267,967. Bottcher, P. March 20, 1926, [Convention date]. Gear-cutting; tools.-In a method of generating toothed cylindrical or bevel gears having longitudinally curved teeth by means of a cutter whose cutting edge moves in a path which is parallel to the pitch plane of imaginary straight-sided rack or crown wheel teeth represented by the cutter, a component movement at right-angles to this plane is imparted to the cutting edge in order to control the depth of the tooth spaces cut in the blank. The cutter is also rotated, during the cutting stroke, about an axis coinciding with the cutting edge so that a constant cutting-angle is maintained. Fig. 3 shows two crown wheel elements 13, 14, which fit exactly when superposed, and which have arcuate teeth which may be generated by cutters 11, 12 respectively mounted on cutter heads 9, 10 having axes parallel to the crown-wheel axes. The inclined edge of the cutter 11 will generate a convex face of a tooth of the wheel 13 while the edge of the cutter 12 will generate an exactly fitting concave tooth face of the mating wheel 14. This arrangement, however, results in a tooth form having teeth of constant height, and in order to produce the usual tapering form of bevel wheel teeth the cutter-heads must be inclined as shown in dotted lines. The tooth flanks generated by the opposite cutter heads will now not fit exactly. According to the invention, the parallel arrangement of the cutter-head axes is retained and the tapering tooth form is obtained by moving the cutting edge longitudinally during the cutting stroke so that theoretically accurate mating gears may be generated. One form of cutter head is shown diagrammatically in Fig. 5. The cutter heads 17, 18 rotate about a common axis and have inclined cutting edges 19, 20 for concave and convex tooth surfaces respectively. The shanks of the cutters are mounted in bushes 23, 24 which may be moved axially by cams 27. 28 respectively. The axes of the bushes are in alignment with the respective cutting edges. By suitable design of the cams 27, 28 any form of tooth root may be obtained. Thus in the application to spur gearing the teeth may be made of less depth at the edges of the wheel to increase the strength. The longitudinal movement of the cutters in the heads by the cams 27, 23 may also be used in order to withdraw the cutters from the work on the non- cutting portion of the travel. Further by making the cams adjustable during the operation, the movement of the cutters may be used to apply depth feed in the direction of the inclined tooth flanks. This feed motion is used in the preliminary gashing of blanks to produce trapezoidal tooth spaces or in producing a non-generated wheel adapted to mesh with a generated pinion. Instead of gashing by means of a number of cutters which act alternately on the right and left of the tooth space and are fed radially to the required depth the cutters may penetrate in the direction of their cutting edges as indicated in Fig. 11. In a modification of this method, Fig. 12, a middle cutter may be used in addition to the side cutters, this middle cutter being arranged to do the bulk of the cutting while the other cutters finish the side surfaces. The invention may also be applied to machines wherein the wheel blank is moved during the cutting stroke, so that, instead of arcuate teeth, cyclic tooth forms are produced. In order to maintain a constant cutting angle when teeth are formed in this way, the shanks of the cutters are fitted with arms 29, 30, Fig. 5, engaging respectively with templets 31, 32 which are so shaped that the cutter shanks are oscillated in the required manner about their axes during the rotation of the cutter head. Auxiliary movements may also be imparted to the cutter head itself so as to modify the tooth curve. If this auxiliary movement consists of a planetary motion of the cutter head axis the cutting edges will describe cyclic curves. Such a cutter head is shown in Figs. 14 and 15. The casing 47 is mounted on crank-pins 49, 50 of crank shafts 51, 52 rotating in unison. The cutter head 53 is carried in the casing on ball bearings and is rotated by a wheel 57 engaging toothed wheels 58, 59 on the crank pins. The cutter head is provided with diametrally-opposed cutter holders 61, 62 which are inclined at the pressure angle to the axis of rotation of the cutter head. The holders are bored to receive outer and inner side cutters 63, 64 whose cutting edges coincide with the axes of the holders. The cutter head is also provided with holders 65, 66 for middle cutters 67, 68, which are located intermediate of the side cutters and are arranged parallel with the cutter head axis. In order to move the cutters axially rails 69 having projections engaging each cutter holder are provided with rollers engaging a cam-groove between two members 76, 77 forming a casing 75. A rise 78 on this cam-groove withdraws the cutters successively on the non-cutting part of their travel. The casing 75 is mounted on trunnions 79, 80 which may be adjusted by means of screws 82, 83 to incline the cam in such a way that the cutters are moved axially during their cutting travel to taper the tooth bottoms when bevel gears are being cut. For giving depth feed to the cutters a feed wheel 87 engages a wheel 86, thus axially moving a non-rotary threaded bush 81 connected to the cam casing. This axial cutter feed is only used for non-generating operations. For maintaining the cutting angle of the tools constant during the stroke, each cutter holder has peripheral teeth engaging a rack 89 having a rectilinear movement controlled by an annular templet 92 mounted in the casing 47. In a modification the auxiliary movement such as that produced by the crank pins 49, 50 may be imparted to the blank instead of to the cutter. The continuous rotary movement of the cutter head may be replaced by an oscillatory movement. In one method of working, all the concave teeth may first finished by means of a cutter in the outer cutter holder only, and the wheel then indexed half a pitch and the convex surfaces finished by means of a cutter in the inner holder having its edge in the same position as the first cutter. If both tooth flanks are required to be cut simultaneously the convex and concave surfaces will be cut to different radii of curvature. If the convex surface has the larger radius, then, in cutting the mating wheel, the tools are arranged so that the concave surface has the larger radius. The method may also be used for cutting teeth wherein the blank rotates continuously during the operation, the cutters engaging successive teeth on successive working strokes. In Fig. 16, one middle cutter F is arranged midway between the convex surface cutter D and the concave surface cutter E, the blank rotating one pitch during each rotation of the cutter head. The edges of the cutters D, E are at the same distance from the axis of the cutter head and the rounded polygonal path of the cutters is obtained by the superposition of a planetary movement on the axial rotation of the head. A rolling generating movement may be imparted to the system for finishing the tooth profiles. A similar process may be used in the production of bevel wheels of the herringbone type.