Holes smaller than they should be

Hello!

I found some questions like this already, but was not satisfied with the answers, so I hope it's OK to ask again...

The thing is, obviously, that my holes are way smaller, than they should be (a 6mm hole comes out as a 5mm one).
My outer perimeters are nearing perfection as far as I can tell (perimeter test blocks fit precisely), so I think I have the flowrate and stuff calibrated quite well. I know it has something to do with shrinkage and the inner arc of the hole getting too much material and there are basically two approaches to this problem:

1.) Stretch setting in the Skeinforge (which I have tried and failed to see any larger difference with many different values inserted...).

2.) The famous polyholes.

Well as I said, I have not been successful with the Stretch, either due to my wrong settings or some other failure...
The polyholes are a nice solution, but I have been unsuccessful in integrating it into Rhinoceros V4, which is my CAD software (if anyone has any ideas on this, I would be VERY grateful!) and creating all holes as polygons manually is very ineffective (resizing becomes a pain, need to manually calculate the number of polygons etc.).
I have seen many technical prints with parts that fitted and included holes - my MendelMax is a shining example. It has many holes fitting the screws precisely and none looks like it is printed as a polyhole. How come everyone seems to have their holes perfect and I don't? I spent quite many hours trying to figure this one out...

So if anyone can help, I would be most grateful.

Thank you,
Matouš

It depends on the the nozzle size relative to the size of filament you are laying down. If you stretch the plastic a lot then holes shrink more.

Also if the holes from your CAD have lots of very short vertices it can choke the firmware and cause over extrusion.

Do the outlines of the holes look wider than normal or are they the correct width but shrunken away from the surrounding infill?

Are you using multiple outlines? If so doing the outer first will result in a smaller hole than doing the inner first.

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[www.hydraraptor.blogspot.com]

Oh thank you for your answer!
So you say that thinner lines of plastic make the holes shrink more? Or do I not understand you correctly?

I measured some of the hole outlines and you were right, they seem wider even by a naked eye, so I'm probably exporting the models too detailed from my CAM, choking the firmware, right?

I am not using multiple outlines, but if I would, how can I tell Skeinforge to do the inner first? Is that the Perimeter->Loop->Infill setting in the Fill section?

Matouš Wrote:
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> Oh thank you for your answer!
> So you say that thinner lines of plastic make the
> holes shrink more? Or do I not understand you
> correctly?

Thinner lines through the same size nozzle means the plastic is stretched more. So I think it pulls towards the centre more when being dragged round in a circle. Another way to think of it is it likes to cut corners when stretched more and a circle is a continuous corner so cutting it gives a circle with a smaller radius.

>
> I measured some of the hole outlines and you were
> right, they seem wider even by a naked eye, so I'm
> probably exporting the models too detailed from my
> CAM, choking the firmware, right?

Yes maybe. There is also an effect where you need slightly less plastic for the same thickness when forming a ring than you do when forming a line. The slicer should take this into account but I don't know if any do other than the reprap host.


>
> I am not using multiple outlines, but if I would,
> how can I tell Skeinforge to do the inner first?
> Is that the Perimeter->Loop->Infill setting in the
> Fill section?

Yes.

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[www.hydraraptor.blogspot.com]

OK, I have tried several different settings and for my 0,5mm nozzle with 3mm fillament I have been able to reduce hole shrinkage from approx. 1mm to about 0,2mm on circle holes and no shrinkage on square holes. I will try compensating the remaining 0,2mm with the Stretch function.
The problem was too wide lines of plastic and too detailed meshes causing firmware chokage, as you implied. I have reduced the line size approximately to the size of the nozzle hole, which helped a lot and played with export settings in Rhinoceros for STL as well. Hope this helpes others experiencing the same problem!

Thank you for your advisement, Mr. nophead, I was getting a bit hopeless on this one!

I'm having the same problem, but I'm using Slic3r. What Slic3r settings can I use to reduce the streching of plastic on my hole perimeters?

Same problem here with slic3r.

I'm sorry, I have been on a vacation this week, so I coudn't have answered.
The problem with holes having too many vertices (too detailed meshes = stl models) can be fixed in a 3D CAD program, such as Rhinoceros CAD or something similar, which can save and edit stl files and meshes. You may find many lighter and free alternatives to Rhinoceros.
In the CAD you must reduce the level of detail on the model you are having trouble printing. This will reduce the number of vertices, that the holes in this model are composed of and therefore help your firmware handle the G-code,
The other problem with holes - shrinkage due to too little fillament or too much fillament in the inner arc of the hole - can be solved by tinkering with the thickness of the line the printer is laying. This can be set in Skeinforge under Carve as "Edge Width over Height (ratio)".
In Slic3r I believe you can set this under Print Settings -> Advanced -> Extrusion width -> Default extrusion width, First layer, Perimeters and so on (the Perimeters setting is the most interesting, concerning this problem).
I think Slic3r has no functions like the Skeinforge "Stretch", so you will have to do with what you have.

I hope this helped! Good luck making your holes nice and wide!

By the way I believe, that the problem with the holes has not been adressed properly in the 3D printing community and there should be some general way to convert holes in the models to printable versions, be it polyholes or just slightly wider and less polygon holes... It could be integrated into the slicers as an option or someone could write a simple script to do this, I think it would help many people out there as it is probably the best way to deal with holes, as is my understanding.

Matouš Wrote:
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> By the way I believe, that the problem with the
> holes has not been adressed properly in the 3D
> printing community and there should be some
> general way to convert holes in the models to
> printable versions, be it polyholes or just
> slightly wider and less polygon holes... It could
> be integrated into the slicers as an option or
> someone could write a simple script to do this, I
> think it would help many people out there as it is
> probably the best way to deal with holes, as is my
> understanding.

This is a tricky thing to accomplish universally for RepRap/FDM, since it will depend on material used (ABS/PLA, and differering characteristics between suppliers), layer height, nozzle size, printing/bed temperature, etc.

A case-by-case solution, as nophead suggested, is the Stretch module in skeinforge just the "script" you're looking for, I think.

Or, a drill. =)

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I wonder if the parts on my printer get nervous when they start creating better looking versions of theirselves.

Yes, you are surely right.

>This is a tricky thing to accomplish universally for RepRap/FDM (...)

Well I believe that this is the case for most of the problems with 3d printing...

>A case-by-case solution, as nophead suggested, is the Stretch module in skeinforge just the "script" you're looking for, I think.

The Stretch does not convert anything to polyholes as far as I know, it only increases the stretch of the plastic by speeding up the feed rate. Converting to polyholes would be additional way to print better holes.

>Or, a drill. =)

Of course do I use a drill! But sometimes you want holes of a different shape than a circle. It is true though, that other shapes are not as problematic as circles.