Unable to select entire surface for unrolling

I’m attempting to flatten a solid (attached to this post) that I have imported from a different CAD software, but am unable to select the whole surface to unroll. I have been able to flatten the various sections that Rhino allows me to select that make up the surface, but I would then have to align them manually to restore the correct outline. I am new to Rhino, so if I have missed something obvious please let me know.

problem solid.step (331.7 KB)

Your model is not a single surface.
It is a “closed, solid, polysurface”, made up of 66 separate, Joined surfaces.

It can not be “unrolled” in it’s present state.
Since you’re new to Rhino, I’m not sure where to begin describing the steps involved.

Is this something you will be doing a lot of, or is this a one-off task?

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Thank you for your reply; flattening curved surfaces (including compound curvatures) is the reason I decided to try out Rhino, having been unable to achieve this using Fusion 360. Whilst I had hoped that importing completed models into Rhino for flattening would be a relatively simple procedure, I don’t mind learning what’s involved to prepare them.

G’day @onihr,

There’s nothing substantially hard about unrolling imported solid parts. The basics are that Rhino cannot unroll a part with thickness, as you have here. You’ll need to decide whether you want to unroll the inside, outside or some mid-plane face to get what you want for cutting.

However, your specific model has some issues if you want to 2D CNC cut this specific shape, which only you can solve.

Edges are bevelled.

There is a bump in one of the faces.

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Thank you very much for your in-depth explanation Ncik. I should have mentioned that all the surfaces that I plan to flatten will be laser cut on thin balsawood, so unrolling the part as depicted in the second of your attached images is the method I had in mind when designing the model (a compromise being made whichever side is selected owing to the laser cutter only cutting perpendicular to the stock).

After playing around with various tools in an attempt to duplicate the arrangement Ncik posted, I tried the Join command with partial success. I’ve attached an image capture showing the misaligned portion.

How accurate does the flat shape need to be? Better accuracy may be achieved by creating a surface mid-way between the two outer surfaces, and then flattening that surface. If the inner surface is used the flat shape may be too small; if the outer surface is used it may be to large.