OD to size, ID always undersize

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over-$-pent
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Joined: Thu Apr 10, 2014 11:11 pm

OD to size, ID always undersize

Post by over-$-pent » Sat Jun 14, 2014 10:44 pm

I have noticed that the OD on cylindrical parts is always within .001, but the ID is always undersize by around .02. I have had to modify the solid models, output a new stl, and then print to get it correct. I have used standard S3D settings with no changes and still have this issue. Material is Taulman 618 nylon. Any ideas as to why this would happen or S3D settings to change for remedy? Thanks in advance for any advice.

Dave

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ednisley
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Re: OD to size, ID always undersize

Post by ednisley » Sun Jun 15, 2014 12:43 am

over-$-pent wrote:Any ideas as to why this would happen
Nophead did the definitive study on why fused-filament circles come out too small:
http://hydraraptor.blogspot.com/2011/02/polyholes.html

Edit: He writes up another effect:
http://hydraraptor.blogspot.com/2014/06 ... wrong.html

You can measure the error on your printer with a set of post-and-hole test pieces from Thingiverse:
http://softsolder.com/2013/04/21/makerg ... t-objects/

His solution: convert vertical holes into circumscribing polygons, with the number of sides determined by the hole diameter. That's trivially easy with OpenSCAD (the sample is for my Thing-O-Matic, but I use the same module with the M2):
http://softsolder.com/2011/11/25/thing- ... libration/

Doing it with a CAD program might be more difficult, but perhaps you can could create a library of pre-calculated cylinders that make common holes.

The slicing program can't fix this, because it sees just a list of triangles. You must build this knowledge into your models: pre-distorted STLs that produce holes of the proper size. Ugly, but that's how it works.

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