wheel, the arbor B to rotate through a corresponding angle; with the result that the bevel blank will have the true rolling motion over the crown wheel circular rack. If G be held fixed, so likewise will the casing, and a rotation of the worm will cause a rotation of the arbor B about its own axis. The same roll cone cannot, of course, be used for different cone angles; but for each cone angle a corresponding rolling cone must be used.

The roll cone G is rolled over the steel bands by a rotation of the horizontal plate D. This may either be by hand or from the driving shaft. The plate D is provided with a circular rack, V, which meshes with a pinion, U, keyed to a vertical stud, T. This stud carries a bevel wheel, Q, which meshes with a wheel, R, which may be rotated by the hand wheel S. Equally, it may mesh with one or other of the bevel wheels P, P (which can be engaged and disengaged by means of a clutch, so that the roll cone can roll in either direction), keyed to a horizontal shaft, N. At its extremity, N carries a ratchet wheel, M, and the intermittent motion of M may be brought about by a click actuated by the rod E driven from the belt pulley F.

To use the machine, the arbor is placed at the proper cone angle in the vertical plane, and the proper roll cone fitted, and by means of the handle S the roll cone is placed in one of its extreme positions, so that the bevel blank is just clear of the tool. During each return stroke of the tool, the roll cone is given a slight motion either through S or through M. The tool consequently cuts out the tooth space in the blank; and these intermittent operationsalternate cutting by the tool, and rolling of the roll cone-are continued until the blank is clear of the tool on the opposite side of the machine. The worm wheel Y is then rotated, by means of the worm, such an amount as to cause the bevel blank to move through an angle corresponding to the pitch, and the operation is repeated. Usually three tools are used, namely, a double-edged tool (such as has been assumed) for roughing, and right and left handed side tools for finishing.

§ 195. The Gibson Bevel-gear Shaper.-In both the Fellows and the Bilgram shapers, the profiles of the teeth are generated, and are necessarily involute teeth, and all wheels cut by the same

cutter will mesh together. In some machines, for example, the Gibson machines used for cutting bevel wheels,' any shaped teeth whatever may be cut, because a "template" or "former" is used. Any errors in shaping the "former" are reproduced in the teeth, but on a reduced scale, since the "former" is usually about three times as large as the teeth to be cut.

Apex of
Pitch cond

0

A

A

The principle of this machine has been already pointed out in § 188. Suppose the wheel blank A (Fig. 356) to be made of butter, and that the trace of the teeth on a spherical surface having the apex O of the pitch cone has been drawn; let it be represented by T. Imagine an extremely thin and rigid knitting-needle, N, pivoted at one end to O, and let the needle be guided over the profile T. The needle will sweep out of the soft blank a perfect bevel wheel space, and every peculiarity of the profile T will be faithfully reproduced, in everdiminishing miniature, as the apex is reached. When the blank consists of metal, it cannot, of course, be cut in this way; but suppose the blank has been roughed out, and that along the direction of the needle a cutting tool, having its cutting corner in the axis of the needle, can reciprocate. if the direction of the needle be fixed whilst the tool is cutting, and if, in the return strokes, the needle (carrying the tool with it) is shifted into successive positions as determined by the space template T, then the cutting edge of the tool will always move along straight lines radiating from O, and the blank will be cut to the proper shape.

FIG. 356.

Clearly

The mechanical arrangement of the machine is shown in Figs. 357, 358; Fig. 357 representing a sectional side elevation, and Fig. 358 a plan. The bevel wheel A, to be cut, is mounted on a mandrel, P1, one end of which pivots in a small trunnion

1 See a paper by Mr. J. H. Gibson read before the North-East Coast Institution of Engineers and Shipbuilders, 1897; also Engineering, March 26, 1897.

bearing at the apex O of the pitch cone. The other end is capable of movement in a vertical plane, and can be fixed on a graduated quadrant B at any desired angle. A dividing gear, C, turns the blank into successive positions for cutting the required number of teeth, and while each tooth is being cut a pinching screw, D, helps the dividing wheel to hold the wheel blank securely in place.

The reciprocating motion of the tool along lines radiating from O is brought about in the following way. A carrier frame, E, is provided with two forks, F, F (Fig. 358), which can turn about horizontal studs, G, G, whose axis passes through O. These studs are carried by a circular plate, H, guided in a horizontal circular groove, I, whose centre is coincident with O. Thus the frame E can swivel about a horizontal axis through O, and this horizontal axis can be turned in a horizontal plane passing through O. The outer end of the frame E carries a roller, J, and this roller is constrained to move round the enlarged profile of the tooth template T (Fig. 356); the roller being kept pressed against the template by the spring K attached at one end to the roller and at the other end to a fixed stud. The outer end of the carrier frame E terminates in a socket, L, which engages a ball-and-socket joint forming a feed nut, M. The feed nut M is attached to a handle, N, and the screw Q can oscillate about the ball-and-socket joint R in the fixed frame of the machine. Thus the spring K keeps the roller pressed against the template T, and by turning N the roller may be moved over the template.

The carrier frame E carries the sliding ram S with the tool box U. The latter is capable of lateral adjustment by means of the screw V, so as to bring the cutting corner of the tool W on the straight line or "radian " joining the centre of the roller J to the apex O of the pitch cone. The ram is reciprocated by a crank shaft, X (turning in bearings in the frame E), through a crank arm and slot, Y, attached to the ram. The motion is transmitted to X by belting from the pulley Z, which rides loosely over a spindle whose axis is coincident with the axis of G (Fig. 358). The spindle of Z is carried by the circular plate H so that the distance between the axis of X and Z is always the same; and it receives its motion from cone pulleys in the usual way. The stroke of