Hi Rudy, there is an area where your 2 pieces have a lot of surfaces that are "coincident" - that is that they have surfaces that are running exactly over top of each other:



This kind of situation tends to make things much more difficult for the boolean calculation code.

If possible you want to have objects that sort of punch more fully through each other rather than having a lot of overlapping surface areas.

In this case, if you can trim off a little bit of the end of your ring, that will get it less confused and make the boolean union work.

What I did was to draw a line here:



Then I used Mirror to make a mirror image of that line on the other side. Then select the ring and run Construct / Boolean / Difference to slice the ring with those 2 lines, this will slice off the ends a little bit so that there is not so much overlapping area with your head piece.

Then you can select the little cut-off ends and delete them (to select them you may want to use a window selection where you click in an open area and then drag to make a selection rectangle).

With this bit of surgery now the 2 pieces look like this:



There is still some overlapping area along the bottom, but it is now a much smaller amount than before. With it set up like this, then the boolean union is able to complete successfully.

I have attached the unioned result here as ring_unioned.zip

Sometimes it may be necessary to do this kind of tweaking when the booleans are having difficulty. Reducing the number of edges and surfaces that are running right through or over top of each other makes things a lot easier for the calculation.

- Michael