Cavity inside a closed polysurface

[James_Walters]

I’m stumped on a pretty simple problem I’m not approaching right.

I’m trying to create the cavity inside of this gear. The highlighted cylinder is a closed polysurface. Boolean operations did not work. Any help would be appreciated.

[maje90 (Riccardo Majewski)]

Boolean difference of a Box in Box

Basically, Booleans do not work on objects that do not intersect.

“Solids” in rhino, breps or polysurfaces, are a “patchwork” of surfaces enclosing space.
When there are no open gaps between the surfaces we can call it “closed solid”, but it still is a patchwork of surfaces.
For a solid boolean operation to work, it needs the surfaces to intersect. To “hit” / “touch”.
If your cavity is completely inside of the solid, there is no intersection.

You can create a very thin hole that will connect the cavity with the outher shell.

See these workarounds:

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

Solid Difference to Make Hollow Sphere

Hi All, I understand there are plenty of solid difference posts on this forum, but my problem seems so simple and I still cannot figure out what is wrong. I am trying to create a hollow sphere by performing a solid difference on two sets of spheres, one bigger than the other. When I use the solid difference component, I get the resulting volume as the same as my original large sphere. When I swap the inputs I get the volume of my smaller sphere. I think this might have to do with the spheres be…

(note also the solution given by PeterFotiadis at the end)

[Gijs (Gijs de Zwart)]

In case it is for rendering or 3d printing:

Extract the meshes.
Invert the inner mesh
Join the meshes