TURNING

TURNING, the art of giving circular and other forms to objects, by making them revolve in various manners in a machine called a lathe, and applying cutting instruments so as to produce the form required; or by making the cutting instrument revolve, when the object to be operated upon is fixed.

This art is of great antiquity, and is of immense importance to the practical mechanic. It embraces many varieties, requiring varied machinery for their proper performance, from the simple throwing wheel of the potter to the complicated and nearly automatic slide-lathe, in which, almost independent of manual dexterity, an indefinite number of copies of an article can be produced with unerring accuracy.

There is probably, in the range of mechanical operation, none susceptible of greater accuracy than the art of turning, and there is none which has so much conducted to the improvement of our machinery. The extent of the applications of this art may be learned by the four following conditions, which comprise the most important of its varieties. A knowledge of the manipulations no mere description can convey; and as, in the limits here assigned us, it would be impossible to describe all the varieties of the art, and the machines made use of, we shall content ourselves with such a selection as may appear to us to possess the greatest interest for the general reader.

1st. The article to be turned may be made to revolve round its own axis, and the cutting tool applied to its surface, so as to produce the requisite form, as in simple turning, performed in the ordinary lathe used by cabinet-turners; the more powerful lathe used for turning metal of large diameter; and the self-acting slide-lathe.

2d. The article may remain stationary, while the cutting

instrument either merely revolves, or, while revolving, has at the same time some other motion given to it.

3d. The article may be made to move in some curve derived from circular motion, and the cutting tool applied as before.

4th. The article may be made to revolve, and at the same time to have a reciprocating motion in the direction of its axis, the cutting instrument being only applied, however, while the article is either in the course of its forward or backward motion, but its application not continued through both; and all these motions may be more or less combined, thus producing almost infinite variety.

The self-acting slide-lathe being the most perfect of the machines of the first class, we have preferred giving a drawing and description of it; from these, the other lathes referred to will be more easily understood than if we had described a less perfect machine. The lathe, when of large size, is generally driven by steam or other power; but when small it is driven by a treadle put in motion by the foot of the workman. The treadle sets on a cranked axle, on which is fixed a fly-wheel; and a band from the fly-wheel passes over the pulley of the mandril or spindle, to which the article to be turned is attached. This is generally the arrangement of the common lathe, which differs from the slide-lathe only in wanting the slide and its moving apparatus, which we are about to describe, and has in place of it a simple rest to work by hand.

The following figures represent the slide-lathe as constructed by Messrs Whitworth & Company of Manchester, and exhibit certain contrivances which have been patented by these gentlemen.

Fig. 1 is a front view; fig. 2 a view of the left-hand end; and fig. 3 a section across the lathe. The same letters refer to

the same parts in all the figures. A A is a strong framing, which supports the bed of the lathe B B. At the left-hand extremity of the bed is fixed the head C C, and through steel bushes in the uprights, or puppets as they are termed, the spindle or mandril works; the projecting right hand end of the mandril is screwed, so that chucks and other apparatus for holding the work to be turned may be attached to it. One of these, the double driver, will be found figured and described afterwards. In brackets projecting backwards from the uprights another spindle E works. One of the racks and the end of the spindle are seen in fig. 2. On the main spindle D there is a conical pulley F, carrying a pinion on its smaller end; the pulley with its pinion turns on the spindle, but independently of it. To the right of the pulley is a spur-wheel H, keyed on the spindle; to this spur-wheel the pulley can be checked at pleasure, so as to be carried round along with it. On the spindle E is a spur-wheel G and a pinion, which cannot be seen in the drawing. These are fitted respectively to gear with a spur-wheel and pulley-pinion of the main spindle. Different materials require to pass the cutting tool with different degrees of rapidity, in order to be efficiently operated upon. Cast iron requires to revolve slowly, so that it may come in contact with a cutting tool at the rate of about 100 feet per minute; wrought iron and steel about twice as quick; and brass is better cut at a still higher velocity. Thus, it is required that provision be made for the lathe for altering the speed, independently of the velocity of the prime mover. It is obvious, too, that the same material must require a change in the speed of the lathe, according to its diameter, that a uniform rate of speed may be maintained by its surface. This alteration in the velocity of the spindle or mandril produced by the pulley and wheels we have just described, in the following manner:—When the belt from the prime mover, acting on the pulley F, causes it to revolve, if a slow motion be required, the pinion of the pulley is made to gear with the spur-wheel I on the spindle E; and the pinion fixed on the same spindle gears into the spur-wheel H of the main spindle D, causes it to revolve with a motion slow as compared with the speed of the pulley. If a higher velocity be required, the wheel and pinion of the spindle E are put out of gear with the pinion and wheel of the main spindle by sliding the spindle E a little to the left hand, and then locking together the pulley and spur-wheel H, so as to cause the main spindle to turn directly by the pulley.

