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I have two objects to make hole. Left one is the object what I want to subtract as you see , boolen make nothing happened

Right one is the mesh cylinder for just boolen test and it works.

What am I missing in my mesh? "Inertersect" , "Union" all works but only "Difference" not working.

It's reproduced. here is link for your better understanding https://cloud.wise-leader.com/index.php/s/jfYy1bshAECtjq8

enter image description here

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Most probably you have flipped normals on the object you want to substract. In this example, both cylinder objects should be subtracted from the plane, but the left one isn't:

boolean not working

If you enable Face Orientation in the Overlays, you can see see faces of the left cylinder are showing red, meaning their inside is pointing outwards, while all the other faces are blue:

face orientation

To change this, select the cylinder and Tab into Edit Mode, select all faces with A, then hit either Shift+N > Recalculate Normals to turn them all blue or if some of them stay red, select those and hit Alt+N > Normals > Flip to make blue as well.

flip normals

all faces blue

You can now disable Face Orientation again, and as you can see on the plane back in Object Mode, both Boolean modifiers are now working:

boolean working

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  • $\begingroup$ Thanks Brinkmann! Actually, it's not my case. But it's also good answer for such an occasion. My case is mesh is unstable with manifold state due to hole design and boolen modifier difference effect is somehow not working. This is happening for me since I am not experieced person for 3D modeling but just beginner with many hole drawing :) Anyway, Thanks for your professional answer ! $\endgroup$
    – mailo-gum
    Commented Feb 11, 2023 at 9:41
  • $\begingroup$ If it's not your case, then we need more information on your objects or maybe you nbeed to upload your file... and you don't have to accept answers that don't solve your problem - because others might have the same and it would be a bit disappointing if this is marked as accepted but it doesn't help. As an optional answer it will stay here as long as I don't delete it ;) $\endgroup$
    – Gordon Brinkmann
    Commented Feb 11, 2023 at 9:44
  • $\begingroup$ Problem reproduced and modleing data linked now ! $\endgroup$
    – mailo-gum
    Commented Feb 12, 2023 at 1:00
  • $\begingroup$ Gordon's answer is correct, I opened the example file you uploaded and turned on Face Orientation in the Overlays and the Cylinder is red, and the rectangular prism is blue. Selecting the cylinder and going to Edit Mode, selecting all the faces and pressing Shift+N to recalculate the normals resolves the issue. It's named Circle.003 so I suspect what happened was you extruded a circle, which by default when you extrude it up, its normals will face inward. $\endgroup$
    – spood
    Commented Feb 12, 2023 at 1:30

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