Nut Seating Files
If you have ever looked at the side of your nut on your acoustic guitar, Les Paul, or Fender, you will see that the nut is seated in a slot in either the neck or the guitar's fretboard. This slot is most commonly referred to as the nut slot. Go figure. It is important before you install a nut that your nut slot is completely clean of hardened glue and it is level or flat. A traditional file will fit in the nut slot on edge, but there is problem. The file has files on both the edges and the flat part of the file. You don't want to file the edge of the fretboard or the edge of the headstock when you are leveling your nut slot. That is why the nut seating file was created.
A nut seating file looks like a standard file with a few exceptions. The nut seating file is completely flat. There is not taper to the edges like in most machining files. Also, the nut seating only has files on the two edges of the file. The flat sides of the file are smooth, so you can't file into the top edge of the fretboard. A nut slot file is also wider than most machining files.
After you have removed the nut from your guitar, you can use the nut seating file to clean and level the guitar nut slot. Simply place the file in the empty slot and file back and forth until the slot is clean. Be very careful not the rock or tilt the file while you are leveling the slot. The nut slot should be perfectly flat and smooth.
There really is no good replacement for a nut seating file when it comes to nut replacement. Commercial machinist files can be adapted for this use, but they don't work as well. You need a file that is made for nut slots. I recommend buying a nut seating file if you do any amount of nut replacements or repairs. If you would like more information about replacing guitar nuts, please see my how to replace a guitar nut article.