Hi, BurrMan!!
My AI wrote you a couple of lines here )))

"Could you post the .3dm with the problem scene (the segmented pipe with the pieces removed, before Blend)?
I'd like to look at the underlying geometry and see whether it's something the command should do differently."

I'm waiting for the file with the problematic scene (cubes, pipe);
let's try to figure out what the problem is.

Hi Burr, I think that you're probably observing how Blend works differently on "natural surface edges" verses "trimmed edges".

For example here with 2 surfaces like this:


If you select those surfaces and use Edit > Show pts to show the surface control points you can see the UV structure of the control point grid.

Notice how the vertical and horizontal edges are going in the same direction as the control point grid. These are called "isoparms".

The diagonal edges are not isoparms.



If you select 2 of the isoparm edges and do Construct > Blend, you get a result like this:





The directions used with edges that are isoparms will use the other isoparm direction like this:



If you select the 2 edges that are not isoparms you will get a blend like this instead:





For edges that are not isoparms the blend direction will be perpendicular to the edge and not in an isoparm direction.

So it will be like this:



And not this:



When I get a chance to overhaul the Blend command I would like to set up an option for controlling this. But currently if it's an isoparm edge it will align to the other parametric direction but if it is not an isoparm edge it will align perpendicular to the trim edge.

- Michael