The ring is simply cut in place, then separated from the rest of the rattle.
Centuries old trick.
Then after separating it, one puts a piece of sanding linen around the narrow part of the rattle with gaffa tape.
When the rattle spins on the lathe, that can be used to sand the inside of the ring.
All it takes is a little manual dexterity.
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That said, making a captive ring with lots of room to work both sides as he does is super easy.
The difficult part is to make it in as small an space as possible, it took me years before I could get the space between the round "head" of the rattle and the part behind the ring down to where it is now.
I don't know how many rattles I've made, but it is a 4 figure number.
It used to be my show piece, that I'd demonstrate at turning gettogethers.
My best time for turning one is 6 minutes.
Now that I don't turn regularly any more, I'm way slower.
This one took at least a ½ hour, but then boxwood is hard to work.