Patent GB-513102-A (GB)
Title: Improved manufacture of gears
IPC: B23F9/10
Filed: 1938-03-31
Abstract: 513,102. Gear-cutting. GLEASON WORKS. March 31, 1938, No. 980/39. Convention date, April 17, 1937. Divided out of 513,094. [Class 83 (iii)] Milling-cutters.-Bevel gears are cut by means of a face-mill cutter having blades arranged only part way round the periphery the cutter and blank being rotated in timed relation with relative rolling movement and the cutter having cutting edges of varying inclination to the cutter axis, the inclination of any one edge depending upon the point in the rolling movement at which said edge is to operate. The 'invention may be applied to the cutting of the gear pairs 30, 31 described in Specification 513,094. These gears have curved teeth which have zero spiral angle at the centre of a gear face as shown by the tangency of the line 38, 37 to the tooth median line 35 at the mid point 0. The radius of curvature R of the tooth line is more than twice the face width W. Both members of the pair may be generated but preferably only the pinion 31 is generated, the gear 30 being form cut with sides which are conical surfaces of revolution. In order to confine the tooth bearing to the centre parts of the tooth flanks the convex pinion sides 43 are of greater and the concave sides 41 of less, curvature than the corresponding gear faces. Also the pinion teeth are preferably relieved at the tops and bottoms as shown at 49, 49<1> in Fig. 6 so that tooth contact in use does not extend to the tops and bottoms of the teeth. The gear teeth may be first roughed out and then finish-cut in a separate operation but preferably both tooth surfaces are finished from the solid in a single rotation of a gapped " single cycle " face mill. In one method the cutter has two groups of blades arranged at slightly different radii. A slot is roughed and one side finish-cut by the first group of blades, then the gear is slightly rotated and the opposite side of the slot roughed and finished by the second group of blades. The cutter has a short peripheral gap between the two series of teeth to permit the angular adjustment. Preferably, however, the cutter is arranged to cut both sides of a slot simultaneously in a single position of the blank. In order to obtain a closer approximation to the correct width taper of the tooth spaces the root line of the teeth may be modified as shown at 53, Fig. 7. To obtain the localized tooth bearing, the usual cutter 61, Fig. 8, arranged on an axis 60 and having equal pressure,angles on the faces 63, 64 is replaced by a cutter 70 on an axis 65 and having unequal pressure angles on its tooth sides. Such a cutter compared with a normal cutter has a smaller radius 67, 71 for the gear faces 42 and a larger radius 69, 75 for the faces 40 so that localized tooth bearing will be obtained. This cutter is preferably used for form-cutting the gear member, the pinion being generated by a tool having equal pressure angles on opposite sides by rolling the pinion pitch circle on a tool representing a tooth of the gear. Preferably .the pinion is generated by a tool having side cutting edges of large pressure angle than the teeth to be cut so as to obtain the required taper while generating both sides of a space simultaneously. Thus a tool shown in Fig. 11 may be employed, the pinion pitch circle being shown at 80 and the rolling circles 112. 113 for the generating motion being determined by the intersection of normals 108 at the points of contact between the tool and blank. Since the point 111 of intersection of the tool sides lies in the root surface, correct taper is obtained without changing the root line of the pinion. The intersection 111 may be within the tooth space, in which case the width taper of the pinion cut will be more than the theoretical bevel taper. A pinion with such excessive taper may be used with a formcut gear cut without modification of the root line. In generating the pinion, Fig. 5, the tool 127 preferably represents a gear 130 having an axis 131 different from the axis 132 of the mating gear, the pinion being rolled as though in mesh with the gear 130. This produces the relief shown in Fig. 6. For generating the pinion by a tool having sides 141, 142 inclined at an angle greater than the pressure angle of the pinion it appears from Fig. 16 that a point 135" at the end of the line of action will not be reached by the tool in its corresponding position 140<1>. The cutters of a single cycle cutter which operate at different points of the roll are accordingly modified by additions to the tooth profiles which are such that they do not interfere with the generating cuts. Thus the cutter which cuts at the central part -of the roll may have tip profiles as shown at 141" and 142" which cut out the tooth roots without affecting the generating cut made at points 146. Similarly the cutter which is effective at one end of the roll is modified as in Fig. 15 so that a generating cut may be taken at point 153<1>. A similar blade but with the edge 152 extended may operate at the other end of the roll. Intermediate blades have progressive modification between the profiles shown in Figs. 15 and 16. The blades have a front rake of 20 degrees. The cutters, Fig. 13, are secured by clamps 174, in the head 171, 179 being the first roughing blade and 155 the last finishing blade. No infeed or withdrawal is used, the blank being indexed while the gap 178 passes the work. The blank is held stationary while the roughing blades 179, 189 are cutting. These blades are of gradually increasing height. Then the blank is rolled in one direction while. the roughing blades 190, 200, which have increasing pressure angle at the outside and decreasing pressure angle at the inside, 'operate. These blades may be modified to conform more closely to the finished tooth space as shown at 202, Fig. 15. The roll is then reversed and the finishing blades between 150 and 155 take their cuts. These blades vary progressively in shape from the form shown in Fig. 15 (blade 150) to the form shown in Fig. 16 (blade 145) and then to the form symmetrical to that shown in Fig. 15 (blade 155), the blades 150 and 155 operating at the ends of the roll and the blade 145 at the middle of the roll. Instead of using curved profiles the blades may be shaped as in Fig.'19. At the centre of the roll the normal blade may be preceded or followed by a blade 230 of greater length and small pressure angle for cutting the tooth roots., At one end of the roll the cutting is done by a normal blade 235 and a blade 240 which performs the function of the projecting portions of blade 155.. The normal blades of greater pressure angle than the pinion to be generated are arranged alternately with the modified blades such as 230 and 240. In order to eliminate bias bearing between the gears the cutting edges of the finishing cutters for formcutting the gear may be of arcuate form, Fig. 20, the inside edges being to a smaller radius 322 than the outside edge radius 325. In cutting the pinion,. the cutter has similarly convex edges, the outer edge curvature matching that of the inner edge 308 of the gear cutter and the inner edge curvature matching that of the outer edge 307. In the generating- operation the cutter is arranged to represent a gear whose axis is inclined to the pinion axis at an angle different from the theoretical mating gear angle in order to localize the tooth bearing. The pinion may also be generated from a cutter having a greater pressure angle than the teeth to be cut.