Well, depends whether you want a start and end tangency of the twisted profile or not.
If you want both, a proper middle shape and perfectly aligned controlled start and end direction, then it takes a bit more steps, but the end result is worth the time.
- Build one straight line between the centers (or edges) of the closed profile curves (you can skip that step if you use snap to “Int” and use the cross lines inside each profile curve as a guide).
- Extrude each profile curve to the opposite side with the “Extrude straight” tool, by using the aforementioned line as a direction constraint.
- Use “Loft” to create a surface starting from edge A1 through line A2 till edge A3. Tick “Match start tangent” and “Match end tangent” options. Repead that with the rest 3 sides of the profile.
- Use “Match surface” with “Tangency” continuity to either edge of each of the loft surfaces.
- You will notice that the 4 surfaces now get a bit off from each other near the middle. This is why you fix that by using “Match surface” with “Position” and “Average surfaces” options.
Twist profile.3dm (362.9 KB)
First, I tried with “Loft”, by using a middle section rotated at 45 degrees (like what Diego Krause showed below), but then I noticed that it failed to properly match the tangency to the extrusions that I created to help me achieve a fixed start and end direction. If start and end direction control is not important for you, then you can use the method showed by Diego.
You can add an extra layer of control over the lofted surface by using two extra profiles, for a total of 5. Adjusting the 2nd and 4th profiles will give you some interesting results. You can use “Divide” with 5 sections to create 5 new points along the straight line that I mentioned above. Then copy the profile curve to the 2nd and 4th points, as well as to the middle of the line. Rotate these extra 3 profiles with the following angles: 2nd: 10 degrees; 3rd (middle): 45 degrees; 4th: 80 degrees. With history enabled, this is adjustable at any moment before “Join”.
Twist profile - Loft with 5 profiles.3dm (80.4 KB)