EE WAS ASSISTING CHANGING ROLLERS ON AN HITACHI BACKHOE. THE #3 ROLLER WAS SET IN PLACE & A BOLT WAS STARTED TO SECURE IT. THE EMPLOYEES BEGAN TO RELEASE THE PRESSURE ON THE ROLLER & THE BOLT DID NOT HOLD. THE ROLLER DROPPED DOWN ON THE EE'S LEFT HAND & PINCHED IT BETWEEN THE ROLLER & AN IMPACT WRENCH. THIS RESULTED IN A 1"" LACERATION TO THE LEFT INDEX FINGER.