One of the most ingenious machines to which the milling tool has been fitted is the well-known Blanchard lathe, which copies, generally in wood, repetitive work, such as the stocks for guns and rifles. The lathe has two sets of centres—one for the copy, the other for the model—parallel on the same bed, and turned at equal speeds and in the same direction by a train of gear wheels. The milling cutter is attached to a frame, from which a disc projects, and is pressed by a spring against the model. As the latter revolves, its irregular shape causes the disc, frame, and cutter to move towards or away from its centre, and therefore towards or away from the centre of the copy, which has all superfluities whisked off by the cutter. The frame is gradually moved along the model, reproducing in the rough block a section similar to the part of the model which it has reached.
The self-centring chuck is an accessory which has proved invaluable for saving time. It may most easily be described as a circular plate which screws on to the inner end of the mandrel (the spindle imparting motion to the object being machined) and has in its face three slots radiating from the centre at angles of 120°. In each slot slides a stepped jaw, the under side of which is scored with concentric grooves engaging with a helical scroll turned by a key and worm gear acting on its circumference. The jaws approach or recede from the centre symmetrically, so that if a circular object is gripped, its centre will be in line with the axis of the lathe. Whether for gripping a tiny drill or a large wheel, the self-centring chuck is indispensable.
PLANING-MACHINES
Not less important in engineering than the truly curved surface is the true plane, in which, as Euclid would say, any two points being taken, the straight line between them lies wholly in that superficies. The lathe depends for its efficiency on the perfect flatness of all areas which should be flat—the guides, the surface plates, the bottom and sides of the headstock, and, above all, of the slide rest. For making plane metal superficies, a machine must first be constructed which itself is above suspicion; but when once built it creates machines like itself, capable of reproducing others ad infinitum.
Many amateur carpenters pride themselves on the beautiful smoothness of the boards over which they have run their jack planes. Yet, as compared with the bed of a lathe, their best work will appear very inaccurate.
The engineer's planing-machine in no way resembles its wooden relative. In the place of a blade projecting just a little way through a surface which prevents it from cutting too deep into the substance over which it is moving, we have a steel chisel very similar to the cutting tools of a lathe attached to a frame passing up and down over a bed to which the member holding the chisel is perfectly parallel. The article to be planed is rigidly attached to the bed and travels with it. Between every two strokes the tool is automatically moved sideways, so that no two cuts shall be in the same line. After the whole surface has been "roughed," a finishing cutter is brought in action, and the process is repeated with the business edge of the tool rather nearer to the bed.
Joseph Clement, a contemporary of Babbage, Maudslay, and Nasmyth, is usually regarded as the inventor of the planing-machine. By 1825 he had finished a planer, in which the tool was stationary and the work moving under it on a rolling bed. Two cutters were attached to the overhead cross rail, so that travel in either direction might be utilised. The bed of the machine, on which the work was laid, passed under the cutters on perfectly true rollers or wheels, lodged and held in their bearings as accurately as the best mandrel could be, and having set screws acting against their ends, totally preventing all end-motion. The machine was bedded on a massive and solid foundation of masonry in heavy blocks, the support at all points being so complete as effectually to destroy all tendency to vibration, with the object of securing full, round, and quiet cuts. The rollers on which the planing-machine travelled were so true, that Clement himself used to say of them, "If you were to put a paper shaving under one of the rollers it would at once stop the rest." Nor was this an exaggeration—the entire mechanism, notwithstanding its great size, being as true and accurate as a watch.[6] Mr. Clement next made a revolving attachment for the bed, in which bodies could be revolved under the cutter, on an axis parallel to the direction of travel. According to the wish of the operator, the object was converted into a cylinder, cone, or prism by its movements under the planing-tool. So efficient was the machine that it earned its maker upwards of ten pounds a day, at the rate of about eighteen shillings a square foot, until rivals appeared in the field and finally reduced the cost of planing to a few pence for the same area.
There are two main patterns of planes now in general use. The first follows the original design of Clement; the second has a fixed bed but a moving tool. Where the work is very heavy, as in the case of armour-plates for battleships, the power required to suddenly reverse the motion of a vast mass of metal is enormous, many times greater than the energy expended on the actual planing. For this reason the moving-bed machines have had to be greatly improved; and in some cases replaced by fixed-bed planers.
It is an impressive sight to watch one of these huge mechanisms reducing a rough plate, weighing twenty tons or more, to a smoothness which would shame the best billiard table. The machine, which towers thirty feet into the air and completely dwarfs the attendant, who has it as thoroughly under control as if it were a small file, bites great shining strips forty feet long, maybe, off the surface of the passive metal, and leaves a series of grooves as truly parallel as the art of man can make them. There is no fuss, no sticking, no stop, no noise; the force of electricity or steam, transmitted through wonderfully cut and arranged gear-wheels, is irresistible. The tool, so hard that a journey through many miles of steel has no appreciable effect on its edge, shears its way remorselessly over the surface which presently may be tempered to a toughness resembling its own. If you want to resharpen the tool, it will be no good to attack it with any known metal. But somewhere in the works there is a machine whose buzzing emery-wheels are more than a match for it, and rapidly grind the blunted edge into its former shape, so that it is ready to flay another plate, one skin at a time.
Planing-machines are of many shapes. Some have an upright on each side of the bed limiting the width of the work they can take; others are open-sided, one support of extra strength replacing the two, enabling the introduction of a plate twice as broad as the bed. Others, again, are built on the verge of a pit, so that they may cut the edges of an up-ended plate, and make it fit against its fellows so truly that you could not slip a sheet of paper edgeways between them. Thus has man, so frail and delicate in himself, shaped metal till it can torture its kind to suit his will, which he makes known to it by opening this valve or pulling on that lever. Not only does he flay it, but pierces it through and through; twists it into all manner of shapes; hacks masses off as easily as he would cut slices from a loaf; squeezes it in terrible presses to a fraction of its original thickness; and otherwise so treats it that we are glad that our scientific observations have as yet discovered no sentience in the substances reduced to our service.