Hi Eric, thanks for reporting the SketchUp problem.

> Is it a user fault or is it a bug?
> I used the default meshing option, and tried both weld edges and not.

It looks like a bug, it will take a bit of examining to see what the problem is, thanks for posting the example file.

One thing you can do right now to tune this up is to alter the model a bit. Right now the top planar face of your curved piece looks like it came from a sweep:



If you delete that top face (and the matching one on the bottom), then select the object that now has open ends, and then run the Construct / Planar command, it will create caps that are made up of simple 4-point planes that have been trimmed to that outline. Simple plane surfaces like that will export better to Skp format, it will become just one single face in the .skp file. (EDIT: same thing that Pilou mentions above)


Another thing is that SketchUp does not seem to like very long and skinny triangles very much, you may get some problems or also just a generally rough look to your model if there are very thin triangles in it, such as in this area:



There are some meshing settings that you can adjust to try and reduce that. One relatively easy way to tune it up is to enter in a value for Aspect ratio limit. Here I entered a value of 6 in there, which will subdivide polygons that have a long side more than 6 times the length of the smaller side. That results in this kind of a mesh structure instead:



Unfortunately that will also drive the polygon count up quite a bit as well and SketchUp is also kind of sensitive to that as well, so it isn't automatically always better to do this. But if you want a smoother look to shapes in SketchUp it can help.

Here is the result in SketchUp with simple plane caps and reduced skinny triangles:



I will be looking at your example file today though to see if I can find out where the problem is, it is probably something like SketchUp not liking to have duplicated points or points that are too close to each other in a polygon outline.

Unfortunately SketchUp is rather more sensitive to how its polygons are arranged than a typical polygon modeling program. It may take a few passes to identify situations that it does not like to handle.

- Michael