Patent GB-311765-A (GB)
Title: Improvements in or relating to gears and a method of and apparatus for hobbing the same
IPC: B23F21/18
Filed: 1929-04-29
Abstract: 311,765. Gleason Works, (Assignees of Head, E. C.). May 16, 1928, [Convention date]. Gear - cutting ; lathes; milling cutters. - A taper or cylindrical hob for cutting gear teeth particularly bevel or hypoid gears having longitudinally curved teeth is provided with side faces which are of continuouslyvarying pressure angle from one end of the hob to the other. A taper hob of this form may be used to produce taper gears having side faces which are of substantially constant pressure angle from end to end of the teeth measured circumferentially of the gear. Further, the proportional taper in the tooth thickness is the same as the proportional taper in the depth. A taper hob 10 preferably having a constant pitch p is provided with straight or curved flutes 12 and the pressure angle a of the edge 16 increases continuously from the large to the small end of the hob, the pressure angle of any particular edge being exactly proportional to the axial position of the edge along the hob. Each tooth of the hob is preferably relieved so that the pressure angle of its side faces varies from the front edge, 29, Fig. 3, to the rear edge 31, this variation being in accordance with the position of the front and rear edges along the helix of the hob. Thus when the hob is sharpened, the new cutting edges will take the same cut as the original cutting edges. The hob may be employed to cut gears in the manner described in Specifications 208,177 and 220,027. The height of the hob teeth may be tapered by turning off the tops of the teeth to a cone angle larger than the pitch cone angle. The hob is relieved by a tool which is continuously tilted relatively to the hob as the operation proceeds longitudinally along the hob to produce the required variations in the pressure angle. Relief grinding is effected by means of a disc grinding wheel which has a similar progressive tilting movement, the extent of this movement being indicated in Fig. 4. In one form of relieving apparatus the blank 10 is mounted between centres 40, 41 and is rotated from the headstock 42. The tool carriage 50 is moved longitudinally by a lead screw 52 driven from the headstock spindle. The carriage 50 carries a plate 67 which is angularly adjustable about a central flange 68 and which carries in guides 71 a sliding carrier 70. A bracket 73 secured to the machine frame 10 is fitted with an angularly adjustable guide plate 74 provided with a setting scale 78, and a slideway 80 on this plate is engaged by a follower 81 connected by a pin 82 to an extension 83 of the carrier 70. Thus by appropriate setting of the guide 80 the tool carrier is constrained to move in an inclined path so that the tool follows a generator of the pitch surface of the hob and the relieving movements are perpendicular to the hob pitch surface. The carrier 70 carries a relieving-slide 85 which is reciprocated in timed relation to the hob rotation and carrier movement bv means of a cam 87 engaging a follower 89 and driven from the headstock through a shaft 61, gears 95, 97, shaft 98, and differential gear 100. The planet wheels of this gear are mounted on a spider 101 secured to a shaft 102 and the loose wheel 103 is driven from the head screw through change gears 104, shaft 108, bevel gears 109 and worm gearing 113. The shaft 102 is connected by spur gears 115 shaft 120 and bevel gearing 121, 123, 128 to the relieving-cam 87. The tool 34 is mounted on an angularly movable support 131 moving about a stud 133 carried by a slide 132 having compound adjustment on the relieving-slide 85. An extension of the arm 138 has clutch teeth engaging similar teeth on an arm 139 carrying an adjustable roller engaging a guide surface 142 secured to the carriage 50. This surface may be straight or curved and is such that during the relieving of each tooth of the hob, the slide 132 is tilted to vary the pressure angle of the tooth. The axis of the stud 133 may be adjusted to be in the pitch surface of the hob so that the hob remains of constant pitch measured on the pitch surface. When the relieving-tool has completed its longitudinal traverse, the carriage 50 may be reversed and a second relieving cut taken, the return movement being preferably made at higher speed. The relieving-slide may be held away from its cam on the return stroke by means of a lever 150 having a cam surface 151 engaging a screw 152. For relieving the opposite cutting edges a second lever 145 carrying a roller engaging a guide 142<1> may be employed to give the required tilting movement. For relief grinding the support 131 may be replaced by a separate support 155, Fig. 5, carrying a grinding wheel 38. In use the hob 160 mav be located as shown in Fig. 16, where 161 represents the blank and 162 the imaginary basic crown gear. Both members of a gear pair are preferably generated by the usual rolling movement about the axis 164. The gears produced may have the root, pitch and top cone surfaces of both members intersecting in a common apex 182, the depth taper being similar to the width taper as in straight toothed bevels. The variations of the hob pressure angles are transferred to the gear as a variation of pressure angle in a section normal to the gear teeth, but the pressure angle in a circumferential direction of the teeth is constant and equal on opposite sides of the teeth. The invention is also applicable to the cutting of hypoid gears.