Rotary Broaching a Keyway on the Lathe with simple materials you have in the shop

[JRock]

It can be done in a mill, just not in the configuration I use in my video.
It'd be as simple as offsetting the bearing / center from the center an 1/8" or so.
The bar would turn and wobble the tool in the stationary workpiece.
Those tools with the double bearings are lovely, but it doesn't have to be that complicated to get the job done.

Yeah, the ability to cut a keyway without a bunch of extra, expensive tools (or breaking my butt) is what got my fire lit