Basic understanding "adjustable curve blend"-tool


I want to model this super basic surface

It is bordered by two curves, which the inner one of is slightly higher (z-axis).

My idea is to:

1. build these curves from fairly simple curves,

2. which I want to blend with the adjustable curve blend tool.

3. Then mirror everything down,

4. make an offset

5. do a two-rail-sweep

My problems already start at #2 with the curve blend tool, which won’t give me a nice G2-blend.

Now my questions:

  • Are there any requirements for the initial curves? Do they need to have a certain type of degree?
  • Can those curves be intersecting like mine, or do they need to be seperated?
  • Will my general process (after I passed #2) give me any good surface I can continue working with?

Thanks for the help!

Well, I’m not sure why the blend tool isn’t giving you a nice result, it should be fine if you have the settins right, but I think you need to back up a bit…what’s your pan for making the actual top “surface?” I don’t see this working.

There is no top surface. It’s just the ring. Maybe a better picture:

I finally got an overall “ok” result, but the curvature seems to have a little dip in the end, which makes the mirroring impossible.

Hi Kaizdd,

I would suggest modeling the smaller curve across the mirror axis with a degree2 / 3cv curve.
This will result in the best continuity:

after this use adjustable blend curve to draw the corner curve. You can use a circle and trim around the intersection to make it more even:

From there you would use 1 rail / 2rail sweep.

Sometimes another way is faster and a bit cleaner:

  1. construct the curves as above
  2. construct the inner surfaces like this:
  3. use _BlendSrf for the corners:

I think the latter is better, since the final goal is always a surface (not curves) and the faster you can get there the earlier you can detect visual flaws/problems with the surfaces. I try to “think” in surfaces when I model, not in a sequence of edges form which surfaces hang of. – if it makes sense :upside_down_face:


Hi Konrad,

this helps me alot. Can’t thank u enough! :slight_smile:

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So after some time I have another question concerning your second method.
You say to construct the curve as above. After trimming with the circle, do I also blend the curves?
and after I constructed the outer curve ring, what is the best way to get the inner?
Offsetting leaves me with with way too many control points

Can you maybe share your file?

Thank you!

Does the inner curve need to be an “exact” offset of the outer curve? If so the large number of control points are necessary because of how NURBS math works. Except for special cases the offset of a NURBS curve cannot be represented exactly by a NURBS curve. Rhino uses enough control points for the offset curve so that it is within the tolerance of the exact offset.

If the Loose=Yes option is selected in Offset then the offset curve will have the same number of control points as the original curve but may deviate from the exact offset.

hm, It was a bit unclear: I would not construct the blend curves( “corners”) – only the sides. Trim with a circle around the corner; then extrude curve ( or extrude curve tapered ). After this you can “blendSrf” the corners. This is not mathematical perfect (the inner curve is not an exact offset) but it is faster and you have a cleaner control point layout. In rhino V7 the blendsrf command is even better an doesn’t add unnecessary control points.

I didn’t know the loose options. Always did the workaround with MoveUVN.
This helps me alot. Thank you!

This makes it clear. Thank you!