These time applications are objectionable in some instances, as the file is left too much at liberty, and the works are liable to be filed hollow instead of flat, especially if the file be rounding, because the unstable position of the work prevents the file from being constrained to act on any particular spot that may require to be reduced.
Some general remarks will be now given on certain practices in respect to economising the wear of files; and these will be followed by other remarks on the modes of holding works that are to be filed, prior to giving, in the next sections, the practical instructions upon filing.
The exterior surfaces of iron castings are usually more or less impregnated with the sand of the foundry moulds, which is very destructive to the tools; and this is in many cases removed by pickling them with dilute sulphuric acid, which dissolves a little of the metal, and undermines and loosens the sand, as explained in vol. i., p. 375. Iron castings become moreover superficially hard from coming in contact with the moist sand of the foundry mould; so that a thin but hard skin envelopes the entire object to the depth of the twentieth or thirtieth of an inch, and as this is very injurious to the files, it is usually chipped off with a chisel and hammer; the pickling is then less required.
The ordinary chipping chisel is about six or eight inches long, and three-fourths of an inch broad on the edge, which is a little convex, that the corners may not be liable to dig into the work. The bevils are ground to meet at an angle of about 80 degrees, and the hammer used with the chipping chisel varies from about two to three pounds in weight. Before commencing to chip the work, it is usual to rub both the face of the hammer and the end of the chisel upon the bench or floor, to remove any grease and leave them bright and clean, as were either of them greasy there would be risk of the hammer glancing off and striking the knuckles. A blow is first given with the hammer upon the angle of the work, to make a little facet upon which the first chisel-cut is made, about the thirtieth of an inch below the general surface of the casting, the chisel being then only raised some 30 degrees above the horizontal line. In continuing the cuts the chisel is elevated to about 45 degrees, the blows are given in quick succession, and the cuts are led gradually over the entire surface, the advance being always upon a line that is convex to the chisel.
Provided the casting is moderately flat, the edge of the chisel is kept at one uniform distance below the general surface of the work, which is occasionally examined with the straight-edge. Should the surface of the casting present any lumps or irregularities of surface, a thicker chip or two thin chips are removed from such high parts, to lessen the subsequent labour of filing, but which process is much less destructive to the file after the hard sand-coat has been removed by acid from the iron.
In some massive works, and also in cases where large quantities have to be chipped off certain parts of castings, much larger chipping chisels are used, which are called flogging chisels; they commonly exceed one foot in length, and are proportionally stout; one man holds the chisel in both hands, sometimes by means of a chisel-rod for greater security, whilst another strikes with a light sledge-hammer. Where much has to be removed, it is also usual to employ cross-cutting chisels; these are about seven or eight inches long, a quarter of an inch wide on the edge, and an inch broad in the other direction; the cross-cut chisel is first used to cut furrows, half or three-quarters of an inch asunder, to the full depth of the parts to be removed, and the intervening ridges are then easily broken off with the ordinary chipping chisel; but since the general employment of the planing-machine, and others of the engineer's tools, the chipping chisels are scarcely required. When iron castings are so near to their required dimensions, that chipping would remove too much, they are either cleaned with a nearly worn-out file, or the outer coat is removed on the grindstone, means that are much less wasteful of the material.
Wrought iron is but seldom pickled previously to being filed, but is either cleaned with an old file, or is ground on a stone to remove the outer scale or oxidised surface; the chipping chisel is only in general required when the nature of the work prevents it from being forged so nearly of the required form as to bring it properly within range of the file.
Brass and gun metal arc, as already noticed in the first volume, p. 375, sometimes pickled, but with nitric acid, instead of the sulphuric acid which is employed for iron; and brass is commonly hammered all over to increase its density, unless a minute quantity of tin is added, say a quarter or half an ounce to the pound, which materially stiffens the alloy, so as to render hammering as unnecessary as it is with good gun-metal.
After a file has been used for wrought iron or steel, it is less adapted to filing cast iron or brass, which require keen files, therefore to economise the wear of the instrument, it is used for a time on brass or cast-iron, and when partially worn, it is still available for filing wrought iron or steel; whereas, had the file been first used on these harder materials, it would have been found comparatively ineffective for brass and cast-iron.
As a further measure of economy, the pressure on the file should be always relieved in the back stroke, which otherwise only tends to wear down or break off the tops of the teeth, as their formation shows that they can only cut in the ordinary or advancing stroke; the file should, in consequence, be nearly lifted from the work in drawing it back, but it is not usual actually to raise the file off the work, as it then becomes needful to wait an instant before the next stroke, to ensure the true position of the file upon the work being resumed: whereas, if it is brought back with inconsiderable pressure, the file is not injured, and the hand still retains the consciousness of the true contact of the file and work, without which the instrument is used with far less decision and correctness than it otherwise would be.