Thanks guys, I understand how complex it must be to run the math on these kind of operations. Hats off to Michael in that regard!
I thought I was keeping things relatively simple in this case as I used the same shape twice and produced the copy via a precise rotation using the center of the circle as the pivot point. In theory the 2 shapes should still be in precise alignment. They certainly look to be aligned when I zoom in like advised. I can only think the sweep operation has produced an object with small surface irregularities that become apparent when you rotate the shapes around the axis.

I attach the model file Michael.

Thanks for the help!

Dave

Hi Dave, yes 2 rail sweep is a kind of general freeform surface construction mechanism. It doesn't make 100% precise geometry it makes freeform surfaces as its output.

If you use Edit > Separate on your shape and then turn on surface control points for the surface you can see that it's kind of stretching out between the 2 rails, so yes it's not symmetrical and uniform like something made up from a cylinder and torus would be:



As the profile moves between your 2 rails it gets stretched out in one direction and so a rotated copy of it will not exactly align to other pieces of itself as an exact pipe that has a circular cross section would.

- Michael

Hi Dave, so for combining these pieces that can be done by cutting each of them with a 45 degree angled line using boolean difference, then delete the common face between them and use Edit > Join to glue the pieces together.

That's what I did to make the closed solid in the attached file.

It will be difficult to try to directly boolean them with each other because of the "barely skimming over nearly coincident surface areas" type of surface intersections that would be required for that.

If you select 2 of the pieces (be careful there's also a duplicate copy of one of them) and create intersection curves using Construct > Curve > Isect, you can see that the intersection curves are quite complex and chaotic:



- Michael

Thank you Michael! That technique worked a treat. In future I'll try to keep my joining surfaces as unambiguous as possible.

Cheers,

Dave

I dl'ed your file and it works if you do what I said earlier. Also: You had 3 objects, I deleted one, positioned the other 2 and presto: a union.

Attached is your file without the 3rd object and the 2 others aligned properly. Select them both and do a union, it works, took only like 3 seconds...