# Creating a hollow 'solid'

Hi all,

I am trying to create a hollow pyramid with approximately inch thick surfaces.
My approach was to create a pyramid and subtract a smaller pyramid using ‘boolean split/difference’.
This doesn’t work since the surfaces do not intersect.

Is there a better approach? If not, what am I doing wrong?

Thanks you very much for your help!
Best regards,

Edwin

You’re not doing anything wrong. You now have two sets of NURBS surfaces, one describes the outer surface, the other describes the inner surface. There is no ‘solid’ material in the space between them.
The best way to demonstrate this for yourself is to slice your pyramid in half. Observe the inner and outer surfaces.

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You cannot directly create “disjoint” solids in Rhino - it’s a somewhat philosophical limitation currently. If you select your inner and outer pyramids and run first _NonManifoldMerge, then _CreateRegions on the result - you will get two objects - one will be the hollow pyramid you want, the other will be the inside volume, which you can delete. It is likely that this object may end up falling apart into the original two volumes if you do certain operations in Rhino with them - as Rhino is not really designed to work this way.

–Mitch

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In other words, since what you call “pyramids” apparently are polysurfaces consisting of 4 triangular surfaces, they aren’t really what Rhino knows as “solids”. If you closed off the bottom of each of your pyramids with a square surface and joined all 5 surfaces you would have two solids which you could probably boolean successfully. I say probably because boolean ops are sensitive to intersections. Your “inner” solid would probably need to have it’s base very slightly outside the “outer” one.

Or, of course, you could start by simply adding the bases as above and doing the boolean on the bases to subtract the inner square from the outer one, then do the join to make the solid if that’s what you want to end up with.

If you want all 5 sides to have 1" walls, you’re out of luck. Rhino doesn’t know anything about that kind of object. If it’s really important to you to have something like that you can always pierce a small hole of some shape through one of the walls to connect the inner and outer surfaces. This will create a valid Rhino solid.

I get this problem all the time preparing models for 3D printing. As AIW says, make a little bridging surface somewhere inconspicuous so that inner and outer surfaces are contiguous.

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