Noob problem: boolean UNION acting like DIFFERNCE (again)

I get this problem quite a bit with boolean merging. I’ve created a model from several pieces and I want to boolean UNION them together as one shape.

My reason for liking UNION is because if I use JOIN then intercepting shapes end up keeping their overlapping areas as mesh and I think that’s going to be messy in the long run (waste of virtual space). UNION is good because when it works, I only get the outer shell of the combined form.

What you see here are two linking shapes (I deleted everything else in my real model to give a simple example). When I try to perform boolean UNION on them, I end up with something more like DIFFERENCE (where one object gets subtracted from the other instead of combined). Why is it doing this? I have tried recalculating normals (worked last time I had something similar) but it’s still messing up. What should I do to fix it?

To download the blender file and have a go at it yourself, go to and click the hyperlink “Click here to download” (file name is “problem.blend”).

Ok i looked at your file and just directly trying to do a boolean i got same problem your getting, checked your normals and they where pointing out on both which is right, so thought it must be getting confused what is inside and what is out side so on one object only i went into edit mode and flipped all the normals ,i put the boolean modifyer on the other mesh and it done union correctly and after applying, all normals where pointing out without doing a recalculate normals.

Hey thanks - that seems to be the trick! Good to know because it’s been happening a lot to me lately, probably because my general technique is to build object on object and merge them all together (don’t see a lot of tutorials doing things this way… many just extrude endlessly - is boolean merging a bad thing?)

Appears that you have to “flip” the correct shape first. When I first tried it out, I think I must have chosen the wrong one because UNION turned into INTERSECTION.

You say you could see the direction of the normals (going outwards). How do you get to see this direction?

When ever ive used boolean (which is not very often as i prefere the result of joining meshes manually) ive had the normal pointing outwards on both meshes and its worked ok, for some reason on your particular mesh shapes you need to have normals faces out on the main mesh and the one to boolean with, needed normals pointing in as it was confused, at a guess to do with angles between faces & the way it calculates what to do ect.

I think you wont find many tutorials to do with boolean as you can end up with some prety random results with untidy triangles everywhere but when you make the joins manualy you have full control over what happens so you can ensure you keep quads ect if required.

Take a look at this post to view the difference of manually joining as opposed to using boolean

You can view your normals by switching on draw normals button which is shown below on the right hand side ‘mesh tools 1’ and the value above the button ‘nsize’ will change the display length of the normal line. Having this on will show as a blue line pointing out from your faces in the normal direction.

you still might want to make a note of this in the bug tracker,