When I was machining the front derailleur for my Raleigh 10sp, I needed to machine a half-bore out of the body to fit the seat tube diameter but didn't have a way to do so on my lathe/mill, so I made a boring head.
From a chunk of brass round stock, I turned a small post on one end (to hold in the spindle of my machine) and then used a hacksaw to cut a small notch in the other end. Next I cut a small piece out of another hacksaw blade and drove it into the notch.
Finally, I carefully ground the cutting "insert" to size so that it would bore the correct size hole.
I chucked it into the spindle of my lathe/mill and stood back while I turned it on. The "insert" didn't fly out, so I carefully made the first pass. It cut beautifully, so I kept at it until I reached the final dimensions needed.
Here's the boring head, sitting on the bed of my little lathe/mill.
And here's a close-up shot of the finished derailleur mounted on the bike.
My favorite part about making the boring head is that I made it on the machine that I would then use it on. There's something very satisfying about using a tool to make a tool, especially when the tool you made will be used on the tool that made it.