The face of the spur-wheel H has drawn on it a great number of concentric circles. The circumference of each circle is divided into equal parts, and such numbers are chosen for the division as are multiples of the greatest variety of measures; at each division of the circles there is pierced a hole, into which a point is pressed by a spring. By means of this apparatus, an object in the lathe can be its surface divided into any number of equal parts. On the end of the main spindle is fixed a pinion K, which, by the interposition of the stud-wheels and pinion L, drives the spur-wheel fixed on the end of a screw N, which extends along the whole length of the lathe, and is called the leading screw. The ends of the leading screws and main spindle are made accurately of the same size, so that the wheels and pinions which have for different purposes to be fixed on them may be exchangeable. On the right-hand side of the lathe is seen the shifting-head or puppet which is capable of being shifted along to any part of the bed, fixed there, and which carries the centre pin. This is adjusted very nicely to its work by means of a screw turned by the wheel P, and it is fixed by the clamping screw Q. The slide-rest consists of an upper and an under slide: the latter lies across the bed, and is moved along it lengthways by means of the leading screw. The upper slide can move at right angles to the under one, and carries the cutting tools: the manner in which the tools are moved by the leading screw is, in this lathe, very ingenious. From the under slide there descends a short vertical spindle, carrying on its lower extremity a peculiarly-formed pinion a, which is constantly in gear with the leading screw. When the leading screw is in motion and the pinion fixed, the pinion serves as a nut or female screw, and the slide is moved along the bed of the lathe to the right or to the left as the screw may be turned. On the upper end of the vertical spindle there is fixed a mitred wheel b: into this a pinion c works, whose spindle carries the slide, and has a handle d fixed to it. On the slide being reached one or other extremity of the bed of the lathe, it may be carried to the other by reversing the motion of the screw; but this is more speedily done by turning the handle d which turns the pinion a, and so the slide is carried along very rapidly.

The upper slide is moved across the bed of the lathe as follows:—A pinion e, which works into the mitred wheel of the vertical spindle, is fixed on the end of a shaft f, on whose other end is fixed the wheel g: this gear into another wheel h, fixed on the end of a leading screw, which crosses the main leading screw, and, by means of an apparatus similar to that above described, causes

the upper slide to move at right angles to the under one. In fig. 3 the under slide is represented as fixed in its position by means of a clamp, whose handle is seen at k in fig. 1, and the main leading screw is, by means of the pinion a, the mitred wheel b, and the pinion and wheels c, g, h, working the cross leading screw of the upper slide, by which it is moved across the bed of the lathe.

In plain cylindrical turning, the motion of the slide is so adjusted in relation to the motion of the article operated upon, that the cutter carried by the slide shall not move over a space greater than the breadth of its point in the time that the article makes one revolution. In screw-turning, the cutter again is made to travel over a space as much greater than the breadth of its point during one revolution of the spindle, as the pitch of the screw requires. The requisite changes in the motion are effected by changing the wheels on the ends of the main spindle and the leading screw.

On the mandril nose there is represented, in fig. 1, what is called a double driver, and we here give the details of an excellent apparatus of this kind, invented by Mr Joseph Clement of London, by which an article can be fixed perfectly true between the centres of the lathe. The figures and description are from the Transactions of the Society of Arts of London, vol. xvii.

Fig. 4 is a side or edge view of the chuck; and fig. 5 is a face or front view. The same letters refer to the same part in both views.

"a a" is a circular plate or chuck, on the back of which is formed a boss b; the boss b contains a female screw for screwing it on the nose of the lathe-mandril. On the face of the chuck or plate a a, and in the middle, is screwed a conical centre c, on which is supported one end of the thing to be turned, as shown at d in fig. 4. c e is a parallel plate in the form of a cross, one side of which is fitted to the face of the chuck a a; in the middle of the horizontal arms of the plate c e are two oblong holes f f, as represented by the dotted lines in fig. 5; g g are two screws which pass through the holes f f, and screw into the face of the chuck a a; the necks of the screws g g are made to fit the holes f f in one direction, but the plate c e is at liberty to slide to and fro beneath the heads of the screws g g in the contrary direction: there are also thin washers beneath the heads of the screws g g, which act as springs, and tend to hold the plate c e in the situation wherever it may be slid to; there is also an oblong hole in the middle of the plate c e, through which projects the centre c; in the vertical arms of the cross or plate c e are six screwed holes; h and i are two studs or drivers, which may be screwed into any of the holes marked j, so as to accommodate the length of the ends of the carrier k. There is a face view of the carrier k, represented in its place (by dotted lines on the face of the plate, c e, fig. 5). The ends of the carrier are acting against the studs h and i.

"Supposing a piece of metal d, to be supported between the centre c of the chuck a a and the centre of the puppet-head of the lathe, and the carrier k to be fixed on one end of it, as shown in fig. 4; let the plate c e (see fig. 5) be slid on the face of the chuck a a, a little towards the left hand; then the stud h will cause the upper end of the carrier k to move or turn in the same direction upon the centre c. But the stud i, and the lower end of the carrier k, will be moved in contrary directions, and will be separated from each other double the distance that the plate c e has slid on the face of the chuck a a; therefore, should the plate c e be in the above position, and the lathe put in motion, so as to turn the plate a a round towards the left hand, the stud or driver h would be the first that would come in contact with the carrier k. But when the turning-tool is applied to the thing to be turned, or any resistance which tends to prevent it from turning round, the carrier k will cause the upper stud h and the plate c e, together with the stud i, to be slid towards the left hand till the stud i comes in contact with the lower end of the carrier k; then the studs or drivers h and i will be acting with equal force against the ends of the carrier k, which will remove the greatest part of the twist or strain from the centre c of the chuck a a, by which means any slender cylinder, &c. may be turned more perfectly than can be done by a single driver. If another stud be screwed into one of the holes opposite the drivers h or i, it will prevent the thing to be turned from overrunning itself, or back-lashing, when it has got a heavyside. The drivers h and i may be made of different lengths, as may be found most convenient."

When it is required to replace in the lathe an article whose