SWEEP 2 RAILS doesn't follow guide shapes - what's the trick?

Hello Community,
I just started working with RHINO 8 for the reason of constructing some models for rapid prototyping to later use them in CAM for making wooden parts for stringed instruments (double bass maker).
I this case I tried to model a tailpiece for double bass using the SWEEP 2 RAILS function.

Because I need the piece to have a distinct height at every point of its length, I’ve drawn arched guidelines every 5 cm of the object (similar to how a ship would be constructed).

When using the Sweep 2 rails function, I have the issue that the resulting object does not follow the guidelines exactly. Between several of these arched guidelines the object bulges up resulting in an uneven surface.

Is there a way to get to a nice and smooth mesh without this bulges?
Maybe there is a different way for achieving a better result?
What is the problem the sweep function probably has with my guidelines?

I would be glad if somebody could drop me a hint! Sorry for bothering with with this newbie stuff!

Cheers, Valentin

Hi @V_R_Stock ,

Try using Loft and then BlendSrf between large changes in the form like this…

You can then finish the form with the Join command to make a polysurface. The issue seen with Sweep2 is due to the result being a single surface that is curvature continuous with itself while also trying to pass through the cross sections.

Surface Method DC01.3dm (2.1 MB)

Create the edge curve and a centerline curve.
Create section curves needed to define the shape. The minimum number needed to defiune the shape is best.

ExtrudeCrv the centerline curve to create a helper surface.

Match the section curves to the helper surface using the SurfaceEdge option. Select Tangent continuity.

Sweep2 using the edge curve and the edge of the helper surface (NOT the centerline curve) as the rails. Select Tangency continuity to the helper surface.

Mirror the surface and Join (or MergeSrf if a single surface is needed).

If a surface normal to a plane then it will have curvature continuity when mirrored across the plane. That is the reason for creating and using the helper surface.

When modeling a symmetric object it is usually better to model half of the object with and curves normal to the mirror plane.