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I am cutting a whole into a block using a cylinder and boolean difference modifier, exactly at the edge of a block. This cuts 50% of the cylinder from the block.

The problem with cutting a whole right at the edge is that it leaves empty faces in the cutout. If I move the cylinder by a fraction away from the edge (either direction), the coutout will close the faces.

This does not seem to happen with a cube, although the 3D preview looks a bit confusing.

Is this expected or a bug? Is there a way to cut a cylinder while keeping the face closed? Or is it actually closed and just looks open? I tried the "Solidify" modifier but that doesn't seem right to do for such a simple operation.

Blender version 2.9 here.

enter image description here

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Boolean will break if there are intersecting edges at the cut. In this case there are the edges on both sides of the cylinder and edges of the object you are cutting. They are intersecting and Blender doesn't know what to do with them.

Overlapping edge

You can fix it by moving the cylinder by a small amount so there are no intersecting edges. If you want exactly half of the cylinder you can rotate the cylinder around z axis by a tiny amount.

Working Boolean

It would be create it the boolean modifier was smart enough to reroute the intersecting edges. Maybe it will come sometime in the future but at the moment there will be many cases where you have to fix bad booleans by hand.

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  • $\begingroup$ Thanks a lot for the quick help and advice! $\endgroup$
    – Janos
    Commented Sep 13, 2020 at 15:39

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