I would like to have a closed NURBS (circle), which has 3 sine waves in its z direction. I started off by using NURBS circle and then deleting 2 segments. Now I can easily make the sine waves by manipulating the segment points z coordinates. Since I deleted the segments, it’s no longer a even circle in the xy plane.
Is there a way to force the NURBS to become round again or is there a better way to model this?
It’s not clear to me what you want, but here’s a way of modelling a sine wave. It’s a NURBS surface with a curve modifier.
Personally, I prefer to work with meshes as they are more intuitive to manipulate.
Attachments
NURBS000.blend (314 KB)
What I wanted was something like this:
A ordinary circle which has its outer z coordinates deformed so they form 3 sine waves.
The way I currently modelled it was:
 created the 1/3 circle in svg
 imported svg to Blender
 converted Shape to Mesh
 deleted all inner Edges (conversion to mesh did a pretty bad job)
 recreated inner Edges/Faces to be concentric
 translated outer Vertices smoothly in z axis (needs some adjustments in properties)
 mirrored 1/3 circle 2 times (1 time on rotated Empty)
I would like to do it via NURBS but using the Curve Modifier I did not even get anywhere near a behavior I understood.
OK, I finally got it. Here is a brief tutorial.

Add a NURBS circle.

Tab into edit mode and W>Subdivide.

Alt+Shift+S followed by 1 to place all control points on a sphere. The control points are not evenly aligned, so here you might want to rotate some of them around the centre.

O to enable proportional editing. Select three control points and G, Z to move up.

O to turn proportional editing off. A to select all control points and E, S to extrude/scale in. Maybe repeat twice.