Скачать или смотреть Pro tip: How to build clean Blend surfaces

A rational surface means that its control points have a variable point weight ratio. This causes lots of issues with the Blend surface tool due to its reliance on identical point weight. The default point weight of free-form NURBS surfaces is 1 (identical for all control points), and "Blend surface" produces clean output geometry only with these.

"Loft surface", on the other hand, ignores the point weight, thus it produces clean geometry even if the input surfaces have a rational (variable) control point weight.

Once the large bottom surface is modified with the “Change degree” tool to match the same amount of control points (4 total) like the opposite surface, it becomes rational, which is causes lots of troubled with certain modeling tools, such like “Blend surface”. The solution is to either:
a. Set the control point weight back to 1;
b. Use the “Rebuild surface” tool. “Change degree” produces errors, so I recommend to be careful with it;
c. Use “Loft” with tangency option to both sides.