Sheet Metal Spiral Stair

Hi guys,

I would like to create a bottom cover for a sprial staircase - similar to the picture below:

Here’s part of my model with the created surface.
doublehelix.3dm (711.5 KB)

Rhino has no trouble unrolling it, however the manufacturer tells me, that he cannot put the lasered sheet metal placed into form.

Does anyone have experience with how to make this work… from the construction point of view? It has been done before (as you can see in the picture), the question is how.


Note: The inner radius on the picture is much larger than in my case.

Just because Rhino will unroll it, doesn’t mean it’s really developable. In this case, it’s not, Rhino will unroll the surface, but also tells you on the command line that it’s 4.61% bigger after unrolling - which means the material needs to be stretched. Helix surfaces are not generally developable, technically they are curves in two directions. If you select the surface and do a Gaussian curvature analysis, and hit the Auto range button, you will see that the surface is not entirely green - which it needs to be if you want to form it from flat sheet without stretching.


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a 3 story coke furnace piped into a 3 story lost wax casting operation, for a total of 6 stories, a 4000 lb handcarved wax model, and 80 pallets of investment?

Just kidding thank you for the useful information on unrolling, helix surfaces and Gaussian curvature analysis. I gonna plug this in my mind for fabricated designs.

Maybe - although hiring a couple of Italian custom coachwork guys to hand hammer some sheet metal to shape in a couple of weeks might be cheaper… and lighter…


Hey guys,

my questeion is probably not so much a Rhino than a manufacturing question.

I have that surface … how do I get it into the real world. (Simple unrolling it doesn’t work) I also couldn’t finde a surface that is a developable Surface that approximates this surface well enough

Are there other strategies to achieve that goal. (I wrote a script that triangulated the edges, like a zipper) It works, but the surface looses some of it’s smoothness and it’s a lof of welding.


Several options:

  1. Use an “English Wheel” to roll the shape into the sheet metal. It would need to be made in sections and welded.

  2. Use a sheet metal brake to can fold sheet metal into the triangles and thus avoid the welding. The surface still won’t be smooth. If it is to painted then it could be faired using a fairing compound.

  3. Shrinking with a torch. May be difficult to find someone with the expertise.

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I would bet that @heath has some real world experience with this.

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Looks like a one-off so maybe you could oversize the surface by 50mm or so all round and accept that the plate will stretch during rolling, tell the fabricator that the plate has a theoretical 50mm allowance and to tack and trim on site. Better to remove material than try to fill gaps.

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