In the UK it is called a thread file, Each side ( 4) has a diferent thread pitch or TPI. Most are double ended so you get a total of 8 thread pitches. As previously mentioned a thread chaser is used on a lathe to finish a thread that has been screw cut on the lathe.. If thread damage is minor then use a triangular needle file and finish off with a stiff steel wire brush then apply a small amount of fine valve grinding paste and wind the nut back and forth. Remember to wipe/flush of the grinding paste when your done.