Hi Andrei, your cutting object is self-intersecting, basically you've got opposite sides of the shape here overlapping and touching each other:




That tends to make for a poorly defined volume, especially if the surfaces actually wiggle through each other slightly.

Then also having that same area trying to overlap on the base object being cut makes for a highly complex type of surface/surface intersection.

You will probably not want to have such a very thin wedge type shape in the cutting object and it will be better for the boolean if you can make a cutting object that pushes some amount through the object to be cut rather than having a "barely skimming" relation with surfaces that overlap over very similar surface area. It can be ok to have overlapping sometimes if the surfaces are very precisely duplicated in the overlap area, but if one of them comes from some fitting operation and just barely waves in and out of the other that makes for a very difficult and complex intersection.


You can instead use Trimming to do a more targeted type of cutting to get the final result, rather than doing a boolean which is based on volumes.

Delete the outer face of your cutting object, then select the main object and run Edit > Trim, then for the cutting objects select the naked edges of your cutting object. That will then trim the surface and you can discard the piece you don't want, and join the results back together which is how I got the attached result.

- Michael