The file differs from the chisel in having a large number of cutting points instead of one cutting edge and in being driven directly by the hand instead of by the hammer. As hand power only is used, it is evident that the amount of metal removed at one stroke will be small, and the amount removed by a single tooth will be exceedingly small.
Fig. 48. Hand File.
Files are made from cast or crucible steel and in manufacture pass through the successive processes of forging, annealing, grinding, cutting, hardening, and tempering. They have three distinguishing features-length, kind or name, and cut or coarseness of teeth. Length is measured from the heel A to the point B, Fig. 48, the tang C not being included. These lengths vary from 3 inches to 20 inches.
There are many kinds of files manufactured. Those in common use are shown in section in Fig. 49 as follows: A-flat file; B-hand file; C-warding file; D-square file; E-three square or triangular file; F-half round file; and G-round file.
The cut of files is in two styles-single and double; and each style has several grades of coarseness, viz, coarse, bastard, second-
Fig. 49. Cross-Section of Files cut, smooth, and dead smooth. The last two grades are sometimes called fine and superfine. As is shown in Fig. 50, the coarseness of each style varies with the length-the longer the file the coarser the cut.
If the cutting surface of a file were perfectly flat, the number of teeth or cutting points engaged with the work would depend on both the width of the file and the width of the piece being filed. To force as many cutting points as would be contained in such a large area deeply enough into the metal to enable each to remove its share of the stock would be beyond the power of the man pushing the file. To avoid this necessity for great pressure, files are usually "bellied" or made slightly convex in the direction of their length, so that, theoretically, the file and the work are in contact only on a line as long as the width of the file. This enables the file to be forced into the metal sufficiently for the teeth to bite, and thus avoids dulling the teeth, which always occurs when the file is allowed to glide over the work without sufficient cutting.
Fig. 50. Diagram Showing Coarseness of Files.
This convexity of files also serves another purpose. The pressure applied to the file to make it bite bends the file more or less, Fig. 51, and if the file in its natural state were perfectly flat, when cutting it would be concave; and this would prevent the production of a flat surface as it would cut away at the edges of the work, leaving a convex surface. Such files might, however, be used on convex surfaces.
Fig. 51. Bench Filing.
Height of Work for Different Classes of Files. Work for filing is usually held in a vise, and, under ordinary circumstances, the surface of the work should be about the height of the elbow. For fine work with small files, where close observation is of more importance than pressure on the file, the work should be higher than this, the height increasing with the refinement of the work. On the other hand, for very heavy filing, where great pressure is absolutely necessary, the work should be several inches below the point of the elbow, so that the weight of the body may be used to good advantage, and also because the workman naturally stoops a little when exerting a great pressure on the file.
Fig. 52. Special File Holder.