A file is a metalworking and woodworking tool used to cut fine amounts of material from a workpiece. It most commonly refers to the hand tool style, which takes the form of a hardened steel bar with a series of sharp, parallel ridges, called teeth. Most files have a narrow, pointed tang at one end to which a handle can be fitted.
Archaeologists have discovered rasps made from bronze in Egypt, dating back to the years 1200 - 1000 B.C., Archaeologists have also discovered rasps made of iron used by the Assyrians, dating back to the 7th Century B.C. Among the drawings of Leonardo da Vinci is a sketch of a machine tool for the cutting of files (the chisel would make one strike, swaging a tooth, then automatically advance into position for the next tooth, and strike again).
Machining in the mid 19th century was heavily dependent on filing, because milling practice was slowly evolving out of its infancy. As late as the early 20th century, manufacturing often involved filing parts to precise shape and size. In today's manufacturing environment, milling and grinding have generally replaced this type of work, and filing (when it occurs at all) usually tends to be for deburring only. Skillful filing to shape and size is still a part of diemaking, moldmaking, toolmaking, etc., but even in those fields, the goal is usually to avoid handwork when possible.
Files come in a wide variety of sizes, shapes, cuts, and tooth configurations. The cross-section of a file can be flat, round, half-round, triangular, square, knife edge or of a more specialized shape. There is no unitary international standard for file nomenclature; however, there are many generally accepted names for certain kinds of files.
The cut of the file refers to how fine its teeth are. They are defined as, from roughest to smoothest: rough, middle, bastard, second cut, smooth, and dead smooth. A single-cut, or mill file, has one set of parallel teeth, while a cross-cut, or double-cut, file has a second set of cuts forming diamond teeth. In Swiss-pattern files the teeth are cut at a shallower angle, and are graded by number, with a number 1 file being coarser than a number 2, etc. Most files have teeth on all faces, but some specialty flat files have teeth only on the face or only on the edge, so that the user can come right up to another edge without damaging the finish on it.
Some of the common shapes and their uses:
Instead of having teeth cut into the file's working surface, diamond files have small particles of industrial diamonds embedded in their surface (or into a softer material that is bonded to the underlying surface of the file). The use of diamonds in this manner allows the file to be used effectively against extremely hard materials, such as stone, glass or very hard metals such as hardened steel or carbide against which a standard steel file is ineffective.
The image to the left shows a selection of needle files in an assortment of cross sectional shapes.
Needle files are usually sold in sets of 6 or 12 (or more) different shapes, packaged in a soft pouch, both for ease of handling and protection of the files' teeth. They are small files that are used in applications where the surface finish takes priority over metal removal rates but they are most suited for smaller work pieces.
They are often used as pictured, however, like all files, they are safest when used with a handle. The handle is often designed around the collet principle which allows the files and handle to be interchanged quickly.
Riffler files are small to medium sized files in an assortment of cross sectional shapes and profiles. The varying profiles and shapes enable them to be used in hard to reach, or unusually shaped areas. They are often used as an intermediate step in die making where the surface finish of a cavity die may need to be improved. - eg; plastic injection moulding or die casting.
The files pictured on the left are designed for use in a filing machine.
A filing machine is similar in appearance to a scroll saw or band saw in that the file is mounted vertically in the middle of a table. When in operation the file reciprocates vertically while the workpiece is presented to the file's face and manipulated around the table/file as the shape requires.
The cone point of the pictured, top two files allows the files to center themselves firmly in the bottom file holder. The pictured, lower two files are required to be inserted into the bottom file holder and physically restrained with set screws, an identical process as for the top holder.
Filing machines are useful tools as they reduce fatigue and improve product accuracy, and although not usually seen in modern production environments, they may be found in older toolrooms or diemaking shops as an aid in the manufacture of specialist tooling.
Files have forward-facing cutting teeth, and cuts most effectively when pushed over the workpiece. Draw filing involves laying the file sideways on the work, and carefully pushing or pulling it across the work. This catches the teeth of the file sideways instead of head on, and a very fine shaving action is produced. There are also varying strokes that produce a combination of the straight ahead stroke and the drawfiling stroke, and very fine work can be attained in this fashion. Using a combination of strokes, and progressively finer files, a skilled operator can attain a surface that is perfectly flat and near mirror finish.
Pinning refers to the clogging of the file teeth with pins, which are material shavings. These pins cause the file to lose its cutting ability and can scratch the workpiece. A file card, which is a brush with metal bristles, is used to clean the file. (The name, "card", is the same as used for the "raising cards" (spiked brushes) used in woolmaking.) Chalk can help prevent pinning.