@geert_2
Thanks for the suggestion on let the parts overlap when making a union.
Yes the part is made by a union of a rod and a spiral, and in fact i had some problems in making a union of them in the beginning, and I solved it by let them overlap as you suggested. After that i could make a union of them without any errors, and then I thought the problem was solved.
But apparently it was not :-/ @kmanstudios
Thanks for the link to the validating tool, I will use this tool to try to find the reason for the error that I am seeing.
I think that I am doing something wrong, rather then there are an error in freecad.