Hi jehan, thanks for posting the file to look at.

There are a few different things going on here that make things difficult for sweep - the biggest one is that your rails don't touch at their ends, there is a slight overlap between them:



Here I moved one off to the side slightly so you can see the overlap:



Having those rails overlapping each other instead of ending at the same point is causing the worst problem - that creates a really crazy type sweep situation in that area where the generated surface will try to fold back inside of itself.

It looks like this may have come from doing a mirror where the mirror axis was not quite in the right place, like maybe the mirror axis got snapped on to some other object instead of the endpoint of this curve like you wanted.


Then actually on top of that there are a few other difficult things, such as having non-planar rails which will cause the profile curves to kind of tilt forward or backward as they move through the sweep. You may need to add an additional profile down towards the end of the sweep to try and keep control of the profile's "up" orientation as it moves along these rail curves that are curved in 3D.

Then also the dense rail curves are ok for some situations, but tend to cause problems in the area approaching the tip which is usually a kind of a problem area.

You may need to redraw those with a fewer number of points, there seems to be some slight wiggling in the ends where the pieces come together, that is getting exaggerated by the sweep and making wiggles in the sweep near that end.

I think that with those very slight wiggles in the rail curves in that area, that tip is just not going to work well with these rail curves.

- Michael