Rhino Gaussian Curvature / Developable Surface Question

[info@mds.org.nz]

Rhino Gaussian Curvature Question.

Hello all you Salty Rhino power users. I have a question regarding the Gaussian Curvature surface analysis tool.
As I understand it and please correct me if I am wrong a surface with zero Gaussian Curvature is a developable surface. I.e can be manufactured using sheet materials such as aluminium or plywood.

In the attached Rhino file I have attempted to model a hull using surfaces that are developable. I have used the Gaussian Curvature surface analysis tool to analyse the surfaces and make sure they are 100% green which should indicate zero Gaussian Curvature (developable). While using the Gaussian Curvature surface analysis tool I have used the “Auto Range” button to set the curvature range values.
The problem I am having is when I invoke the ‘Unroll Developable Srf” tool within Rhino and select the bottom surface (under the waterline) I get an error stating “Unrolling doubly curved surfaces will produce inaccurate results” but the surface is 100% green….?
Upon playing with the curvature range values and setting them to + 1e-07 - 1e-07 I start to see some concave (blue) curvature up forward but with the curvature range values set this fine I would think the amount of Gaussian Curvature is so minimal it would render it negligible? What I don’t understand is the unit of measure the curvature range values represent. If the range is set to + 1e-07 - 1e-07 dose that mean the surface is + 1e-07 of a millimetre away from being truly zero Gaussian Curvature?

What im worried about is I intend to CNC cut all the frames and hull panels and when I come to lay them over the frames they wont fit!.
Im wondering if some of you Rhino experts out there that use this software to design your hulls can have a look at my hull and let me know if it is developable give me some pointers and keep me from making a very expensive mistake?

Some notes on my hull model that you might need to know

• I have used a plug-in called Orca3D to develop the base hull shape then modified the surfaces to achieve my desired hull shape.
• In the attached rhino file the STBD side is one single surface and the PORT side I have evoked the rhino command ‘Divide Surface on Creases” to divide the hull surfaces into the 4 single surfaces that make up the hull.

Any help greatly appreciated.
Regards
Dax

[Joe Petrich]

Rhino%20Gaussian%20Curvature%20Question-2.3dm

It seems you have introduced some concavity in the bottom. This has resulted in a non-developable surface. If you do a 2 rail sweep using the chine and centerline profile (Mergedge this edge first) as rails and the aft edge of the bottom as the swept curve you will get a developable surface which will unroll. I have done this in the attached model with the resulting surface in red and cut sections (in blue) to compare the two surfaces so you can see the difference.

If this boat is to be built of aluminum and you intended to "torture" the concavity into the plate as you build the boat then you could use the free Advanced Flattening "squish" plug-in from Rhino 4.0 Labs (http://wiki.mcneel.com/labs/advancedflattening). It flattens undevelopable shapes.

[info@mds.org.nz]

Hi Joe
Thanks for your reply. After reading your post I had one of those a-ha moments where the penny dropped. Why didn’t I think of a 2 rail sweep before.
That is just the sort of advise I was looking for and I can’t thank you enough.
The only part I haven’t grasped yet is you mention using the mergeEdge commend. Why have you suggested using this command? I’m not familiar with this command and the help file did not help me any?

I don’t intend on torturing the concave shape into my hull. I will remove it to make sure it is truly developable as you suggest.

[Joe Petrich]

Sometimes through various modeling processesan edge will get split into two or more sections. When you try to use the edge for a sweep the sweep will only use the section you pick, not the entire edge. Mergeedge joins the edges together again. If for some reason you would like to keep the edge split but still use it for the sweep there is an option you will see in the sweep command called (chain edges). When this is invoked you can pick each individual edge without having to merge them.

[info@mds.org.nz]

ok I got it. Cheers for that little bit of advice.
Another question, have you got any clues regarding what the unit of measure is the curvature range values represent in the Curvature surface analysis tool? . If the range is set to + 1e-07 - 1e-07 dose that mean the surface is + 1e-07 of a millimetres (or whatever the model units are set to) away from being truly zero Gaussian Curvature? (see attached screenshot Joe)

[shellexpansion]

have you tried the software <Advanced Smart Unfold for Rhino3D> from solid3dtech? It should solve your problem.
se