Can sketch up output smooth cylinders and radiuses for CNC?

I need to model some columns, which I intend to cut on my CNC.
I’m going to carve them in two halves (then glue up).

The columns have beads (full bullnose rings).

Is there a way to output these so that carve with no pesky facets?

IMG_20210508_075741 (1)

From what I recall reading, there is an export format supported by SketchUp that will represent circles and arcs as true mathematical curves. I think it is via the 3D DWG format, but perhaps others can explain the true story.

Yes. Exporting a 3D model to DWG creates true AutoCad arcs and circles out of SketchUp arcs and circles. However, this has no effect on the exported faces, they remain facetted and are not converted into NURBS surfaces or other geometrically correct representations of revolved or extruded surfaces.
I don’t know what kind of surface descriptions modern CNC software supports, but directly from SketchUp the only workaround is to add sides to your circles and arcs until the resulting facets are smallert than the resolution of the CNC machine.

This is the half column I’m trying to section, then cut fyi…
Thanks again for your help.


Ahh, sorry I did not read the original post clearly enough. I somehow had the impression that the goal was a 2D CNC device, such as a router or laser cutter.

There is a method for outputting true circles and arcs in 2D for interpretation by a CNC router or plasma cutter or other sheet goods manufacture process.

There is not a method for exporting curves in 3D data. If you are “carving” in 3 dimensions then the method would be as described above, increase the number of circle sides until the facets are not noticeable.

Would you happen to know the real world results as a 3D carve on a CNC?

Hoping to get the radiuses very close to looking like they were turned on a lathe, or concave router bit.

What material are you carving in? If you make the facets very small you might be able to make a passable version with a little processing after. A bit of hand sanding might be enough in MDF or styrofoam.

96 facets is a good start depending on the diameter of your object, it’s generally best to use multiples of 4 to work with circles, SketchUp will accept up to 999 sided circles. I have used 360 facet circles before, one face per degree. This yields extremely smooth circles of course. The trade off is that the numbers of edges and faces quickly climb in to millions making models very large and difficult to work with. In that situation the smallest facets often go below SketchUps edge resolution making tiny faces disappear and causing problems, there are ways around this limitation but you need to know what you’re about to work that way.

Having CNC both true arcs and facets from SketchUp, I can say that without post sanding it is remarkable how much the facets do show. Even when I hav thought the curve looked smooth onscreen the eye can still see the flat edges in the real world object.

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Wow thanks Riley!
This will be carved out of red oak, in one of the columns would be 8 ft tall…

Thank you for the real world advice!

At that scale and that material the facets will show. You would need to sand or turn the whole thing I would think.

I agree with @endlessfix. I have also found that your machining strategy has as big an influence on the amount of post sanding. The smaller the step over the smoother, but also the longer the machine time. Whether you use a sweep, raster or offset toolpath can also have a big influence on the time and quality, all of which depends on material. If you can, machine a scaled down version in red oak first to find a good machining strategy.