I aligned it with .01 select tool
It can tend to be a pretty annoying thing with negating unions. I would just recommend that you de-union the part that has that infinitely thin part and extend/expand the size of 1 of the parts in the specific direction to fill in that gap/space. Simplest fix that I can think of at this time. Hope this helps and that you kind of understand what I’m saying.
Here is a visual example if this helps, what I did was I basically extended the size of the square part to fill in the space (in my case extended it along the Z-axis). By the looks of the part, it may be either along the X or the Z-axis that you would have to make the adjustment to fill in the thin space with the part.
that’s what I did it doesn’t wanna work
I’m confused, are you trying to add a negative part to fill in a gap between other negative parts? If so, put them in the air. Delete the thin part. Then, click on collisions and slowly move one piece to the other, and then you have a perfectly aligned part