Offset of swept surface problem

New here. so I hope I am posting this in the right place.

I wonder what the best way to model this kind of swept surface is, to get it to offset properly.
I guess the problem has to do with the curves tangents.
I prefer not to trim a surface to this shape, but would like to build it with curves.
Anyone got an idea? :slight_smile:

offsetsrf issue.3dm (330.8 KB)

Hi Pelle - as you have it there it will seldom if ever work - the problem is that you have adjacent edges of the surface tangent, or so nearly tangent that the calculation for the normal needed for the offset falls apart there.

These edges are tangent:

If you can make those hard corners and trim to the correct shape, your offset will work.

offsetsrf issue_PG.3dm (112.2 KB)


Thank you!
I understand.

Are there any way to more accurately control the result of a trimmed, complex surface, such as this one?
Simply trimming a surface with a curve in top view, for example, makes it real tough to know what it will look like in front view so to speak.
I guess I’m looking for some sort of “live projection” of a curve onto a surface.

Anyway, I’m looking for a way to get the surface edges identical or at least as close as possible, to the curves in the file.

Thanks again!

Hi Pelle - in this case, the original curve and the trimmed edge are .002 apart - and I was only a little bit careful in moving he corner point. to make the hard corner- the corner point in a case like this only needs to move a small amount. I also extended the edges of the surface (ExtendSrf) so that the trim curves would be completely within the boundaries of the surface. In any case, I think you’ll find the trimmed result is pretty close to what you were shooting for.

To answer your question about trimming though:

  • When trimming a surface with a curve in a plan parallel view like the default Top, Front, and Right view, the cutting curve is projected on the surface in the view direction.

  • When trimming a surface with a planar curve in an angled parallel or a perspective view like the default Perspective view, the cutting curve is projected on the surface in a direction perpendicular to the curve plane.

  • When trimming a surface with a 3-D curve in an angled parallel or a perspective view, the cutting curve is pulled on the surface by closest points.

Also, possibly helpful for:

Both the Project and Pull commands pay attention to History, so if you Project with History recording on, you can edit the original of the Project operation and the projected result will update.