A file is a bar of high-grade crucible steel, pointed at one end for a handle and having cutting edges or teeth extending from a point near the handle to the opposite end. The mechanical principle of the teeth of a file is that of the wedge. The handle acts as a lever. In the course of manufacture, files pass through the successive processes of forging, annealing (gradually heating and cooling), grinding, cutting, hardening, and tempering. They are annealed before being ground and cut, and thus the hardness is reduced. File teeth are like a series of small chisels cut at an angle to the sides of the file, as shown in Fig. 126. Cutting on the return stroke dulls the teeth and injures the file. It is possible to destroy some of the teeth of a brand new file in one minute's careless work.
(a) Hot Chisel.
(b) Square Punch Chisel.
(c) Round Punch Chisel.
(d) Side-Set Chisel.
Fig. 125. - Handle Chisels.
Many new kinds of files of all shapes and sizes, have recently appeared, so that there are now at least 104 different varieties on the market. All may be divided into three general classes, namely single-cut, double-cut, and rasps (Fig. 127). The files in each of these classes vary in length, in shape, and in coarseness of teeth.
A single-cut file has the teeth all running diagonally across the face in one direction only. A double-cut file has the teeth criss-crossing or running across the face in two directions, making a surface covered with small, sharp points. Each style or shape of single-cut and double-cut file has several grades of coarseness. These grades are called coarse, bastard, second-cut, and smooth, the coarseness varying with the length of the file. The longer the file, the coarser the teeth and the cut. Single-cut files are generally used for cutting soft metals and for lathe work. Their coarssr grades are sometimes called float files, or "floats."
Fig. 126. - Action of File Teeth.
The double-cut files are used for all kinds of hand-work. The teeth of a rasp are entirely separate. They are round on the top and are formed by raising with a punch, small portions of stock from the flat surface of the file flank. The rasp is used for removing large quantities of stock quickly and will work well on soft metals and even on wood. When a good job is wanted a rasp must be followed up with a file of finer grade.
Fig. 127. - File Cuts.
(a) Single cut.
(b) Double cut.
Files are made convex, i.e., rounding, as shown in Fig. 128, for three reasons: (1) to overcome the effect of the spring down or bending of the file due to the pressure of the hands in making a cut; (2) to overcome the spring or warp caused by heating and hardening the file when made; (3) to make the file bite or cut with only a few teeth in the middle of its length.
Figure 129 shows the end view of sections of files. A is the flat file, B the hand, C the square - the most commonly used - D the triangular or 3-square, E the half-round, and F the round.
Fig. 128. - Shape of File.
The length of a file is the distance from the heel H to the point P (Fig. 130). The tang T is not included in the length. In ordering a file from the toolroom it is necessary to state the length, the degree of coarseness, and the shape. For example, you may want a 14 in. flat bastard, or a 16 in. half-round float.