ELLIPTIC TURNING.
This machine is frequently called, by those who do not understand the ellipse, an oval chuck; but it is not oval, for an oval expresses an object that is smaller at one end than at the other. Fig. 1,plate 3, is a front view of the machine. I K is the iron plate to which all the parts (except the ring, hereafter described) are fastened. A screw, similar to that on the nose of the mandrel, is riveted tothis plate, fig. W, and upon it whatever you wish to turn is fixed. Fig. 2 exhibits a back view of this machine; at each of the four corners there is a short square pillar, marked D. Within these are two narrow ribs of steel, reaching the whole length of the plate I K. Each of them, being bevelled, forms an angular groove, reaching all its length. By means of these grooves the slider, F, moves up and down.
Plate 4.
Plate 4.
When the slider is in its place, two pieces of steel, M M, bear upon the side pieces, to which, and to the plate, I K, they are firmly attached by four screws, X X X X. The plate, F, being cast in the same piece as the slide, cannot be thrown out of its place, but moves in a longitudinal direction only, the nut, L, acting as a stop to prevent its going too far. The space between the end pieces, M M, is just equal to the diameter of the ring, O, Fig. 3, upon the outside of which they revolve when the nut is screwed upon the mandrel. Two arms, R R, are connected with it, and in each there is a groove extending nearly their whole length. This machine is connected with the lathe, and its motion obtained as follows:—E E, fig. 4, representsa headstock, through which two holes are drilled, the centres of which are precisely in a line with the centre of the mandrel, M; the ring is fastened to the headstock by two screws, I I, the shanks of which pass through the grooves and through the holes in the headstock. When the ring is in this position, it will be perceived that it can only move from side to side, and its centre must always be in the same horizontal line with that of the mandrel. Now screw upon the mandrel the nut, L, fig. 2. The plate, I K, if set exactly opposite the mandrel, will revolve in a circle; but if the centre of the ring be the least in the world on one side of the mandrel, it will revolve in an ellipse. When, therefore, the work is fastened to the screw, W, it is quite as easy to turn an ellipse as on the ordinary lathe to turn a cylinder.
Of course, the slider must move with great steadiness and freedom, to effect which, very great accuracy in the workmanship must be observed. Figs. 5 and 6 are two different views of the machine,—5 is the side, 6 the end view.
To turn a hollow sphere, the convex surface is first turned, and perfectly smoothed and finished; itmust then be bored with a centrebit, to make an aperture sufficiently large to admit the tool, fig. 7,plate 1, with which the interior must be worked away. As, however, it would require a large aperture to enable you to hollow out the whole sphere, it is preferable to make six openings with the centrebit, each in a line with the centre; they must also be made at equal distances from each other, and every hole must be at right angles with all the rest, except one, which is exactly opposite to it. Place the sphere in a chuck, with the middle of any two of the holes in a line with the axis of the mandrel; turn out as much as you can of the first hole, then bring the other holes forward and do likewise; at last the excavations will be cut through.
To turn the Chinese balls, which are so much admired for their beauty and curious workmanship, we are told to proceed as follows:—As they are composed of spheres one within the other, the holes must be just deep enough to leave the thickness between each little more than the diameter of the smallest sphere. The work must be begun by forming the innermost sphere, and afterwards it must becontinued regularly on to the larger, till at last the outer sphere is completed.
An immense, almost an endless, variety of figures may be worked in the lathe by carefully regulating the tools. Beautiful flowers in ivory, equal to the Chinese carving, are formed by the experienced turner, with small wheels and other instruments: medallions, even, are executed in the lathe, with machinery so constructed that the tool follows on the wood the exact lines of the head that is being copied.
Gold and silver ornaments, such as watches, snuff-boxes, and other trinkets, are worked with what is called a rose engine; plates with patterns indented upon them are fastened upon the mandrel: the screw regulates the tool, which produces an exact counterpart of the pattern.
Many of the copper cylinders used in printing calicoes afford curious specimens of engraving in the lathe. It is impossible to imagine any thing more beautiful than the effect produced, and a whole web of linen is printed by them in a very few minutes. The methods employed in this kind of turning by the various artists who practice it are butlittle known; and, indeed, to look at the patterns produced, it would be supposed that the graver must have been used to form them. A general idea of the nature of the work is all we are enabled to give. The pattern intended for the cylinder must be cut upon a small steel wheel, which revolves upon an axis. This wheel is then to be held against the copper cylinder, which, quickly revolving in the lathe, carries it round, and receives from it the impression of its pattern. The wheel operates like a punch, but the roughness it makes on the edge of the work is easily polished down.
Having now finished the directions for concentric and elliptic turning, we will, before beginning the ornamental parts of this interesting art, beg to impress upon the minds of our readers that they must never be discouraged by failure in their first attempts, even though they may be subjected to many disappointments; for, as the Bard of Avon expresses it:—
“Oft expectation fails, and most oft thereWhere most it promises; and oft it hitsWhere hope is coldest, and despair most sits.”
“Oft expectation fails, and most oft thereWhere most it promises; and oft it hitsWhere hope is coldest, and despair most sits.”
“Oft expectation fails, and most oft thereWhere most it promises; and oft it hitsWhere hope is coldest, and despair most sits.”
“Oft expectation fails, and most oft there
Where most it promises; and oft it hits
Where hope is coldest, and despair most sits.”