Blend surface needs some help please!

:yellow_circle: USING Blend Surface this pipe can be blended with the curved surface without any difficulty, but the result looks like a neat and narrow piece of welding. The resultant weld has plenty of room below the pipe and to the left, but is much cramped on the upper side and at the right, under the acute angle of entry.

The aim is to create a recessed area above the area of entry, roughly in the area I’ve marked with a red oval, and smoothly blended—with the much or all of the curved side (green).

In order to create more space for a blend, I have cut out increasingly large chunks from the curved side but haven’t solved the contradicting need to have beneath the pipe a conventional, broadly convex blend and above the pipe a concave blend. I have tried all sorts of techniques including creating an intermediate surface, which is only slightly successful, being un-smooth and unlovely.

I think this has to be doable one way or another; I wonder if anyone can suggest the best or fastest way please?!

A simplified file and illustration is alongside!


Pipe entering curved surface at acute angle.3dm (3.0 MB)

Dear Clive - can you please post a sketch / hand-drawing / scribble or a section of what you expect the blend or transition should look like. sorry - that i can not imagine the surface you tried to describe.
what do you expect the red section to look like (left) ?
one approach, trimming based on rolling ball (middle)
one approach, trimming based on distance from edge ( right)
but it seams you search something more fancy :thinking: ?

kind regards -tom

Hi Tom,

Thanks for your interest!

I’ve attached a screenshot that may help describe the problem ~


:white_large_square:—a cross-section in wire frame, being a vertical plane, set in Top view at 45°
:black_large_square:—represents the original, overall convex rounded surface
:blue_square:—represents the cylinder entering that surface, split on the surface (and with lower part hidden)
:purple_square:—represents a sketch of the desired recessed area above the area of entry. N.B. this recess is within the original convex surface, i.e. is lower than the original black profile
:red_square:—I have left in the red oval, as the indicated area to recess (to compare with earlier screenshot)

Rhino’s default options for Blend Surface do not seem to allow for the customised recess. I suspect that, either there is some wrinkle in BS (add shapes?) that I’m unfamiliar with or, that the task needs dividing in some way to meet the contradictory goals.


so you want a transition like this ?

i think you need to build a custom sweep2 with a more complex setting of cross-Sections.

if you want to use _blendSrf for what ever reason - you will need some helping-surface.
do a first _blendSrf (pink) from the cylinder to the helper. G2-G1 / Curvature to cylinder, tangent to helper.
(forget the helper)
do a second _blendSrf from the first to the sphere…
just very fast to show the concept - still not sure if this is, what you search:


Many thanks for your suggestions.

You’ve given me a few ideas. I’m trying to create that significant recess, while avoiding the look of a weld and creating more of an organic shape.

The attraction of Blend Surface was the smoothness possible over a large area but I now think I was hoping to do too much with a single automated tool.

Only the outer edges of the green (your blue) area need to stay fixed. I’d like to use as much as possible of the rest, in order to create the smoothest shape.

I’m now thinking that lofting or sweeping between four or more rebuilt curves (?) may produce the customised result I’m seeking.

:yellow_circle:—FURTHER research has yielded a relevant tutorial on YouTube. Though the cylinder equivalent is sitting on a horizontal base and slope is more shallow, it is still the closest I’ve seen to the case I mentioned ~

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