Hi Jeff, well after browsing through that thread a bit I can see that you're very familiar that offsetting a polyline is not the same thing as offsetting an arc! ;)

It is surprising that there is so much disagreement over there about this - I mean if you just draw a snapped "half circle" arc in Sketchup and then offset it, the offset result is not a half circle anymore... But it's because you didn't really have a half circle in the first place, you really just had a polyline in the general shape of a half circle.

That's all a consequence of having all geometry made up of little line segments and facets. If you're trying to do stuff involving curved shapes sketchup becomes problematic in a lot of ways for those kinds of tasks, particularly if you're trying to do "curved shapes with accuracy"...

- Michael