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I'm trying to perform a boolean difference to cut a piece out of this treasure chest lid to change to a pop-in hinge: Original lid The model is an STL from Thingiverse. I created a shape that I want to cut out of the lid in Fusion360, exported it as an STL, and imported it into Blender: Cutting shape I lined them up in Blender and then added a boolean difference modifier to the treasure chest lid, setting the cutting shape as the object to subtract. Shapes lined up Instead of cutting a piece out of the lid, it looks like it performed a union instead: Result of difference The original cutting shape is hidden, so what is visible here is the result of the operation. It looks like it unioned the two shapes.

I checked all the common reasons for problems:

  • Normals - Normals look fine when I show them. To be safe I opened each object in edit mode and did Mesh > Normals > Recalculate Outside. No change.
  • Duplicates - I attempted to eliminate duplicate points using Mesh > Clean Up > Merge By Distance for each object. No change.
  • Non-Manifold Geometry - When I go to Select > Select All by Trait > Non Manifold, nothing is selected for either object.

What else could be causing my boolean difference to fail? What can I do about it?

Here's a Dropbox link to the Blender file.

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Your treasure chest 113Mb blend file was to large for my machine to handle you may want to "clean up" the STL before you upload files. Do a "Limited Dissolve". http://youtu.be/-OynHqYd_1I

Blender stackexchange has it's own file attachment system to help prevent files being "lost / moved" http://blend-exchange.giantcowfilms.com

Also are you going to 3D print this out? if so there's certain offsets when doing a Bool cut you need to take into account. Like making the cutting object about 0.2mm larger.

Are you using Blender 2.91? The "Bool Tool" using the Exact option may also help.

Here's an animated gif of the process using a box and your lid cutter.

Animation of process

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    $\begingroup$ Upgrading to 2.91 (from 2.90) did the trick. I chose "exact", but it still didn't give a correct result until I checked the "Self" box to allow self-intersection. I'm not sure what that means, but since it wasn't necessary in your example with the cube, it must be an issue caused by the treasure chest lid STL. I was unable to get blend-exchange to work, perhaps because my file is too big. Since my problem is solved I wasn't going to bother cleaning up and trying again, unless you think it would be helpful for future askers. $\endgroup$
    – Robert
    Dec 14, 2020 at 21:06
  • $\begingroup$ One further question: could you elaborate on what you mean about making things 0.2 mm larger? Is that just to compensate for normal 3D printer expansion, or is it something specifically related to doing boolean offsets in Blender? $\endgroup$
    – Robert
    Dec 14, 2020 at 21:07
  • $\begingroup$ @Robert "Self" option is new to me, but this option probably helped because your mesh was very very dense. Document: docs.blender.org/manual/en/latest/modeling/modifiers/generate/… I can't stress enough how doing a "limited dissolve" on imported STL's will help in the long run with many issues, size just being one of them. This video will help with this youtu.be/-OynHqYd_1I Yes 0.2 mm is just for some 3D printers (deals with thermal expansion of some material PLA, PETG, etc..) Good video to watch on this youtu.be/qEtIACrEk-I $\endgroup$
    – Rick T
    Dec 15, 2020 at 1:32

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