Thanks for your suggestions,
in response to pilous comment i would like to point out, that a a fantastic CAD program like SU should not be limited to only "box-design"
With respect to the construction of the model, i took a lot of care to create "real" solids. Still it wouldnt work out with 3d printing. I added a png
s showing the model and an enlargement of an intersection.
I marked "open lines", that SU still considers part of a solid - and here seems to be the problem, that can only be solved with a true Circle-geometry