How to Cut a Key Way Using a Lathe

If you have need to cut a keyway, and you don’t yet own a milling machine. You may use your lathe with a milling cutter in the chuck or suitable collet, and put the shaft that needs the keyway into the lathe’s toolpost.

You can feed the shaft against the rotating cutter. It’s a slower operation, but it will get you the keyway you need.
I don’t remeber who gave me this idea, but it works.