Hello Michael,
thank you for explanation, now I understand the problem...
I was confused and I focused on spring, while it was also very tricky to get a solid.
The cage was quite simple construction, so I believed the core of the problem is in spring :-)

I like the "clean" constructions avoiding such mess by principle... therefore I have less experience with fixing such problems...
My next challenge is to re-construct the cage to avoid those slivers...

Just one more question:
if i have 1 solid, then i cut it by boolean diff with a single line to get two solids, then I edit both parts, but not connecting faces...And the if i try to join (bool union them again) - it is not possible, moi does nothing, I still have 2 separate solids... what to search for?

Jozef