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A knife file is a kind of knife-shaped, tapered file used to file V-shaped channels or the joining point between two acute-angled surfaces. Knife files are generally hand tools made of hardened steel with rows of elevated "teeth." The teeth on the knife file allow it to fulfill its function of filing wood and metal. They have an arc shape and are tapered from base to point, but the knife edge has a uniform thickness for the length of the file.
Knife files range from those with a “rough” cut of teeth, to those with a “dead” cut of teeth. A rough knife file will have twenty teeth per inch, whereas a dead knife file will have one hundred or more teeth per inch. The full range of teeth, from least number to highest number per inch, is: rough, middle, bastard, second cut, smooth and dead. A rough knife file shaves off material quickly and approximately, whereas more teeth per inch make the knife file smoother so it will file more slowly and produce a smoother end product.
Knife files work by making fine cuts into the wood or metal being worked with. This allows the worker to shave off and refine parts of the material being molded to the design specifications and shape needed. Knife files are optimal for hard-to-reach acute-angled corners and are also effective for sharpening other tools such as chisels and knives.
Over time, a knife file’s teeth can become clogged with fine bits of material, either metal or wood. The pieces of debris left amongst the teeth of the file are called pins, so clogging of the file is called pinning. Pinning makes the file much less effective and can scratch the surface being worked on. A file card, with hard metal bristles, removes the debris and makes the file effective again for further use. Rubbing chalk onto the teeth of the knife file can also help prevent excessive pinning.
The best way to store knife files is to hang them from a rack. They can also be stored in a drawer, but it is important that each file is carefully separated from the other. Storing files properly helps ensure that the files do not lose teeth and become less effective when used. The life of a knife file will depend on how much it is used, proper storage and care and the softness of the wood and metal it is used on.