Unroll Surface Help

I have a boat hull that I created that is developable. I created a solid by offsetting the side by the thickness of the plywood. I now need to send this to a CNC machine. My question is, how do I maintain alignment of the two sides so that the proper rolling bevel can be cut. You will find the file attached.
Unroll Help.3dm (113.1 KB)

I don’t think you can unroll a polysurface.
You can unroll one of the large surfaces and that’s what the mill will follow.

You lost me on the “proper rolling bevel” comment.

A polysurface which represents a single developable “surface” can be unrolled by UnrollSrf or UnrollSrfUV.

A solid cannot be unrolled by Rhino and remain a solid.

A reasonable assumption is when a thin sheet which has a developable shape is unrolled, the center surface is the neutral surface and does not stretch, compress or shear.
To model this and the bevels:
Use TweenSurfaces to create the center surface. Do not trim this center surface
Pull the borders of the inner and outer surfaces onto the center surface.
UnrollSrf the center surface and the edge curves.
OffsetSrf the unrolled surface 1/2 the thickness of the thin sheet on either side to create unrolled inner and outer surfaces.
Trim the unrolled inner and outer surfaces using the pulled unrolled curves.

Pull the unrolled edge curves to the unrolled inner and outer surfaces.

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Thanks for your replies. I fear that I have not asked my questions properly so let me try again. I created a developable surface then offset it to the thickness of the plywood, in this case 1/2" (I think). I then had to trim the bottom edge of the surface to the hull bottom. My plan was to extract the outboard and inboard surfaces then unroll those. When I do, I lose alignment of those two surfaces. So my question is, how can I unroll these surfaces and maintain alignment. I can update this file to show my workflow progression if I need to.

Hi Jay -

The answer is still that there is no automated way to do this. The workflow that David outlined should do what you need. Did you try that?
-wim

@majors101 First consider what physically happens when the a sheet of plywood is bent or flattened.

Lines through the plywood perpendicular to the surfaces of the plywood stay perpendicular when the plywood is bent of flattened.

The portion of the plywood on the outside of the bend stretches. The portion of the plywood on the inside of the bend compresses. The surface in the middle of the plywood stays the same size.

When the plywood is allowed to return to its flat shape the opposite happens. The portion of the plywood on the outside of the bend compresses back to its original size. The portion of the plywood on the inside of the bend stretches back to its original size. The surface in the middle of the plywood stays the same size.

(To be exact the actual behavior is more complicated, but the description above is consistent with the assumption that the plywood bends into a developable shape.)

When Rhino unrolls a surface it does not stretch or compress the surface. The surface stays the same size. So trying to unroll the inner and outer surfaces will not result in the correct bevels on the flattened surface. Simple example - Start with a portion of a cone. All edges are 90 degrees to the adjacent surfaces. A piece of plywood with that shape would have 90 degree edges when unrolled. But if the inner and outer surfaces are unrolled individually the outer surface will be larger than the inner surface. Create a solid using those unrolled surfaces and the edges will not be 90 degrees.

The method I described above is an appropriate method to create the unrolled sheet with bevels.

Thanks David. I completely missed your first response. This makes sense and should work. I will post if I run into any problems. Thanks again!

That worked perfectly! Thanks again David.