Did this once by mounting a motorshaft between centres and grinding a parting tool to right shape/size and mounting side ways in tool post and manually advancing and feeding untill keyway cut,essentially scraping the material out,slow but worked,this was on HMS Sheffield back in the 70s,cant remember what the motor was from,the keyway was cut on the other side to the chewed up original,worked ok when refitted.Ah, Memories,---wheres me teef/glasses gone,
Chris