The art of giving circular and other forms to solid substances, in the fabrication of innumerable articles, by the aid of a machine called a lathe. There is perhaps no contrivance with which human ingenuity has aided the dexterity of the mechanic more entitled to our admiration than the lathe; especially when we take into the account all the improvements it has undergone, from its simplest and most ancient form in the potter's wheel, to that adaptation of varied and complex mechanism, by which not merely circular turning of the most beautiful and accurate description, but exquisite figure-work, and complicated geometrical designs, depending upon the eccentric and cycloidal movements, are daily produced.
The operation of turning differs very essentially from most others, in the circumstance, that the matter operated upon is put in motion by the machine, and is wrought by means of edge tools, presented to it, and held fast; whilst in most others the work is fixed, and the tool put in motion. In ordinary turning, the work is made to revolve on a stationary straight line as an axis, while an edge tool, set steady to the outside of the substance in a circumvolution thereof, cuts off all the parts which lie farthest from the axis, and makes the outside of that substance concentric with the axis. In this case, any section of the work made at right angles to the work will be of a circular figure; but there are methods of turning ellipses and various other curves, distinguished by the name of engine-turning.
Lathes are made in a great variety of forms, and put in motion by different means: they are called centre lathes where the work is supported at both ends; mandrel, spindle, or chuck lathes when the work is fixed at the projecting extremity of a spindle. From different methods of putting them in motion, they are called pole-lathes, and hand-wheel lathes, or foot-lathes; for great works they are turned by horses, and water-wheels, but more generally by stean.-engines. The lathes used by wood-turners are usually made of wood, in a simple form, and are called bed-lathes; the same kind will serve for turning iron and brass: but the best work in metal is always done in iron-lathes, which are usually made with a triangular bar, and are called bar-lathes. Small ones, for the use of watch-makers, are denominated turn-benches; hut there is no essential distinction between these and the centre lathes, except in regard to size, and that they are made in metal instead of wood, and the workmanship being more accurate and better finished.
The centre lathe is now very little used hut by country turners, to make articles of household furniture in soft wood, as table-legs, staircase-rails, bedposts, etc. It consists of the following parts: 1st. The bed, which is composed of two beams bolted together at a small distance asunder, and parallel to each other; it is supported horizontally on legs at the ends, and forms the support of the whole; the groove is the narrow opening between the two halves or chucks of the bed, to receive the tenons of the puppets, which are two short upright posts fastened down upon the bed at any place by means of wedges, driven through mortices in the tenons of the puppets beneath the bed; one of the puppets has a pike or pin of iron fixed into it, and the other one has at the same level the centre-screw, working through a nut fastened in the puppet; both the screw and the pike have sharp points made of steel, and hardened and tempered that they may not wear away; they must be exactly opposite, and in a line with each other. The piece of wood which is to be turned, suppose, for instance, a pole of wood, is supported by its ends between the points of the pike and the screw, that it may turn round freely, and the screw is screwed up, till it has no shake.
The puppets can be placed at any distance asunder, according to the length.
The rest is a rail or bar, extending from one puppet to the other, for the support of the tool; it lays in hooks projecting from the faces of the puppets; the work is put in motion by means of the treadle, which is worked by the turner's foot; the string or cat-gut is fastened to the treadle, and passing two or three times round the work, it is fastened to the end of an elastic pole, fixed to the ceiling over the turner's head: now as the turner presses the treadle down by his foot, the string turns the work round, and a sharp chisel or gouge, being held against the wood upon the rest, will cut the wood to a circular form. When he has brought the treadle to the ground, he releases the weight of his foot, and the elasticity of the pole draws up the treadles, turning the work back again; during which retrograde motion he withdraws the chisel from the work, as it would not cut in this direction through it, and might impede the motion of the wood; and the pole is fastened to the ceiling of the room, where the lathe is placed by a pin, upon which it can be turned about as a centre, and it rests upon a horizontal bar fixed at some distance from the centre: it is placed in a position nearly perpendicular to the axis of the work, so that, when it is turned upon its centre pin, the string at the other end may be brought over any part of the length of the work where it will be most convenient for the turner to have the string put round it: in the same manner the end of the treadle is placed, with one end over a centre pin in the floor, that its opposite end may be moved under the work to the proper place for the string.
It is held in this position, while moving up and down, by a second treadle, perpendicular to the first, which moves in a loose centre on the floor at one end, and the other is perforated with a number of holes to receive a pin fixed in the first treadle, and thus to confine the treadle to move up and down under any place it is set to: the end of the principal treadle is turned in the lathe, and made like a pulley, to hold the line or string which is wound upon it, and the turner winds the string on or off this end of the treadle, to adjust its length to the diameter of the work round which the string passes; the string is fastened to the end of the spring-pole in a similar manner. The workman stands, or is seated before his lathe, having one of his feet on the treadle, to give the motion; it must be very moderate and equal; he places his tool on the rest, and approaches the head of it gently to the piece, performing his work gradually without leaving any ridges, and when he meets with a knot, he must go on still more gently, otherwise he would be in danger both of splitting his work, and breaking the edge of his tool.