the same (only small round holes should really be affected)?
Re: Slic3r Generates Wrong Dimensions?
July 22, 2013 12:47AM
I maintain it's an issue with Slic3r. When I first mentioned holes being too small people came up with a hundred things that might be the issue. Fact is Slic3r does not calculate something right. It sounded like I was alone and no one else ever heard of it but I have since heard many, many people have the same issue.

I have since given up and just make a guess at how much bigger to make a hole. I am getting better at it.

It's got nothing to do with shape of the hole and I am pretty sure it's not shrinkage but just plane old malfunction in Slic3r calculation.
Re: Slic3r Generates Wrong Dimensions?
July 23, 2013 04:10PM

I was having the same issue, and I thought to be the only one with this problem, but now I know that it is a general issue.
I have also created this object to test the cylinders: [www.thingiverse.com]
They are basically 6 small couples of cylinders of 3, 4, 5 mm of diameter, one with hole, one not, because slic3r (but also skeinforge) creates different sizes based on the fact the cylinder is a block or it is holed. So, two objects of same CAD size, gives two different printed objects in size.

Try it, and post some results, I am curious. Since the objects are really small, macro photos give much more impression that the printer is out of alignment (and it is, when you print at 0.1mm layer and the layers are not 0.1mm perfectly aligned in vertical angry smiley ; even a 0.1 of shift is well noticed !)

I printed in PLA and the cooling is the key of success. I recognized that my second print with a fan VERY near the object gave better results.

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