For turning light work, a bow, such as is used for shooting arrows, is susperded by its middle over the lathe; the string is then tied to the middle of the bow-string instead of the pole, and acts in the same manner. The continued rotary motion given by a wheel is so much superior for turning to the reciprocating motion of a treadle and string, that regular turners seldom make use of the latter; yet the simplicity and cheapness of the whole is a great recommendation, especially among country workmen, who are not so careful of their time as in the towns, where competition obliges every one to use the best and quickest means of despatching his work.
The common centre-lathe becomes a powerful machine when worked by means of a large wheel, turned by one or more labourers; the wheel should be heavy, that its momentum may be sufficient to overcome any trifling obstacle in the work, and the frame in which it is mounted must be of sufficient weight to stand steady, and not be liable to move by the exertions of the man turning it. An endless line is used to communicate the motion of the wheel to the work; it passes round a groove in the circumference of the wheel, and, after crossing like a figure of S, goes round a small pulley fixed upon the work; by this means, when the great wheel is turned, it gives a rapid rotary motion to the matter to be turned, and with a much greater power than can be obtained from the treadle, with the additional advantage of the work turning always the same way round, so that the turner has no need to take his tool off the work; the small pulley is perforated with a square hole, to receive a square made on the end of the work, and the turner has many different pulleys, each with a different sized hole through it, to suit work of different diameters; but there is an inconvenience attending this method, for if the four corners of the square on which the pulley is fitted be not all equally distant from" the centre of the work, the pulley will not turn round truly, and the band will be liable to slip round upon it.
To obviate this, the pulley in the annexed figure is often used; it has a square hole through it to receive the work, and is made to fit upon it by means of four screws a a a a, passing through a part of the wood by the side of the pulley, and their point pressing into the work; in this manner one or two pulleys can be made to serve work of any dimensions, and can always be set truly upon it; it has, as shown in the edge view, two different sized grooves, in either of which the band may be worked when required
There is a kind of centre-lathe, which is generally employed by millwrights and iron-founders, in turning heavy metal work, such as the gudgeons of mill-shafts, rollers for sugar or rolling-mills, pump-rods, which are to pass through stuffing-boxes, or, in short, any work which will admit of having both its ends supported on centres; it is in many respects similar to that we have described, but is adapted to give a continued rotary motion to the work; it has legs which support it from the floor, and the bed is formed by two parallel beams or cheeks, bolted to the legs; one of the legs stand up above the bed to support the main, or left-hand centre point, instead of having a puppet on purpose. The centre pin is fastened into it, by a nut and screw behind, and upon this pin two wooden pulleys are fitted side by side, close to each other, so that they appear but one; either of these, at pleasure, is caused to turn round by means of an endless strap, going round a drum, extending over head or under the floor, and which is turned by horses, or a steam-engine; the strap being only the breadth of one of the pulleys, will turn but one of them at a time, but it can easily be shifted from one to the other at pleasure, and then the other will standstill The front one of these pulleys gives motion to the work.
The back puppet is fixed upon the bed of the lathe, by a tenon projecting downwards, and entering the space between the two chucks of the bed; it is fixed at any place, by means of a screw-bolt, which passes down through the puppet, and goes through a piece of iron, which takes its bearing on the under side of the bed; a nut is fitted in this screw, and thereby the whole puppet can be drawn down upon the cheeks so firmly that it will not move by any strain the work may occasion: the back puppet has a back centre screw, which has a steel point to support the work.
The work is turned about in this lathe by means of an iron pin, projecting some inches from the flat surface of the front pulley, which, as before mentioned, is fitted on the centre point: a piece of iron, called a driver, is screwed upon the work near its left hand end, so as to project perpendicularly from it, and the pin in the pulley intercepts this as it turns, carrying the work round with it.
The other pulley, which is fitted on the centre pin, is only of use when the lathe is wanted to stand still, in the same manner as the live and dead pulleys used in cotton-mills. When the workman wishes to put the lathe in motion, he presses the handle of his tool, or any other smooth piece of wood, against the edge of the endless strap while it is in motion, and pushes it towards the front pulley; in a very short time the strap will get completely on the pulley, and shift itself to a fresh place on the drum corresponding to the pulley; this causes the pulley to turn round, and by the pin pushing round the end of the driver screwed on the work, communicates its motion to the work to be turned. When he wishes the motion to cease, for the purpose of examining his work, he pushes the strap back again on to the other pulley, which has no communication with the work, as it slips freely on the centre pin: the driver is simply an iron ring, having a screw tapped through one end of it, to pinch the work so fast as to prevent its slipping.
The side opposite the screw should be angular, that it may fit any sized work; this driver may be fixed on either end of the work, while the other is turning, but when it is necessary to fix the driver on that part of the work which is finished, the end of the screw is apt to pinch and bruise it; it is therefore proper to use a driver composed of two bars of iron screwed together by two screws, passing through one bar tapped into the other; both bars are somewhat hollowed out in the middle, that they may encompass the work. If this should be found to injure the work, a piece of sheet-lead wrapped round it before the driver is put on, will prevent the possibility of its damaging the work, and if the screws of the driver are drawn very tight, it will carry the work about with sufficient force to bear turning.
The manner of mounting and giving motion to a piece of work in the centre lathe is thus: - the back puppet is first fastened on the bed of the lathe, at the proper length to receive the work; the workman then places one of its ends against the points of the front centre, with the points as near the centre of the work as he can guess; he then brings the centre of the other end of the work opposite the point of the centre screw, and screws it up so as to hold the work just tight enough to prevent its falling down. In this state by turning it round by one hand, while he holds a piece of chalk against it with the other, he finds whether it is pitched nearly concentric on the points; and if it varies much any points, he turns back the screw and tries again, observing to shift the centre-point nearer towards that side which appears to project farthest in revolving, and therefore gets marked with the chalk. When he has found the true centre, he screws up the point so hard that it may mark the end of the work; then, taking the work out of the lathe, he punches or drills holes in the end, where the screw and centre points have marked, and when the work is returned into the lathe it will run nearly concentric; the driver being screwed fast on either end of the work as is most convenient, the work will be turned round by the pin projecting from the pulley as before described.
The turning of heavy iron-work, for which these lathes are used, is performed by various tools chiefly called hooks, but these will be further described.
The centre lathe will perform any kind of work which can be turned upon centres made in the ends of it; but a great portion of the articles formed in the lathe, must have one of their ends at liberty, to be operated upon while they are turning, as cups, boxes, and all kinds of hollow articles; these are turned in