"When however pieces of these sections, but of greater lengths, have to be produced by means of the file alone, it is more usual to make them in two or three pieces respectively, as shown detached in figs. 877 and 878; and which pieces are first rendered parallel on their several edges, and are then united by screws and steady pins; or rather, they are united before being actually finished, in order that any little distortion or displacement occurring in fixing them together may admit of correction.
In works of these kinds, which have rebates, grooves, internal angles, or cavities, the square, with a sliding blade, shown in fig. 876, is very useful, as the blade serves as a gage for depth, besides acting as a square, the one arm of which may be made of the precise measure of the edge to be tried. This instrument is often called a turning-square, as it is particularly useful for measuring the depth of boxes, and other hollowed works turned in the lathe.
In making straight mortises, as at s s, fig. 879, unless the groove is roughly formed, at the forge, or in the foundry, it is usual to drill holes nearly as large as the width of the mortise, and in a straight line; the holes are then thrown into one another by a round file, or a cross-cutting chisel, and the sides of the mortise are afterwards filed square and true.
For a circular mortise c c, the mode is just the same, with the exception that the holes are made on a circular line; and that, instead of a flat file being used throughout, a half-round or a crossing file is used for the concave side of the mortise.
Short rectangular mortises, or those which may be rather considered to be square holes, as in fig. 880, would if large be prepared by forging or casting the material into the form; and then the six exterior faces faces having been corrected, the aperture would be filed on all sides under guidance of some of the various tests before referred to. And in such a case, it is convenient to employ a small square s, in the form of a right-angled triangle to which is attached a wire that may serve as a handle, whereby the square may be applied at any part within the mortise without the light Of the workman being intercepted by his own fingers. Sometimes also, a cubical block filed truly on four of its faces to the exact dimensions of the aperture, is used as a measure of the parallelism and flatness of the four interior faces.
These miscellaneous examples of filed works with plane surfaces, will be concluded by others of somewhat frequent occurrence, and in which different tools are judiciously employed in conjunction with files. The method first to be described, is one that is considerably used in thick pieces of metal, for making holes differing from the circular form, such as square, hexagonal, triangular, elliptical, and other holes, by first drilling a round hole, and then enlarging and changing the section of the circular hole by a taper punch, better known as a drift, which tool is made of steel, and exactly of the same section as that required in the hole; the drift is hardened and tempered before use.
The drift for a taper square hole is made as in fig. 881, or simply as a square pyramid, considerably longer than the hole required: a round hole is first drilled in the work, just large enough to admit the small end of the drift, which is then driven in, its angles indent and force out the metal, making it first like the magnified line m, and ultimately exactly square, unless by mistake the hole were drilled too large, when the- circular parts would not be quite obliterated. If admissible, the endlong blows on the drift are mingled with a few blows on the sides of the work, as at b b,or parallel with the sides of the drift, which cause the metal to adapt itself more readily to the tool. The drift must not however be used too violently, for as it acts as a wedge, it may burst open the work, and which latter is therefore mostly left strong and rough before being drifted; and generally, when the angles have been somewhat indented, they are partly filed out, and completed by the alternate employment of the file and drift, the marks made by the latter serving continually to indicate the parts to be removed with the file.
Taper square holes, such as those in the chucks for drills, are made with some facility. The chuck is first drilled on its own mandrel, and the drift is put in the four different ways in succession, that the errors incidental to its form may be scattered and lost; the chuck is also placed on the mandrel at intervals, with the drift in its place, that the drift may show as it revolves, whether or not the hole is concentric. When it is required that the drifted hole should be parallel instead of taper, the drift is made as in fig. 882; that is, parallel for a short portion in the middle of its length, and the extremities alone are tapered so as to make the tool smaller at each end; the work is therefore first gradually enlarged to admit the largest part of the drift, and the parallel part is then driven through the work, and renders the inner surface of the same a true counterpart of the drift, if proper care have been taken. In some few cases, the sides of the drifts arc notched with a file, so as to act as teeth; but this is not general.
When drifts are used, the process of working is often reversed, or the interior surfaces are completed before the exterior. The holes are first drifted whilst the work is larger than its intended size, and afterwards the exterior part is filed or turned, as the case may be, from the hole, that is, the hole, (sometimes filled with the drift,) is made the basis of the measurement of the exterior portions of the work. Frequently, as in a square washer, the drift itself, or else a square arbor of similar form, with a center hole at each end, is made to serve as the chuck by which the work in placed in the turning lathe for completion.
In the concluding example of this section, that of making by hand the key-ways in the round holes of wheels, it is to be observed that it is common to turn a cylindrical plug exactly to fill the hole, and to make a notch in the plug as wide as Unintended key-way and parallel with the axis: the plug is shown at g, fig. 883. A piece of steel f, is then filed parallel, and exactly to fit the notch, and its edge is cut as a file, and used as such within the guide-block, the latter being at the time inserted in the hole of the wheel. In this case the block becomes the director of the file, and the notches in any number of wheels arc made both parallel and axial, and the only precaution that remains to be observed is in the depth of the notches, and this is not always important; the depth may however be readily determined, by making the grooves at first a little shallower than their intended depth, and then, the plug having been removed from the hole, a stop is attached to the side of the file, parallel with its edge, as at s, to prevent its penetrating beyond the assigned depth.
The method of cutting key-ways in large wheels, that was frequently employed prior to the introduction of machinery for the purpose was as follows. Supposing the wheel to have been bored with a three-inch hole, and to have required a key-way halt-inch wide and half-inch deep. The guide-block g, fig. 884, of three inches diameter, would have had a groove say half-inch wide and one inch deep, and a cross-cut chisel c, exactly to fill the groove would have been made. The chisel having the same section as the groove, when driven through would produce no effect; but if a piece of sheet steel s, 1/16, thick, were laid at the bottom of the groove, the chisel would then cut a groove half-inch wide and -1/16 deep; and if two, three, four, and ultimately eight such strips were successively employed together, as in the section and detached views, fig. 884, the hole would be accurately chiselled out by the repetitions of the process. The hole would require to be finished with a parallel thick file, called a key-way or cotter-file, which has already been described on page 822 - 3, of the present volume.