Fret Crown File
When you look at the frets on you guitar or a piece of fret wire, you will see that the fret bead is crowned or perfectly rounded. As you play your guitar, the strings wear down and flatten the fret. This is where a fret crowning file comes into play. Luthiers use a fret crowning file to reshape the rounded bead of the fret wire. The fret crowning file is most often used while leveling and dressing guitar frets.
The fret crowning file is not like an ordinary file. Instead of having the files on the broad, flat sides of the file, the fret crowing file's sides are smooth and the files are located on its edges. These edges are concave in shape to fit the rounded fret bead. This is what gives a flattened or worn down fret its new crowned shape. Once the fret is properly leveled and the fretboard is masked off, you can take your fret crowning file and file back and forth until the fret has the proper crown.
There are many different styles and sizes of fret crowning files. Some files are made for skinny fret wire, while others are made for jumbo fret wire. The less expensive fret crowning files have normal files; whereas, the nicest ones are encrusted with diamonds. The diamond files will cut the fret faster. They will also stay sharp longer than traditional files.
Every luthier or guitar repairman should have at least one fret crowing file. Don't mess around with trying to crown your frets with a standard file or triangular file. It's a waste of time and, in most cases, produces poor results. I recommend spending the money and buying one of these luthier tools. If you would like more information about how to use a fret crowning file, please see my how to dress guitar frets article.