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In the attached I duplicated a torus and edited many of the faces out, essentially get windows onto the original Torus object. I've tried various things like adding a Solidify or Extruding the faces on the edited object, which doesn't seem to work. Nor does trying many variation on the Boolean parameters. I would think I'm overlooking something fairly obvious. enter image description here

I have searched the other questions and they seem to mostly have to do with normal directions, which I don't think apply here.

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    $\begingroup$ Blender's Boolean operators don't work well with surfaces that coincide and don't do well with very large objects. I thought to shrink/fatten the Holes object to make it smaller than the original Torus and then add a Solidify but 2.93.6 is taking forever to perform operations on it on my Windows box. $\endgroup$ Nov 21 '21 at 22:58
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    $\begingroup$ If you're using a boolean, it expects a manifold mesh so it can cleanly distinguish inside from outside-- a mesh without borders, identifiable by select non manifold. A solidify modifier should eliminate border problems, as you mentioned, but the modifier order matters. A solidify after the boolean does nothing useful for the boolean, because the boolean doesn't see the solidify. A file demonstrating what you want to boolean would probably help people identify your particular problem. $\endgroup$
    – Nathan
    Nov 21 '21 at 23:42
  • $\begingroup$ Thanks @Nathan, I did include the blend file in the first sentence of the question. $\endgroup$
    – james_t
    Nov 23 '21 at 17:21
  • $\begingroup$ Oh, I'm sorry for missing that! I'll take a look. $\endgroup$
    – Nathan
    Nov 23 '21 at 17:26
  • $\begingroup$ @MartyFouts -- thanks I have seen other comments related to the Boolean being quite an imperfect solution for may things. I'll try other directions. $\endgroup$
    – james_t
    Nov 23 '21 at 19:10
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Your Torus is a manifold mesh, like it should be. Your Holes object, which you are using as the target of the torus's boolean modifier, is not, so the boolean doesn't make any sense to Blender. And, it's laying on the surface of Torus, with aligned edges, so there's really not any difference between meshes to boolean away.

We can give Holes a solidify modifier, which will turn it into a (nearly) manifold mesh, at which point our boolean ought to work:

enter image description here

Notice the settings on the solidify. I set the offset to 0, so that Holes would thicken in both directions (it's no good if it just thickens outwards, it's lying on the surface of Torus.) I tuned the thickness to eye, and you can see the results in the left window, where I've just got Torus visible: Torus now has thickened Holes cut from it.

I said almost manifold. Holes has some faces like these:

enter image description here

When we solidify Holes, that turns into an edge shared by 4 different faces, which is a big no-no. Calculating normals from a mesh with edges like that is unreliable at best. In your file (exact solver), in 2.93.5, it works. I would expect it to break in older versions of Blender. It still leaves the post-boolean Torus with some non-manifold geometry, which might be a problem for further operations.

If you want to fix that, you might give some thought about exactly what you want these kinds of holes to be, and either clearly join or separate Holes's faces that are joined by only a single vertex.

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  • $\begingroup$ Thanks, Nathan. I'm investigating whether I can do something like Select Interior Faces and then Rip Vertex, to create separate panels on the Holes object. $\endgroup$
    – james_t
    Nov 23 '21 at 19:27

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