The general aim of this present section, is to show, by way of contrast, how several of the pieces advanced as illustrations of works executed with files, in sections iv. to vii. are produced by the planing-machine and analogous contrivances.

The comparison of the modes of producing flat and rectilinear works with the file and with the planing-machine, is greatly in favour of the latter method, in respect to facility, expedition, and accuracy; and the modes are besides entirely different.

For instance, the laborious and tedious mode of filing a flat surface has been spoken of at some length, and it will be remembered the work is fixed, and the tool is moved in a variety of directions upon the surface to be filed.

So far as the action of the smith's hand-plane for metal, (page 483,) and carpenters' planes may be brought into comparison, they are used in many respects as the files, but are applied generally parallel with the one side of the superficies that is being wrought.

But the engineer's mode of planing works in metal is entirely different from either of the above, and is strictly analogous to the mode of turning works in the lathe with a slide rest, if we consider the axial motion of the lathe to be replaced by the rectilinear motion of the planing-machine, as was briefly explained in general terras in the introductory chapter to the present volume, more particularly in reference to the " guide principle" see pages 468 and 469.

The work to be planed, is there briefly described as fixed on a carriage or sledge, that is made to move to and fro in a true right line, as upon a very accurate railway; whilst the cutting-tool, which is just like those used in the turning-lathe was supposed to be affixed to a bridge, standing across and at right angles to the railway. Such a fixed tool would plough a furrow in the work, which furrow would be accurately straight as the railway or guide from which the work itself received its direct of motion.

If the reader will only conceive the work to be continually moved to and fro upon the slide or railway, a distance equal to its own length; and that by a subsidiary contrivance, or another slide placed horizontally, the tool between each reciprocation of the work were moved a small distance to the right continually, a series of grooves would be ploughed, all individually right lines; and these grooves would shave off all the asperities and irregularities of the work, leaving it finely grooved in parallel furrows. Or provided the end of the tool were flat, and ever so little broader than the small interval between the successive strokes, the tool would leave a plane or smooth surface; and the perfection of this surface would mainly depend, on the railway or the cutting-slide, and the horizontal or position-slide, being each truly rectilinear.

Supposing now, by means of a third slide placed exactly perpendicular, the tool were depressed the tenth of an inch, and the process were entirely repeated, the surface would be reduced with perfect uniformity, one-tenth of an inch all over, and the new surface would be mathematically parallel with that existing immediately previous. Pursuing this idea, let it be further supposed that the surface just planed is the upper surface of the carriage or sledge of the planing-machine, technically known as the bed or table, upon which the work that is to be planed is fixed by screw-bolts and clamps.

It will now be shown how the piece represented by A, B, C, in fig. 868, page 879, would be treated in the planing-machine. in order to make its sides strictly parallel, in pairs, and also at right angles to each other, in short, to convert it into a true parallclopipcdon.

The side A, of the piece, would be first placed uppermost and correctly planed; afterwards the side A would be inverted, and placed on the bed of the machine, care being taken that no shavings intervened to prevent their coming into absolute contact; and the second face, or a, would be planed strictly parallel with A, and that without any especial care on the part of the operator.

Next, to plane the edge B, it would be necessary the third slide of the planing-machine should be placed truly vertical, and that between each reciprocation of the bed of the planing-machine on its railway, the tool should be depressed a small quantity by the vertical slide, so as in the end to make it slowly descend, by intermittent steps, down a vertical and right line, exactly equal to the perpendicular height of the side B.

All things now remaining fixed as before, if the tool were traversed horizontally until it touched the second edge or b, this edge of the work would, on pursuing the same gradual depression of the tool, be planed also vertical and in strict parallelism with its opposite, or B.

Continuing the same order of work as in the hand process, the ends C, c, would require the work to be released from the bed of the machine, shifted just 90 degrees, and then refixed, when the ends C, c, would be treated exactly as B, b, had previously been, and would thence be made parallel and square.

In some instances, indeed, the analogy to the railway is strictly maintained for a farther stage of the work, as such a piece as A, B, C, fig. 868, would sometimes be mounted, as it were, upon one of the turn-plates by which the railway-carriage is twisted one quarter round, preparatory to moving it from off the principal to an adjoining line. In the planing-machine, the turn-plate on which the work is fixed, is a supplementary circular bed having a horizontal or azimuth motion, so that by leaving the tool unaltered, except in its vertical path, the several edges of any regular polygon, whether a square plate, or a triangle, pentagon, hexagon, etc, may be planed with strict accuracy. The sides of any irregular polygon, may be also planed, by moving the tool so much on the transverse or horizontal slide, as the differences in the radial distances of the sides of the unequal polygon from the center of the turn-plate.