Hi Rudy, it seems to be working over here...

First thing, just a note - it looks like on piece of your rectangular head piece is missing (one plane is not there) here:



The outer piece is not there, maybe it got deleted by accident?


Anyway, here is what I am doing after loading your file. First I start by selecting the top head piece:



Then run Construct / Boolean / Diff, and then select the cylinder as the cutting object and push Done (or right-click in the viewport). That should cut the head like you want:



Here's a view from the front to see it more clearly:



Are you seeing something else when you do this on your machine?

Note that the outline at the bottom are a couple of the original curve objects that you used to create the head, just select those and delete them if you don't need them any more.

If you are seeing something different than this, maybe you can attach a screenshot of what is different on your screen? To make a screenshot push the "Print Screen" key on your keyboard, to capture an image to the clipboard, and then paste it into an image processing program (windows paintbrush will do if you don't have anything better) and save as a .jpg file and attach here.

- Michael

Your octagonal object is missing a face, so it's not a closed volume...

I see Michael beat me to it... :)

jonah

Thank you Michael,
Thank you Shorty.

First, I was making the MAJOR mistake by selecting the wrong base object (was selecting the ring...shame on me!!).

Yes, the rectangular shape has one side very slightly longer than the other (it is a real model...an emerald cut gemstone), so it gives me some problem when I try to connect or have some tapered form of the same.

Anyway, I really thank you much guys!
Rudy