In following out the subject of the instructions for the use of the file, it is proposed, first to explain that which may be called the manual process of producing a true or plane surface on a piece of cast iron of moderate dimensions, say four or five inches wide and eight or ten inches long; and although the entire routine is only required for surfaces of the most exact and finished kind, the same general treatment, when discontinued at certain stages, is equally suited to various other works in mechanism, that only demand by comparison an inferior degree of precision: the routine is also nearly the same for surfaces larger or smaller than that referred to.
Before any effective progress can be made in filing flat works, the operator must be provided with the means of letting the progressive advance of the work, he should therefore possess a true straight-edge, and a true surface-plate. The straight-edges used by smiths arc generally of steel, and although they have sometimes a nearly acute edge, it is much more usual to give them moderate width: thus, in steel straight-edges from one to four feet in length, the width of the edge is from one-sixteenth to one-fourth of an inch, and in cast-iron straight-edges from six to nine feet in length, the width is usually two to three inches.
The straight-edge is used for trying the surface that is under correction, along its four margins, across its two diagonals, and at various intermediate parts, which respective lines, if all exact, denote the surface to be correct; hut the straight-edge alone is a tedious and scarcely sufficient test, and when great accuracy is desired, it is almost imperative to have at least one very exact plane metallic surface, or surface-plate, (the plano-metre of the French,) by which the general condition of the surface under formation may be more quickly and accurately tested at one operation: and to avoid confusion of terms, it is proposed in all cases, when speaking of the instrument, to employ the French appellation piano-metre or rather planometer, which is exact and distinctive.
The flat piece of cast-iron, intended to be operated upon, having been chipped all over, as described in page 850, a coarse hand-file, of as large dimensions as the operator can safely manage is selected, and in the commencement, the rough edges or ridges left by the chipping-chisel are levelled, those parts however being principally filed, that appear from the straight-edge to be too high.
The strokes of the file are directed sometimes square across as on a fixed line, or obliquely in both directions alternately; at other times the file is traversed a little to the right or left during the stroke, so as to make it apply to a portion of the work exceeding the width of the file. These changes in the applications of the file are almost constantly given, in order that the various positions may cross each other in all possible directions, and prevent the formation of partial hollows. The work is tried at short intervals with the straight-edge; and the eye directed on a level with the work to be tested, readily perceives the points that are most prominent. After the rough errors have been partially removed, the work is taken from the vice, and struck edgeways upon the bench to shake off any loose filings, and it is then inverted on the planometer, which should be fully as large or larger than the work. As, however, it cannot be told by the eye which points of the work touch the planometer, this instrument is coated all over with some colouring matter, such as pulverised red chalk mixed with a little oil, and then the touching places become coloured.
In all probability the work will at first assimilate so imperfectly with the planometer, that it will only rest thereon at its two highest points, most likely at the two corners of the one diagonal, and when pressure is applied at the two other corners alternately, the work will probably ride or rock on the two points of temporary support. The work is slightly rubbed on the surface-plate, and then picks up at its highest points some of the red matter; it is refixed in the vice, and the file in principally used in the vicinity of the coloured parts, with the occasional test of the straight-edge, and after a short period the work is again tried on the planemeter.
Should the same two points still become reddened, they are farther reduced with the file, but it is probable the work may be found to rest upon larger portions of its surface, or upon three or four points instead of two only; and if so, all the marked places arc reduced in a small degree before the succeeding trial. This process is continually repeated, and if watchfully performed, it will be found that the points of contact will become gradually increased, say from two to four, to six or eight, then to a dozen or more, and so on.
In this, or rather an earlier stage of the work, the smith's plane for metal is often advantageously used in connexion with the file. The general structure of the plane is shown by the figure and description on page 488, and it is employed much after the manner of the joiner's plane, but it may be used at pleasure lengthways, crossways, or diagonally, without any interference from grain or fibre as in wood work. The grooved or roughing-out cutter is employed in the commencement because it more readily penetrates the work, and a few strokes are given to crop off the highest points of the surface, the furrows made by the serrated cutter are then nearly removed with the file, which acts more expeditiously although less exactly than the plane, and in this manner the grooved plane iron and the coarse file are alternately used. In the absence of the planometer, the metal plane assumes a greatly-increased degree of importance.
As the work becomes gradually nearer to truth, the grooved cutter is exchanged for that with a continuous or smooth edge, a second-cut, or bastard hand file, is also selected, and the same alternation of planing and filing is persevered in, the plane serving as it were to direct the file, until it is found that the plane iron acts too vigorously, as it is scarcely satisfied with merely scraping over the surface of the cast-iron; but when it acts it removes a shaving having a nearly measurable thickness, and therefore, although the hand-plane may not injure the general truth of the surface, it will prevent the work from being so delicately acted upon, as the continuance of the process now demands; a smoother hand file is consequently alone employed in furthering the work.