Files either cut by being abrasive or by being sharp. Abrasive files work like sand paper. Diamond is the most common abrasive file. (Another realated tool is the Rasp.
Cutting files come in different shapes:
- Machinists' Flat
And in different tooth patterns (Listed from Coarse to Fine):
- Coarse double-cut
- Bastard double-cut
- Dead Smooth
Files can be sharpend a few times by being placed in an acid bath. If you use lots of files it is worth the cost if you only need to replace a couple of files each year don't bother sharpening.
After the teeth are cut on a file the file is hardned. This makes them easy to break. That can be used to yor advantage if you need to shorten a file. After shortening, grind the end making sure not to heat the file. (Use a bucket of water to cool the file after every few seconds of grinding.
Choose course files for softer materials and for leaving rougher surfaces with quicker stock removal. Choose finer files for harder materials and finer finishes with slower stock removal.