* The teeth of some single cut files are much loss inclined than 55 degrees, those of floats are in general square across the instrument.
Eight courses of cuts are required to complete a double-cut rectangular file that is cut on all faces, but eight, ten, or even more courses are required, in cutting only the one rounded face of a half-round file. There are various objections to employing chisels with concave edges, and therefore in cutting round and half-round files, the ordinary straight chisel is used and applied as a tangent to the curve, but as the narrow cuts are less difficult than the broad ones, half-round and round files are generally cut by young apprentice boys. It will be found that in a smooth half-round file one inch in width, that about twenty courses are required for the convex side, and two courses alone serve for the flat side. In some of the double-cut gullet-tooth saw files, of the section K, as many as 23 courses are sometimes used for the convex face, and but 2 for the flat. The same difficulty occurs in a round file, and the surfaces of curvilinear files do not therefore present, under ordinary circumstances, the same uniformity as those of flat files, as the convex files are from necessity more or less polygonal.
Hollowed files are rarely used in the arts, and when required it usually becomes imperative to employ a round-edged chisel, and to cut the file with a single course of teeth. Sir John Robison's curvilinear file will be hereafter noticed, in which the objections alluded to in both hollowed and rounded files are nearly or entirely removed.
The teeth of rasps are cut with a peculiar kind of chisel, or as it is denominated a punch, which is represented also half size, and in two views in fig. 812. The punch for a fine cabinet rasp is about 3 1/2 inches long, and 5/8 square at its widest part. Viewed in front, the two sides of the point meet at an angle of about 60 degrees, viewed edgeways, or in profile, the edge forms an angle of about 50 degrees, the one-face being only a little inclined to the body of the tool. Different sized rasps necessarily require different sized punches, the ends of which would much resemble the ordinary point tools for turning wood or ivory, but that they are more obtuse, and that the edge of the punch is rounded, that the tool may rather indent than <
In cutting rasps, the punch is sloped rather more from the operator than the chisel in cutting files, but the distance between the teeth of the rasp cannot be determined as in the file, by placing the punch in contact with the burr of the tooth previously made. By dint of habit, the workman moves or, technically, hops the punch the required distance; to facilitate this movement, he places a piece of woollen cloth under his left hand, which prevents his hand coming immediately in contact with, and adhering to the anvil.
The teeth of rasps are cut in rather an arbitrary manner, and to suit the whims rather than the necessities of the workmen who use them. Thus the lines of teeth in cabinet rasps, wood rasps, and farriers' rasps, are cut in lines sloping from the left down to the right-hand side; the teeth of rasps for boot and shoe-last makers and some others, are sloped the reverse way; and rasps for gun-stockers and saddle-tree makers are cut in circular lines or crescent form. These directions are quite immaterial; but it is important that every succeeding tooth should cross its predecessor, or be intermediate to the two before it; as if the teeth followed one another in right lines, they would produce furrows in the work, and not comparatively smooth surfaces. Considering the nature of the process, it is rather surprising that so much regularity should be attainable as may be observed in rasps of the first quality.
In cutting files and rasps, they almost always become more or less bent, and there would be danger of breaking them if they were set straight whilst cold, they are consequently straightened whilst they are at the red heat, immediately prior to their being hardened and tempered.
Previously to their being hardened, the files are drawn through beer grounds, yeast, or other sticky matter, and then through common salt, mixed with cow's hoof previously roasted and pounded, and which serve as a defence to protect the delicate teeth of the file from the direct action of the fire. The compound likewise serves as an index of the temperature, as on the fusion of the salt, the hardening heat is attained; the defence also lessens the disposition of the files to crack or clink on being immersed in the water, see vol. i. page 253.
The file after having been smeared over as above, is gradually heated to a dull red, and is then mostly straightened with a leaden hammer on two small blocks also of lead; the temperature of the file is afterwards increased, until the salt on its surface just fuses, when the file is immediately dipped in water. The file is immersed, quickly or slowly, vertically or obliquely, according to its form; that mode being adopted for each variety of file, which is considered best calculated to keep it straight.
It is well known that from the unsymmetrical section of the half-round file, it is disposed on being immersed, to become hollow or bowed on the convex side, and this tendency is compensated for, by curving the file whilst soft in a nearly equal degree in the reverse direction; by this compensatory method, the hardening process leaves the half-round files nearly straight.
It nevertheless commonly happens, that with every precaution the file becomes more or less bent in hardening, and if so, it is straightened, not by blows, but by pressure, either before it is quite cold, or else after it has been partially reheated in any convenient mode; as over a clear fire, on a heated iron bar, over a hooded gas flame, as in tempering watch-springs, or in any other manner. The pressure is variously applied, sometimes by passing the one end of the file under a hook, supporting the center on a prop of lead, and bearing down the opposite end of the file; at other times by using a support at each end, and applying pressure in the middle, by means of a lever the end of which is hooked to the bench, as in a paring-knife. Large files are always straightened before they are quite cooled after the hardening, and whilst the central part retains a considerable degree of heat. When straightened, the file is cooled in oil, which saves the teeth from becoming rusty.