Hi Andras, also like Danny mentions the area to be cut being numerically small is also part of the issue - when distances to be cut are too close to the intersection tolerance it can tend to make the cut sort of "glom on" to the existing boundary rather than being distinct.

One way to solve it is like I wrote above where doing an "untrim" to restore the full original surface can then be cut more easily.

But also if you scale your objects up by 100 times in size so that the width of the area to be cut is not such a small number then it can be cut in place without doing an untrim first.

- Michael