Circles made of n sided ploygons cause endless issues

Is there any work around the issue of circles being made up of n x straight lines? I spend more time fixing imprecise / broken models because of this issue than doing anything constructive. Even with 100 sided circles, it’s an issue. Reading multiple forums on this issue, it would appear to be the no 1 deal breaker with Sketchup. Trying to get lines to be tangential to polygons is not geometrically possible, so your model will always be a compromise - no issue for architecture or woodwork - but with machine components, it’s unworkable… since most Laser / water jet cutters only work with DXF files, you then have to convert to get the machines to run…

The big problem I ran into early on was not the segmented nature of SU circles but forming structures with mismatched circles. That is, a cylinder, tube, pipe, etc. where one end was a circle with, say, 60 segments and one with 59 or 61. This lead to endless frustration with hidden geometry lines like the cobwebs of spiders on crack, and other annoying problems.

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nope. it’s part of the core. most (all?) geometry-oriented 3d softwares have to do that because it gets rid of π and thus allows the creation of faces. an actual circle would rely on π to get a perimeter, and since π is irrational, so would be the perimeter.
It’s ok in a 2D tool, it’s a computational nightmare in 3d.

Some software will lie about it. They won’t tell you they use segments, and visually round everything. Or do parametric design and nurbs. Sketchup is truthful about it and allows you to pick your degree of precision.

really ? I can think of many more pressing matters. :sweat_smile:

3 things :

  • if you’re making 2d work, and you expect 2d limitations, then you probably should either work on a 2d tool or use one as a bridge. I cut paper with a cameo (and few years back, stuff with a laser), I go from SU to illustrator to the machine. Using a 2d software allows you to quickly adjust your SU output. Using a 24 sided circle, I can quickly round up all the corners between the segments.

  • you’re not limited to 100 segments. I just checked, a 2m diameter circle with 800 segments works fine. And the segments are about 0,78mm long. My laser kerf was about half a mm, I’m guessing waterjet will be thicker. and if I model the elements bigger (say in m instead of mm) then scale down, the segments will become even smaller. The smallest I got was 0,031415 mm (hey, π !) that’s 31 micron. the kerf of my laser was - if correctly tuned, about 10 times bigger.

  • export as a 3d dxf. You see, when you make a circle, even though it appears as a polygon, SU knows it’s a circle. Exporting it in 3d dxf instead of 2d dxf will conserve this property. Alas, you’ll need to clean up the triangulation, so go back to my first point and use a 2d tool as a cleanup bridge.

(this is a 24-sided circle exported in 3d dxf. the circle is still a circle, and the triangulation is a polyline I’ll have to erase)

Capture d’écran 2023-09-05 à 11.26.50
(this is a Ø2m circle with 800 segments scaled down to Ø2cm. now the segments are 31 micron long.)


Something to add to the export as 3D DXF or DWG, turn off Faces when you do that. The edges will export as true curves, but faces will superimpose line segments as well.

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Yeah, I saw it during lunch wondering how long you had this one i reserve, and if you were waiting for this exact question :smiley:

impeccable timing for sure !

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Click in sequence on the scenes tabs of these old SU files for ideas.

Tangent to a curve.skp (93.9 KB)

Tangent to circle.skp (222.4 KB)

There is at least one plugin that creates true tangents in SU - on SketchUcation by @TIG iirc.

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I have an extension that creates DXF files with true arcs with directionality etc. SimpleDXF creates very simple files that can be properly read by most CAM software.

I created this to work along side my CabMaker and CutMaster software.
If you want to send me a PM I would be happy to show you.


Hi Garry

Many thanks for the reply – that would be very helpful – much appreciated. As you can see my email address is REMOVED

Best regards

Tim Graham

I have sent you an email with my skype handle

You shouldn’t post your email or personal information on any forum unless you want your information to be visible for all the bots that wander around the web looking for them.

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Thanks John – much appreciated - I will check it out…

Keep well and best wishes


Thanks to everyone who responded - very useful suggestions - I will try implementing them and see how things work out.

Hi, for DXF export with the arcs/circles for CNC machining, you can try my plugin (here in the middle of the page).
Please read the english help after installation to fully understand the tools.