Since I’m a lazy brewer and make 10 gallon batches with typically over 20 pounds of grain, I use a drill to power the grain mill used to crush the grain. Cranking the grain through by hand would take much longer. My new 1/2″ corded drill (replacing an old 1/2″ cordless Makita with tired NiMH batteries) was powerful enough to slip off the small flat on the drive shaft, polishing it round.
I first enlarged the flat spot on the shaft using a grinding attachment on a Dremel tool, hoping to provide enough grip for the drill chuck. In the picture below you can see the polished area of the shaft where the drill chuck had spun around and actually reduced the diameter of the shaft.
The unexpected downside of this was that the drive axis of the chuck was angled away from the axis of the drive shaft, causing a wobble that tended to loosen the chuck which would then grind away still more of the mill’s drive shaft. This video shows the wobble
I made a brief attempt to grind down other sides of the shaft to better align the drill with the mill, but since I am not equipped for precision milling I decided to try another approach. A flexible rubber coupling only cost about $10 (made in the USA!) and securely attached to the mill with a set screw. At the other end I used an extra drive bit, padded out inside the coupling with some scrap sheet metal, for the drill chuck to grab.
As you can see in the video below, the coupler worked remarkably well at eliminating the wobble, up to an ridiculous RPM