Turning, the art of shaping wood, metal, or other hard substances into forms having usually curved, and most commonly circular outlines, and also of executing figures composed of curved lines upon plane or cylindrical surfaces, by means of appropriate tools and a machine called a lathe. Theodore of Samos, named by Pliny as the inventor of turning, may have originated the application of the process to the shaping of hard substances. The principle of turning is simple. A piece of wood or other hard substance being so fixed in a horizontal position by pivots or otherwise at its two ends as to be allowed to revolve freely about an axis in the direction of its length, and caused to turn rapidly in this manner, while a chisel or other cutting tool is approached to the piece so as to meet it as it advances on one side, and held firmly to it, the tool will cut away from the piece at that place all the material outside of a circle whose radius is the distance of its point from the axis of motion; thus it will give to the part a circular outline, and will reduce the diameter of this circle as its point is advanced further into the material.

The tool being gradually moved along the length of the turning piece, it can thus be made to reduce successively the entire length to the circular outline, and, by cutting to different depths in different parts, to produce a turned piece marked with circular grooves, or other forms of curved surface. Outside circular turning is the most common, and is known as "centre work." With lathes of peculiar construction the work may be turned hollow, or bored or reamed, or turned both inside and out. - The most simple lathe for wood turning is called the pole lathe, made of two horizontal and parallel planks or beams, having a narrow space between them in which there are two uprights or puppets, one stationary, at the end of the bed, the other having a tenon passing through the space and secured by a wedge beneath, so as to be movable along the bed, to accommodate it to the length of the work. Near the upper end of each puppet, on the sides facing each other, is a conical iron or steel point, the two being in the same line. The piece is placed between the points, and the movable puppet brought up so that both points are pressed slightly into its ends; the line between these is the axis about which the piece will revolve.

The fixed rest is placed conveniently for steadying the tool; while to turn the work, a groove being cut about it, usually at the left end, a stout cord or catgut is passed twice or oftener about the piece, then drawn straight and attached below to a treadle to which the workman's foot is applied, and above to an elastic pole or lath fixed at one end to the ceiling overhead, whence, probably, the name pole lath, or simply lath, or lathe. The workman holds the gouge or chisel to the work, and pressing down the treadle with his foot, the work is caused to spin rapidly round toward the point of the tool; and so long the latter takes effect. When the treadle, having come quite down, is released by the foot, the recoil of the lath carries back the cord and work in the reverse direction, and the tool does not cut. If it is required to turn the entire length of the piece, the cord must be shifted after a time to the finished part. This contrivance serves for ordinary wood turning; but on account of its imperfections, and especially the loss of time during the return or rising of the treadle, it is now little used. For light or fine work, the pole is often replaced by an elastic bow and string overhead, the cord giving the revolution being attached to the middle of the string.

But the forms now more usual, and especially for heavy work, are: 1, the foot lathe, in which the treadle is by a link made to give motion to a crank, from a larger grooved wheel upon which a cord crosses in form of an 8 to a smaller grooved wheel or pulley fixed upon an axis at one end of the work, and giving motion to it; 2, the hand-wheel lathe, in which the power is applied by the action of one or two persons in turning a wheel, from which a band communicates movement to the axis and work; and 3, the power lathe for the heaviest work, moved by horses, water, or steam. Any wooden lathe, such as is used by turners in wood, is also distinguished as a bed lathe; while those of iron, for the best work in metals, are called bar lathes. In any form of lathe such as those now considered, the turning axis at one end of the work, to receive the power and give motion to the piece, in place of the simple point which can be used in the pole lathe, becomes indispensable. This axis is called the mandrel, and sometimes the "live spindle." The chucks, or contrivances fixed upon the end of the mandrel, are of various forms and construction, according to the kind of work they are intended to secure; the most common being the screw chuck, the hollow, drill, universal, concentric or die, and ring chuck, and the carrier and driver.

The crowning improvement in the adaptation of the lathe to accuracy of work, and to that of all varieties, is attained through the invention of the slide rest, a carriage upon which the tool is supported, and by the construction of which it can be moved along the work by the machine or by hand, and at the same time advanced toward it, or set at any angle, as the character of the work may require. - In the cases thus far considered, there is but one axis or centre line of the work; that is, the centre line is a fixed direction throughout the process of cutting. But it is desirable to execute work in which the cutting in different parts or moments shall be in reference to two or more different axes through the solid or surface acted upon. A simple mode of effecting this is that of fixing the work successively with the different axes, and turning it at two or more operations. But practically, a far more complete and satisfactory result is attained, the forms executed being variable at the pleasure of the workman, by incorporating into the lathe devices which shift the place or direction of the single axis of motion, and in a defined manner and degree, while the work is rotated and the tool continually acting upon it.

