The former instructions have been restricted to the supposition that only one of the superficies of the work was required to be made plane or flat; but it frequently happens in rectangular works, such as the piece A B C, fig. 868, that all six surfaces, namely, the top and bottom A, a, the two sides B, b, and the two ends C, c, all require to be corrected and made in rectangular arrangement (the surfaces a, b, c, being necessarily concealed from view), and therefore some particulars of the ordinary method of producing these six surfaces will be added; and the former remarks on pages 500 to 508 on squaring thick and thin works in wood may be also consulted.

The general rule is first to file up the two largest and principal faces A and a, and afterwards the smaller faces or edges B b, and

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C c. The principal faces A a, especially when the pieces are thin, must be proceeded with for a period simultaneously, because of the liability of all materials to spring and alter in their form with the progressive removal of their substance, and on this account the work, whether thick or thin, is frequently prepared to a certain stage at every part, before the final correc-tion is attempted of any one part.

The straight-edge and surface-plate are required, to prove that each of the faces A and a is a plane surface, and the callipers or a similar gage is also needful to prove them to be in parallelism. Callipers, unless provided with set screws, are very liable to be accidentally shifted, and it is needful to use them with caution, otherwise their elasticity, arising from the length of their legs, is apt to deceive. There are gages, such as fig. 869, with short parallel jaws that open as on a slide, and arc fixed by a side screw; and a still more simple and very safe plan, is to file two rectangular notches in a piece of sheet-iron or steel, as in fig. 870, the one notch exactly of the finished thickness the work is required to possess, the other a little larger to serve as the coarse or preliminary gage.

Sometimes the one face of the work, or A, having been filed moderately flat, a line is scored around the four sides of the work with a metal making-gage, the same in principle as the marking-gage of the joiner, fig. 342, page 487. At other times the corrected face A, is laid on a planometer larger than the work, as represented, (neglecting the inversion,) and the marginal line is scribed on the four edges, by a scribing-point p, fig. 868, projecting from the sides of a little metal pedestal that bears truly on the surface-plate.

Chamfers or bevelled edges are then filed around the four edges of the face a, exactly to terminate on the scribed lines, the central part of a can be reduced with but little watchfulness, until the marginal chamfers are nearly obliterated. This saves much of the time that would be otherwise required for investigating the progress made; but towards the last, the callipers and planometer must be carefully and continually used, to assist in rendering A and a, at the same time parallel and plane surfaces.

The two principal edges, B b, are then filed under the guidance of a square; the one arm of the square is applied on A, or a, at pleasure, as in joinery work: or if the square have a thick back, it may be placed on the planometer, as at s, fig. 868; if preferred, the work may be supported on its edge B upon the planometer, and the back square also applied, as at s, in which case the entire length of the blade of the square comes into operation, and the irregularities of the plane B, are at the same time rendered obvious by the planometer.

Another very convenient test has been recommended for this part of the work - namely, a stout bar, such as r, fig. 868, the two neighbouring sides of which have been made quite flat and also square with each other. When the work and trial-bar, (or rectangulometer,) are both laid down, the one side of the bar presents a truly perpendicular face, which may, by the intervention of colouring matter, be made to record on the work itself, the points in which B differs from a rectangular and vertical plane.*

When the edge B has been rendered plane and square, the opposite edge b, may in its turn be marked either with the gage or scribing-point at pleasure; the four edges of b may be then chamfered, and the entire surface of b is afterwards corrected, (as in producing the second face a,) under the guidance of the square, callipers, rectangular bar, and surface-plate, or some of these tests.

See Smith a Panorama of Science, Vol. I, page 30.

The ends C c, now claim attention, and the marginal line is scribed around these by the aid of the back square alone: but the general method so closely resembles that just described as not to call for additional particulars.

Should one edge of the work be inclined, or bevelled, as in the three following figures, in which the works arc shaded to distinguish them from the tools, the rectangular parts arc always first wrought, and then the bevelled edges, the angles being denoted by a bevel instead of a square: either with a bevel having a movable blade, as in fig. 871, or by a bevelled templet made of sheet-metal, as in figs 872, or 873, which latter cannot get misadjusted. The bevelled edge of the work, is also applied if possible on the planometer; in fact the planometer and bevel are conjointly used as the tests. Bevelled works are either held in the vice by aid of the chamfer-clamps, fig. 855, page 859, or they are laid in wooden troughs, with grooves so inclined, that the edge to be filed is placed horizontally. Triangular bars of equilateral section are thus filed in troughs, the sides of which meet at an angle of 60 degrees, as in fig. 874.

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The succeeding examples of works with many plane surfaces are objects with rebates and grooves, as represented in figs. 875 to 878. Pieces of the sections 875, and 876, supposing them to be short, would in genera] be formed in the solid, either from forgings or castings, as the case Bight be; the four exterior and more accessible faces would be tiled up square and true, and afterwards the interior faces, with a due regard to their parallelism with the neighbouring parts, just after the mode already set forth. The safe-edge of the file is now indispensable; as in filing the face b, the safe-edge of the file is allowed to rub against the face a of the work, and which therefore serves for its guidance; and in filing the face a, the side b becomes the guide for the file. The groove in fig. 876 requires a safe-edge square file.