If you have a router table, a birdsmouth jig works great for this. You could also jerryrig your router so it sits on top of the birdsmouth.
Either way, you want your round piece to sit in the V of the jig and you rotate your piece by hand. Leave the router fixed (in a table or some other fashion from the top).
As you have a pattern block, you would want the pattern block to ride on the edges of the jig.
I did a crude sketch of how this would look.
Of course, this assumes I followed the question you asked
Jim
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"Why is it that I loathe washing dishes, but I absolutely love sitting out here in the shop washing parts with kerosene?"
Roger Welsch, Busted Tractors and Rusty Knuckles