This species of turning, with a variable centre of the work at different moments, takes several names according to the devices employed or the particular results secured, as eccentric, geometric, oval, and rose-engine turning. Such work is said to be figure-turned; and the general principle is that of employing some form of chuck which shall give an oscillation or lateral movement to the axis or the work during its revolutions, so as to insure those deviations from a simple circular application of the cutter required for the intended form or figure. The chucks employed are designated as the eccentric, the geometric, the oval, or more properly elliptical, and as rosettes; while a straight line chuck can also be employed to cut plane surfaces or square work. In all work of this character, the amateur turner prides himself not only on the delicacy and elegance of his results, but quite as much on the difficulty of execution; and the value of turned work is often estimated by the degree in which it departs from the circular figure.

For eccentric turning, a single eccentric chuck is one fixed upon a strong plate that can slide laterally within straight guides screwed upon the face of the mandrel, and which, having upon it a toothed wheel and click, is called the click plate; the slide, and the nose upon it for carrying the work, can be shifted into various positions out of centre, before applying the cutter. In the double eccentric chuck, a second plate or slide at the back of the first, and at right angles to it, can further vary the position of the axis. By aid of such a chuck, any required part of a disk can be brought in line with the centre of the mandrel, and holes bored in any part of it, or the edges hollowed out in curves of like or different radii, or polygons with curvilinear sides accurately produced. Ornamental work with these chucks consists mostly in the execution of variously curved figures on surfaces, without cutting away or changing the general outline, as in ornamenting ivory or fine work in wood. The ivory turner often employs a small instrument called an eccentric cutter, in which the tool revolves rapidly, being moved by a bow, and with which, by means of a single eccentric chuck and a separate adjustment of the cutter, involved figures like those ordinarily requiring the double eccentric chuck are produced.

For geometric turning, a chuck of similar name is employed. In this, a wheel concentric with the mandrel, while the latter and the chuck revolve, gives, by means of a train of smaller wheels, an independent movement to the click plate and axis of the work. By varying the relative sizes of the wheels, by introducing an added wheel to cause the work to turn at the same time in a direction the reverse of that of the mandrel, and by changing the position of the tool, or giving movement at the same time to it, an almost infinite variety of curiously involved curved figures may be engraved or marked upon a plane surface to which motion is thus imparted under the point of the tool. By this machine some of the most perfect rosettes and other lathe work of bank notes, in the United States largely relied on as a means for the prevention of successful counterfeiting, are executed. The figures will vary with the construction of the machine; of which, save by actual inspection or model, no duplicate can be constructed.

The geometric lathe of the American bank-note company of New York, the single one of its kind, is a foot lathe of highly complicated and perfect workmanship, the construction of which is said to have occupied three years' time, at a cost of $10,000. Elliptical turning is accomplished by means of the elliptical or oval chuck, in which the pressure of an eccentric ring, moving within and clasped by rubbers, is made to draw the slide out of centre alternately in the opposite directions, so that a stationary tool, held to a plate to which this movement is imparted during a revolution of the mandrel, will describe an ellipse instead of a circle; while the size, direction, and form of the ellipses can also be varied; and, as in the other forms of lathe here described, the micrometer screw may be introduced for determining the accuracy of proportions in the work. - Of the few machines which have been invented for turning irregular forms, in heavy or ordinary work, that of Blanchard is perhaps the best known and most successful.

The principle of this machine is, that forms are turned by a pattern the exact shape of the object to be produced, which in every part of it is successively brought in contact with a small friction wheel; this wheel precisely regulates the motion of chisels arranged upon a cutting wheel acting on the rough block, so that as the friction wheel successively traverses every portion of the rotating pattern, the cutting wheel pares off the superabundant wood from end to end of the block, leaving a precise resemblance of the model. This remarkable machine, with modifications and improvements, is in use in the national armories as well as in England, and in various forms is applied to many operations in making musket stocks, such as cutting in the cavity for the lock, barrel, ramrod, butt plates, and mountings, comprising, together with the turning of the stock and barrel, 13 different machines. Besides gun stocks, it is also applied to a great variety of objects, such as busts, shoe lasts, handles, spokes, etc. - For further information respecting the tools and materials used in turning, see Holtzapffel's "Turning and Mechanical Manipulations" (3 vols., London, 1847-52); and for the general subject, see the article "Lathe" in "Appletons' Dictionary of Machines," etc. (New York, 1857), and Valicourt's Kouveau manuel complet du tourneur (3 vols., Paris, 1848-'53).