I picked up a DeWalt 1/2" drill at a garage sale, but discovered that the chuck was shot (the key pivot holes were so worn that the key couldn't be used to close it with any real force without slipping out).  Easy fix I thought.  Picked up a new chuck, unscrewed the left handed keeper screw inside, then put the key into a hole and hit with a big hammer.  Nothing.  Its supposed to just unscrew, but got totally stuck.

I opened up the gearbox, and the gear on the same shaft as the chuck had two holes to grab with a pin spanner.  So I grabbed the chuck with a chain vice-grip, and tried several makeshift spanners that all broke.  I even welded two rods to the sides of a socket from the junk drawer, but the rods sheared from the strain.

So, I'm thinking "I need a tool that can beat on this chuck, but in a direction to get it to unscrew", and it hit me.
I grabbed the largest torx socket I had on hand (the same one I hammered into a spline lugnut that got stuck), grabbed the flutes with the chuck jaws, put it onto my 1/2" impact wrench, and put the drill back together.  A few hits of the trigger on reverse, and the chuck spun right off.  I guess a 1/2" or so allen socket would have worked as well, or anything else that wouldn't slip.
It will want to turn, so you need to pulse the trigger, but that still had WAY more power than I could even exert with a 3# sledge, and worked in seconds.