EE AND LEADPERSON WERE WORKING ON A CRUSHER REPAIR. THE LEADPERSON HAD A SLEDGEHAMMER AND THE EE WAS HOLDING A WEDGE. THE EE THOUGHT THE LEADPERSON WAS DONE & PUT HIS HAND OUT TO REMOVE THE WEDGE WHEN THE LEADPERSON SWUNG THE SLEDGEHAMMER HITTING THE EE'S THUMB THAT WAS ON TOP OF WEDGE. LACERATION TO THUMB REQUIRED STITCHES.