They are essential for many shaping and smoothing jobs on metal, wood and plastics, and they’re often needed for sharpening tools and garden implements. Though files are designed in dozens of different sizes and styles, the home handyman need only concern himself with a couple of basic types. These differ according to size (length), shape (cross section) and coarseness of cut (the kind of teeth it has).
The teeth on most files are cut at a certain angle across the face. Files that have a single row of parallel teeth are known as single-cut. Those which have a second row of teeth crisscrossing the first are called double-cut. Single-cut files generate the smoothest finish. Double-cut files would cut faster, but they don’t leave as smooth a surface.
Both the single-cut and the double-cut files differ as to coarseness. The most widely sold types are the grades named as bastard-cut. Used on wood, leather, lead and other soft materials, rasp-cut files (generally called wood rasps) are also available. They have individually shaped teeth and give a rough cut which gets rid of stock rapidly. Home handymen will find them valuable for shaping and smoothing lumber, as well as for shaving joints to a snug fit. Like the metal files, these wood rasps also come out in various degrees of coarseness.
In addition to the type of cut, files differ in cross section and shape. The most commonly used shapes are rectangular (flat files or mill files), square, triangular or round. Each comes in assorted lengths, and as the files increase in length, they likewise become larger in cross section. They might be either tapered (narrower at one end) or blunt (consistent in thickness from end to end).
The home handyman will discover that two rectangular (flat) files will handle a lot of his metal-smoothing and sharpening jobs: an 8 or 10 inch mill file (these are single-cut) and a flat bastard file (these are double-cut) of about the same length. The mill file will be utilized for sharpening tools and for final smoothing of hard metals, while the double-cut bastard file will be involved for all-around heavy work when great amounts of metal must be removed.
Other files which prove handy include the triangular file, which is useful for sharpening saw blades, filing latches, straightening damaged screw threads and similar jobs. Round files are most often used for reaming out or expanding holes and for inside filing of tight curves on wrought iron pieces and metal moldings, the half-round file simplifies the job of smoothing out concave surfaces, while the general-purpose file is convenient for use on rotary lawn mower blades and similar jobs. It’s double-cut on one side and single-cut on the other. The combination shoe rasp is likewise half-round. It has two gradations of rasp teeth, as well as single and double-cut teeth on both sides.
Landis, Scott. THE WORKBENCH BOOK
T. Jeff Williams,Book,Basic Carpentry Techniques