TURNING, in mechanics, a very in genious and useful art, by which a great variety of articles are manufactured, by cutting or fashioning them while they re volve upon an axis or line, which in most cases remains immoveable. 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. so nuine rous, and so universally in use, that it would be superfluous to enumerate or point them out. In the present article we shall describe the art in a general way, sufficient to show its principles, and may be of utility in practice.
The simplest process of turtling is that. of the potter, who, in the first stage of forming his ware, sticks a piece of hu mid clay upon a wheel, or flat table, while it revolves horizontally; and in this state of rotation of the clay, he fashions it , with the greatest facility into vessels of every description. But in most operations of the art, the revolving body is cut or shaved by applying a chissel, or other suitable tool, to its surface, while in mo tion; a condition that requires firmness in the axis of rotation, and also that the tool itself should be steadily supported. The instrument or apparatus for these purposes, is called a Lazar. See the arti cle. Among the great varieties of lathes, it is indispensably required, for circular turning, that the work should be sup ported by two steady centres, or by parts equivalent to two centres, at a distance from each other in the axis of rotation, and that the tool should be supported by a steady bar, or piece, called the rest. The mechanism for causing the rotation has been described in the article just referred to.
A great number of turned articles ei ther have, or will admit of, a perforation through their axis. All wheel-work, and most of the articles turned in wood, are of this description. Clock and watchma. kers accordingly use a very cheap, sim ple, 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 is the case ; and lastly, there is a small rest with its support, slidable and adjust able along the bar, as in another lathe.
These instruments, which cost five or six shillings at the watch tool shops, will therefore support any piece of four, or five inches long, and three inches diame ter between the centres, and the method of producing the rotation is by passing the catgut 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.
Such pieces as have a hole through the centre are drawn tightly upon an arbor or mandrel, having a pulley or ferril fix ed upon it, to carry the gut or bow-string, and the mandrel itself is turned between the centres upon its own pointed extre mities. There are mandrels fitted up in different ways for holding the work firm ly-, and if flat, at right angles to the mo tion ; but we cannot consistently with brevity enter upon a description of them, which will immediately be understood by inspection in a workshop.
The common lathe zif the turners in wood, called the pole-lathe, is the same thing as the watchmakers' 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 sheers, forming what is called the bed of the lathe, and its two poppet-heads are up right blocks of wood, mortised in be tween the sheers, above which they rise and carry the centre screws, and between which they are moveable, and 'may be wedged firmly at any required distance from each other. The work itself is ei ther put between the centres, or upon a • wooden mandril, and itt, is° made to re volve by a string or band, proceeding from a long springing pole at the ceiling or roof of the shop, round the work, and thence tol.a treadle or foot-board, which acts by alternate presiure from the foot, While the workman applies tool with his hands.