Sweep2 creating irregular shape (Rhino5)

I am trying to create a dome on the end of this block.
I have created 7 cross section curves and 2 semi circular rails, however, when I do a sweep2 I end up with a bumpy twisted mess. I have aligned the control points, but the link up the cross sections in an odd way and I can’t work out how to fix it.
Basic.3dm (205.1 KB)
Added 3dm file

Can you upload a .3dm file with the geometry? You can use the vertical arrow above where you type posts.

RailRevolve using one curve. may give a better result if the shape is acceptable.

CrvSeam can be used to adjust the seam locations in close curves. Try aligning the seams in your cross sections.

Thanks for that I couldn’t spot the file upload.
The problem with revolve is that the upper and lower half are not symmetrical

Hi @dave6

First of all, your curves are, well… not curves. They are polylines (short, straight segments), so unless you are going for a very faceted look, that’s not going to work. Other than that, “straight surface with G1 blends” tend to look not so good when they taper, because of the inherent “flat spot” on G1 transitions. The effect becomes even more pronounced when it tapers. But… in the attached file are three ways of doing it: Loft (with the “Loose” option), Sweep2 and BlendSrf. I redid your curves (or polylines, rather) really quickly, so they might not be super precise. You can clearly see that the G1 “flat-to-curved” segments look somewhat brutal, but the Blendsrf approach, I reckon, is the best bid. The “block” is also not quite symmetrical, but I’m not sure of this is on purpose or not.
Basic_JN.3dm (227.0 KB)
HTH, Jakob

Edit: The flat sides also have some nasty, little sliver surfaces, that you might want take a look at!

RailRevolve does not need symmetry.

I assume you are trying to use the two semi-circles which cross as rails, and the closed curves as cross sections. That is a very different arrangement of curves than Sweep2 is intended to work with, and I am surprised the command gives any result. Sweep2 produces a surface, and like any surface in Rhino it has four sides, though one, two or three sides can be zero length. The rails form two opposite sides of the surface and the first and last cross section curves form the other two sides of the surface. Other cross section curves control the interior shape of the surface.