By Joseph A. McGeough
The file’s many tiny chisel-like teeth point in the direction in which it must be pushed in order to be effective. Because little material is removed with each stroke, the tool is well suited to smoothing a rough workpiece or altering its details. The file was unknown in early antiquity, during which time smoothing was done with abrasive stone or powder or with sharkskin, the granular surface of which approximates sandpaper.
Files of copper are unknown, but bronze was shaped into flat files in Egypt in 1500 BCE. A combined round and flat file of bronze was produced in Europe by 400 BCE. The file became popular in the Iron Age and a number of specimens survive from Roman times. The longest is flat, one-inch-wide, about 38 cm (15 inches) long including the handle, and has about 20 cm (8 inches) of working length. A number of shorter files of about 10-cm (4-inch) working length are particularly interesting because of the notch they carry near the handle. The V-shaped cross section (called knife-shaped today) indicates that these files were intended for dressing saw teeth. The notch enabled the worker to set the teeth—i.e., bend successive teeth to alternate sides to gain a free-running saw. These files had straight-across and coarse toothing, but the advantages of obliquely cut teeth and of double-cut (intersecting) teeth were appreciated early.
A treatise written in 1100 mentions files of square, round, triangular, and other shapes. At this time files were made of carburized steel that was hardened after the files were cut by either a sharp, chisel-like hammer or a chisel and hammer. An illustrated manuscript of 1405 that was copied by a succession of later authors shows a polygonal file; the screeching of the filing operation is commented upon too, with the curious suggestion that files be made hollow and filled with lead to eliminate the noise. In 1578 a writer asserted that the only way in which threads could be cut in screws was with the file.
Although Leonardo da Vinci had sketched a file-making machine, the first working machine was not produced until 1750, and it was a century later before machine-cut files substantially replaced those cut by hand. Power-driven, hand-cut rotary files are still used on dense metals because hand-formed, discontinuous teeth dissipate the heat well.
The ordinary file, in terms of its material and cut, is primarily used on cast iron and soft steel. Other materials—various nonferrous alloys, stainless steels, and plastics—are better accommodated with files of special composition and tooth formation (cut). A wide selection is manufactured.
Rasps, or, more correctly, rasp-cut files, have a series of individual teeth produced by a sharp, narrow, punch like chisel. Their very rough cut is suited to the fast removal of material from soft substances, such as wood, hooves, leather, aluminium, and lead.