wheel of the same or double the number of the teeth of the required wheel on the lathe-spindle end, and moving it forward one or two teeth in gear with the wheel it drives, for every cut taken.

For rack-cutting the piece of metal to be cut is attached to the face plate of the lathe in a horizontal position, the cutter slide placed exactly vertical; the spindle is then horizontal. Motion is given to the cutter in the same manner as before, except that the gut need not pass over the two guide pulleys, but direct on to the pulley i. The traverse for the cut is of course obtained by moving the screw b, and the pitch is divided by moving the screw of the slide-rest.

With regard to spur-wheels, they are generally circular, but elliptical and other shaped spur-wheels are frequently required to transmit variable motion. Wheels of irregular shape can be turned by the apparatus, and in the manner previously described; they can also as readily be cut by using the wheelcutting apparatus, combined with the shaper-plates. Dividing the teeth must, in these cases, be done by hand, the division-plate not being applicable.

The diameter of the wheels to be cut in the lathecentres is of course limited; but those who wish to cut larger wheels can do so with the same cutting apparatus, but by having another spindle fastened to the lathe-bed at right angles to it. One end of this spindle can carry a division plate, and the other end be screwed to receive various sized studs for carrying the wheels to be cut. The wheels must project and come just outside the edge of the lathe-bed, the cutting apparatus is placed in the same relative position with the

work as before, and the whole operation is conducted in the same manner.

By this means wheels of any required diameter can be cut, up to 6 or 8 feet, but the pitch must not be very coarse.

MILLING OR CIRCULAR CUTTER MAKING.

The very small cutters used for cutting watch wheels are small pieces of steel turned to shape, and portions of the metal filed away to convert the circle into a cutting instrument. This method is also occasionally used for larger wheels, and for cutters for other purposes, but only in the absence of the proper instrument. The larger cutters, whether for wheel-cutting or any other purpose, are, according to the best practice, carcfully milled or serrated in a machine for the purpose. The cutters made by the machine are much better than the ordinary cutters in every respect. Their cutting principle is almost perfect, they are far more durable, and are hardened with greater certainty of not flying or warping. Of course, the work produced by them is also of a superior quality.

A wheel-cutter is shown at Fig. 164, and an ordinary tap-cutter at Fig. 166. All these are cut by means of a machine

FIG. 166.

or instrument applied to the lathe. Some of these cutters, of very good workmanship, are exhibited at the Patent Office Muscum, South Kensington, by Mr. Bodmer, of Manchester, and a machine for their

manufacture was shown by Messrs. Fairbairn and Co., of Leeds, in the Exhibition of 1862.

The contrivance described at Figs. 167 and 168 is different from, and I believe somewhat simpler than, those in ordinary use. It consists of a base plate a, which is fastened to the receiving plate of the slide-rest by two dovetail bolts b, in the same manner as the rest of the apparatus. The nuts of these bolts are, however, fitted into recesses to be out of the way of the slide. The bottom of the plate a is cast with a rib, which is planed to fit lengthwise in the groove of the receiving plate of the slide-rest. This rib answers the double purpose of strengthening the plate a, and keeping the apparatus in proper position, with one of its slides parallel to, and the other at right angles with, the line of the lathe-centres.

The top face of the plate a has two cheeks, forming the sides of a short slide, and into which fits the lower part of the slide c. This slide is at right angles to the lathe-centres, and is traversed by the handle d at the back of the instrument, the link e forming a connection between the handle d and moveable part of the slide c. The top of the slide e also has cheeks forming the sides of another slide ƒ at right angles to the under slide.

The slide f is a casting of the shape shown, and forming a frame or standard for the cutter spindle g. This spindle is of hardened steel, and runs in the neck of the frame, and also in a hole in the lower part of the slide f, as will be understood from the dotted lines.

The cutters are small serrated pieces of steel, form

ing frustra of cones. One is shown in place, its lower edge is just level with the lathe-centres. The cutters do not screw directly into the spindle, but into a receiving piece which is screwed to the spindle, and forms a part of it. The receiving pieces vary in shape, according to the cutter to be used. Some of the cutters are very small, and therefore will not admit of a hole through them; they are therefore made solid, with a small projecting screw, which is screwed into a receiving piece made for the purpose. The pulley for driving the spindle and cutters is placed between the two bearings of the spindle, and being screwed against a slight shoulder on the spindle, acts as a collar to keep the latter in place.

The two small pulleys at the side are merely used as guides for the driving gut, which is brought from a large pulley on the shaft overhead, under the guidepulleys and around the pulley on the cutter-spindle.

If we had to cut teeth longitudinally on and around a cylinder, a simple revolving cutter carried by the slide-rest, and moved along the cylinder by the leading screw, would answer our purpose, and we could dispense with the slides of the instrument just described ; but the reader will see, by referring to the figures of circular cutters, that the teeth have to be cut on curves of different radii and peculiar shape. The slides of the milling instrument are devised to enable us to pass round these curves, and their action is very simple.

Whatever shape cutter we wish to serrate, we place a shaper-plate h on the raised sides of the base plate, of such a shape that, on the handle d being pulled out, the bottom slide shall be traversed from the lathecentres a distance equal to the height of the curve of