I have created the 3D-model by Moi3D v2. I tried to combine solids using boolean feature "Union". Why solids wasn't combining?

The small pieces you like to union with the big piece have missing faces - hide the big peace and you can see it.. redraw them and boolean will work

Hi JL82, yes as mentioned above there seems to be something wrong with the structure of the small objects there, the areas that look like holes are damaged somehow like their trimming boundary information is messed up in some way.

I've attached a version with those objects reconstructed and this should now boolean ok.

- Michael