When we wanted to bore a hole into a round piece of wood we used a boring machine, and we created a set of blocks on either side of the round piece for the machine to rest on. This held the machine and piece so that we could bore the hole and then chisel it out to make a mortise.

Here is a snapshot of the setup:


You could do the same think with a longer piece you just have to create a set up so that you have a flat surface to reference off of for the machine to sit on, that is 90° to the mortise holes.

I hope this helps you to understand how you could create a mortise into a round piece of wood.

Jim Rogers