Hi Paul, this is the "auto-place" mode kicking in when you don't want it to.

If you have the profile curves for a sweep positioned away from the rail curve (specifically if they are outside of the bounding box around the rail), then auto place mode will kick in.

This is to allow for drawing shapes on a plane and having them moved and rotated automatically into place on the rail, like in the example that Pilou shows above.

You can also have multiple profiles for auto-place mode if they are all on the same plane, see here for another example. MoI is interpreting your curves in the same way.

To make your example work the way you want, you will need to make your circle slightly larger, so that it is touching or going through the profile curves instead - you can do this by scaling it up slightly from its center point. That will cause MoI to use the profiles exactly from their current positions instead.

I have been meaning to add some UI widget for controlling auto-place mode more explicitly, that's something that I am going to be looking into for v2 when I get a chance to do a batch of sweep related stuff.

Maybe I can also improve the automatic "auto-place" mode detection to turn it off when your profiles are on opposite sides of the bounding box like in your case here.

But a slightly larger circle rail will fix it up for you now.

- Michael