I'm building a Thickness Sander and I spent a great deal of time making a drum for it out of MDF. ( https://www.instructables.com/id/Thickness-Sander/ )
As luck would have it, I overlooked milling the keyway into the steel shaft of the 25 lb. sanding drum until I was almost ready to install the drum into the sander.
It took a little bit of doing, but I was able to hold the sanding drum securely (this is 1/2 the battle), and mill the keyway slot.
Step 1: Use a Edge Finder to Help Determine the Center of the Shaft
This made for a challenge, since the sanding drum is 25 lbs + and its three feet long. The placement of the slot for the keyway is critical as it needs to be dead in the center of the shaft since that is the way that the pulleys are made. You have to be accurate. You only get one shot.
I used a 2 x 6 board to support the drum and clamped the supporting board and the drum to the X axis of the vertical mill. I also clamped the shaft to the vice on the X axis. This assures that the shaft won't move while I was milling the keyway into the shaft.
Next, I used a edge finder (the second red circle) in the mill and adjusted the mill so that the edge finder had just kicked out a hair, thereby indicating the edge of the steel shaft.
The digital read outs (DRO) in the last photo make this a really easy process. Once you find the edge of a part, if you know the dimensions of the steel (in this case, a 1" shaft), you can move the Y axis of the mill (forward/backward travel), by 50% of the distance and the spindle in the mill will be right over the center of the shaft.
The DRO is accurate to within 0.0001" ;-)
Step 2: Use the Vertical Mill to Cut the Slot
Again, the DROs make this a very precise operation. Click below to see a video of the actual process:
After I milled the slot, I used a metal file to clean up any burrs on the top edge of the shaft. I then tested the key in the keyway slot.
I then put the pulley on and made sure that everything fit.
This took me about an hour to get everything set-up and practice on a scrap shaft to make sure that I knew what I was doing as this was the first time that I was going this operation.
I made it at TechShop.