The number of cuts per inch on a file range from about l0 on the coarsest to some 250 on the finest. The softer the metal to be filed, the coarser the cut of the file should be. For harder metals such as tempered steel the finer double-cut files are preferable.
The quality of a file is judged by the amount of use it will stand before it is worn too smooth for cutting. However, some files are known to cut fairly fast for a certain length of time, but at a later stage the better file may still continue to serve for a long time while the inferior will go from bad to worse and will quickly reach a useless stage.
The horologist should acquaint himself, through catalogs, with the different types of files which are available. One may thus always obtain the correct file for a particular job. A flat file for instance may be had parallel and wide, parallel and narrow, parallel edge, tapered edge, coarse cut, fine cut, short, or long, to cut on up stroke for regular work or to cut on down stroke for filing machine. It may also be of interest to know that round files, in addition to the regular needle file may be had in parallel shapes as small as one millimeter in diameter. The so-called escapement files are used for various jobs excepting escapement work.