TURNING is a very ingenious and useful art, by which a great variety of ar ticles are manufactured, by cutting or fashioning them while they revolve upon an axis. Every solid substance in nature may be submitted to this process ; and, accordingly, we have articles turned in the metals, in wood, in pottery, in stone, in ivory, &c. Among the great varieties of lathes, it is indispensably required, for circular turning, that the work should be supported by two steady centres, or by parts equivalent to two centres, at a dis tance from each other in the axis of rota tion, and that the too] should be support ed by a steady bar, or a piece called the rest.
Clock and watch-makers use a very cheap, simple, and portable lathe, called a turn-bench, consisting of a straight bar of iron, about five inches long, with two cross bars or heads, about two inches long, one fixed at the end of the long bar, and the other capable of being shifted by means of a socket and screw. In each of these heads is a centre-pin, terminating in a point at one end, and in a central hole at the other, like the centre-pin in the poppet-head of any other lathe ; the use of which is to afford point centres when the points are turned towards each other, or hole centres when the contrary is the case ; and, lastly, there is a small rest, with its support, slidable and ad justable along the bar, as in another lathe. These instruments will, there fore, support any piece of four or five inches long, and three inches diameter, between the centres; and the method of producing the rotation is by passing the cat-gut string of a bow once or twice round the work, and drawing the bow backwards and forwards with one hand, while the other is employed in applying the tool. The turn-bench itself is held steady in a vice fixed to a bench or stand.
The common lathe of the turners in wood, called the pole-lathe, is the same thing as the watchmaker's turn-bench, but upon a large scale, and a little varied. Instead of the horizontal bar, it has two long stout bars of wood, called shears, forming what it called the bed of the lathe, and its two poppet-heads are up right blocks of wood, mortised in be tween the shears, above which they rise and carry the centre-screws, and be tween which they are movable, and may be wedged firmly at any required dis tance from each other. 'The work itself
is either put between the centres, or up on a wooden mandrel, and is made to revolve by a string or band, proceeding from a long spinning pole at the ceiling or roof of the shop, round the work, and thence to a treddle or foot-board, which acts by alternate pressure from the foot, while the workman applies the cutting tool with his hands.
The velocity of rotation may be ex tremely swift in wood, slower in brass and bell-metal, still slower in east-iron, and slowest of all in forged iron or steel. Steel and iron require to be kept wet.
If the poppet-heads, supporting the mandrel, be made regularly to move from side to side, during the rotation, or the rest be made to approach to, and recede from, the work, any number of times in a turn, the cuts will not be circular, but undulating, indented, or waved in any curve that may be required. The mo tion is commonly regulated by certain roundplates of brass fixed on the man drel, called roves, which have their edges waved, and are called roses. The largest columns, the most ponderous artillery, and the minutest pivots of wateh-work, with all wheel-work, rotatory machines, vessels, &c., are worked by this ma chine.
Turning of earthenware and porcelain is requisite, to give to circular vessels their correct figure, and proper size and thickness. The lathe has similar ment to that for turning wooden-vessels, ; with this addition, the treddle has a cross-piece, for convenience of a tread er, giving motion more readily to the wheel and spindle, and of properly re gulating that motion, while the turner is steadily engaged in his operations. Outside of the bead-stock of the lathe, on the screw, is fixed a chuck, of the size proper for the inner surface of the edge of the vessel, to fit easily. The turner stands looking towards the headstock (not exactly linear with his lathe, as does the wood-turner), and is separated from the wheel by a wainscot partition, so as to prevent any of the clay-turnings fall ing on the machinery. The treader stands at the end of the spindle, on a raised position, so that the foot requires the treading, to give force to the treddle. At the side of the treader is a board, on which are the vessels from the thrower, one of which is constantly handed over to the turner as he needs it.