Wood and other substances are turned into an elliptic form by means of a chuck, which is applied on the common turning lathe. This chuck is on the principle of the trammel, Plate CCXVIII., fig. 1. The grooves in the chuck are much wider than in the trammel, and the points of the chuck that correspond to C, D of the trammel, fig. 1, remain fixed, and in one horizontal line; whereas in the trammel these are the points which are put in motion.

If two straight lines, crossing each other, be drawn on a piece of transparent paper or mica; if this transparent paper be laid upon a sheet of white paper, with two principle points marked on it; if the transparent paper be moved round, so that the cross lines shall travel over the two points, in like manner as the two points C, D in the trammel, fig. 1, travel over the cross grooves of the trammel; and if the point of a pencil be held fixed, and touching the transparent paper, so as to leave a trace on the transparent paper when the paper is moved; then after the transparent paper has made a revolution, the trace left on it by the point of the pencil will be an ellipse. This method of describing an ellipse represents the action which takes place in the chuck for turning ellipses; the point of the pencil which remains unmoved is in the same situation as the turner's gouge; the transparent paper which receives the trace of the ellipse from the fixed pencil is analogous to the wood, which is to be turned into the form of an ellipse by the fixed cutting gouge.

In fig. 2, the chuck is represented as fitted on a common turning lathe, of which A is the pulley of the maundrel; B and C are the sides of the frame supporting the chuck; P the rest, D the frame in which the rest slides; E, F the feet of that frame, I the nut and screw which serve to fix the rest; G, H are the continuation of the sides B, C. K is the elliptic chuck, with two grooves, through which the knobs of the slider pass. These knobs are connected by a strong bar of iron screwed into their ends; and on this bar of iron is seen the screw for fastening the board, to which is fixed the wood or other substance which is to be turned elliptically.

Fig. 3 shows the other side of the chuck, which in fig. 2 is turned towards the side of the frame C. N, in fig. 3, is the board with the grooves which contain the slider O. In the middle of N is seen the end of the screw, which is fixed to the maundrel. The board N has a circular motion, being fixed on the axis of the maundrel, whilst the slider O, at the same time that it is carried round by the circular motion of N, is constrained to perform other motions by the grooves in N and by the groove in O, fig. 3, which slides on the ring M, fig. 4.

In fig. 4, L is a part of the side C of the maundrel frame, with the ring M fastened to it. On this ring, the broad groove in the slider O, fig. 3, moves when the lathe is set going; and this groove is at right angles to the grooves in N, fig. 3, in which the knobs of O move. In fig. 4 it is seen that the centre of the ring M may be made to coincide with the centre of the spindle of the mandrel, in which case a circle is described. If the ring is fixed, so that the centre of the ring does not coincide with the centre of the mandrel, an ellipse is described by the wood screwed upon the bar of O in fig. 2; and the most eccentric ellipse that the machine describes is obtained when the mandrel is at the circumference of the ring. The centre of the spindle of the mandrel, and the centre of the ring M, are always in one immovable horizontal line, and are analogous to the points C, D of the trammel, fig. 1. In fig. 3 it is seen that the sides of the grooves may be brought nearer together by means of screws; so that the sliders and the cylindric ring may fit exactly to the grooves.

The best elliptic chucks are made of brass. See Mechanical Exercises, by Peter Nicholson, London, 1812. (n.)