Hi Jesper, just to help clarify what Martin is explaining, here is a screenshot from the bottom of your objects:





So the because the rod is hanging off the bottom slightly it means the ring does not fully divide it into pieces, that's why you are running into some trouble there.

Like Martin wrote you probably want to move the rod upwards a little. And then instead of using Trim which is more of a surface modeling operation (which will slice only the surface area of objects) when you have solids it can be more convenient to cut things using booleans instead, since Booleans with solids will keep the results as solids as well.

So in this case using booleans you would first select the rod and then do a boolean difference using the ring as the cutting object with the "Keep objects" checkbox enabled so that the ring still stays around, that will then cut up the rod into solid chunks and you can discard the outer solids and then you can do a boolean union between the interior rod piece and the ring.

You could use Trim as well, but Trim is a somewhat more low level operation that only slices up surface area, once you use it on a solid the result won't be a solid anymore it will be open surfaces, the booleans are kind of like a "higher level" cutting operation that also cuts things but then automatically decides which pieces to discard based on which volume they are in, and then also automatically joins the pieces together, so it's not unusual for the booleans to be a bit more convenient than using Trim since with Trim you can then need to do some more steps later on. Not always though, it's just a general rule of thumb that with solids prefer the booleans unless you know that you are going to be working at the surface level for a while and then forming up solids later.

- Michael