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This question already has an answer here:

When using a Boolean modifier on an object, the "Operation" choices (Difference, Union, Intersect) are all not working normally. Choosing "Union" makes it do the effect of "Intersect" and "Difference" makes it do what "Intersect" is supposed to do. What's the deal here?

This is happening with following a simple tutorial. It's a simple primitive cube and primitive cylinder. No normals or doubles problems.

My specs are: Macbook Pro 13" Retina early 2015 512GB i7 with 16gb RAM upgrade OS-X version 10.11.4

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marked as duplicate by GiantCowFilms, David Apr 13 '16 at 13:18

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ You should at least provide a link to the tutorial or better your blend file with the problem or tell us about the steps you took where you run into the problem. I bet, you have manifold geometry. $\endgroup$ – Samoth Apr 12 '16 at 20:18
  • $\begingroup$ Related: blender.stackexchange.com/a/46809/2843 $\endgroup$ – Samoth Apr 12 '16 at 20:20
  • $\begingroup$ Please show screenshots of mesh or upload your .blend file. The most common reason for Booloean operations behave vice versa are flipped normals. $\endgroup$ – Mr Zak Apr 12 '16 at 20:24
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    $\begingroup$ This is most likely a problem with normals, open non-watertight or non-manifold meshes. Can you post a Blend file? Try recalculating the normals on both your meshes with Crtl+N in edit mode, and try cleaning them up with remove doubles. ALso check for wholes, gaps or open parts on your meshes. $\endgroup$ – Duarte Farrajota Ramos Apr 12 '16 at 23:07
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The direction of the Normals determines how the objects combine when using a Boolean Modifier:

On the first image the cube and the torus have their normals facing out:

enter image description here

On the second image the Torus has the normals facing in:

enter image description here

On the third image both objects have their normals facing in:

enter image description here

Quote from the manual:

  • Difference

The modified mesh is subtracted from the target mesh.

If the target Mesh has inverted normals, Blender will Intersect the modified mesh.

If the modified Mesh has inverted normals, Blender will add both meshes (Union).

If both Meshes use inverted normals, Blender will Intersect the target Mesh.

  • Union

The target mesh is added to the modified mesh.

If the target Mesh has inverted normals, Blender will Intersect the target Mesh.

If the modified Mesh has inverted normals, Blender will subtract the target Mesh.

If both Meshes use inverted normals, Blender will Intersect the modified Mesh.

  • Intersect

The target mesh is subtracted from the modified mesh.

If the target Mesh has inverted normals, Blender will subtract the target Mesh.

If the modified Mesh has inverted normals, Blender will intersect the target Mesh.

If both Meshes use inverted normals, Blender will add both meshes (Union).

To recalculate normals enter Edit Mode (Tab).

Select all faces and press CtrlN To have the normals face out

Or CtrlShiftN to have normals face inside the mesh.

For an explanation on Face Normals please read:

What are face normals?

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