AutoCAD - Settings that may increase resolution for 3D printing

[Brian McKenzie]

Another 'variable' for AutoCAD users to experiment with. 'FACETRATIO', in addition to 'FACETRES', may further help smooth ugly faceting when creating STL files for 3D printing.



The default setting of the system variable FACETRES is 0.5
Change this to 10 - by entering FACETRES on the command line, and then type 10.

The variable 'FACETRATIO' controls the aspect ratio of facets on rounded 3D bodies. The default setting is 0. Change this to 1, after entering FACETRATIO on the command line.

As can be seen above, the file size grows massively when FACETRATIO, now changed to 1, is used in addition to the FACETRES variable when set at 10. I couldn't see any visual improvement when trying it with FACETRES left at its default setting of 0.5 (as for the coarsely faceted cylinder shape above left), but the file size did increase from 5 to 26kb.

Whether tinkering with FACETRATIO translates into improved 3D prints remains to be seen. It doesn't, see next message.
(Note, to see these changes within exported STL files, it may be necessary to view with 3rd party software.)

-Brian McK.

 

[Brian McKenzie]

An update:

Sadly, changing the FACETRATIO variable is of no benefit to increasing resolution of STL files for 3D printing. While it did enhance the onscreen view of the cylinder shape at right in the previous message, it does not actually increase the number of Facets in a model, as we would welcome, but merely increases the number of Triangles making up individual Facets.

Of interest, and desirable, is that by changing the FACETRES variable from 0.5 to 10, the number of Facets around the circumference of the cylinder example increased from 24 to 98.

Each of these Facets on the circumference comprises of a pair of Triangles. When the Facetratio variable was changed from the default value, the number of triangles increased to 39 pairs of triangles on each Facet, distributed along the length of the cylinder* - all describing the same flat face as the original pair did - so they will be of no additional benefit for improving 3D prints.
*100mm long x 20mm dia

-Brian McK.

 

[mickoo]

It might have an impact if it were curved or coiled, or the shape were more complex?

MD

 

[Big Train James]

All it appears to do is sub-divide any given facet into additional triangles, so it shouldn't have any additional impact on more complex models, like a spring. Basically, a facet is divided into a "grid" of triangles, with Facetratio=0 being 1 triangle wide by x high. Facetratio=1 apparently divides a facet into a grid y triangles wide by x triangles high, by virtue of limiting the size of the triangles, so more triangles are required to comprise the same facet. We need something that increases the number of facets, which is what Facetres appears to do.

What is counter-intuitive to me is why this would be something desirable? It would appear to have no benefit unless your goal is to increase file sizes. Unless it's something to do with increasing control points for surface editing, but that doesn't exactly sound right either .

Another question I have regards the comparative image above with the three variable settings. Is this from an STL viewer? Increasing Facetratio clearly has had an effect on the representation of the object, as well as the file size. But that doesn't make sense if the number of facets hasn't changed (meaning the appearance, not the file size).

Jim

 

[Brian McKenzie]

Jim,

The views in the comparative image are from when the STL files ex AutoCAD were opened in SolidWorks. The extra 'smoothing' displayed is probably no more than the graphics card working harder, because it has more triangles to deal with. The files were viewed later as wireframes, after installing the free software, 'Open3dmod'. The 'quiltwork' of the extra non-beneficial triangles was plain to see.

This odd AutoCAD function (Facetratio) is not replicating other software, where more triangles yields a smoother finish with less lumpy facets.

-Brian McK.