Patent GB-546494-A (GB)
Title: Improved method of and apparatus for producing gears
IPC: B23F9/10
Filed: 1941-01-15
Abstract: 546,494. Gear-cutting ; milling-cutters. GLEASON WORKS. Jan. 15, 1941, No. 576. Convention date, March 19, 1940. [Class 8 3(iii)] In a method of and apparatus for generating tapered gears, particularly bevel or hypoid gears with longitudinally curved teeth the tool and gear blank are rolled together first in one direction and then in the other to generate opposite sides of the teeth the ratio of the rolling movement being different in the two directions. This enables pinions to be rough cut with a taper slot by using a single cutter and without change of cutter position and gives a roughed-out pinion which closely approximates to the final form so that the life of the finishing cutters is prolonged In a bevel pinion 40, Fig. 3, the projected tooth normals to opposite sides of a pinion at a median point 48 along the tooth length are shown at 54, 55. These normals will also be perpendicular to the inside and outside cutting edges of the cutter. If the pinion is slightly rotated about its axis the normals 54 and 55 can only be maintained as cutter normals if the pressure angles of opposite sides of the cutter are changed. Such a change in the cutter necessitates, if equal pressure angles are to be cut on opposite sides of the pinion teeth the use of a different generating roll ratio for the cutting of opposite sides of the pinion teeth. It is shown that if a generating movement corresponding to the pinion 'rotation is imparted to the cutters such movement results in a displacement of both the normals 54, 55 towards and into coincidence with the mean normal 57. Thus by suitable adjustment of the cutter pressure angles and modification of the rolling ratios for opposite sides of the teeth a taper pinion may be cut without changing the position of the cutter axis in the cradle. The modified cutter is shown in Fig. 4 where A a represent the normal pressure angles and A<SP>1</SP> a<SP>1</SP> the modified pressure angles which are respectively less than and greater than the normal. The sum of the pressure angles may be made greater or less than the sum of the pressure angles of opposite sides of the teeth by known modifications in the rolling ratios. In some cases the pressure angle of the gear may be modified on one side only. The method of the invention is also applicable to hypoid gears. The method is further illustrated in Fig. 8. During the roll in one direction the cutter which starts the cutting at the small end of the pinion moves through the distance 97 to generate the convex sides 95, the work having corresponding rotation from 851 to 85<SP>11</SP>. The concave faces 96 are also partially cut on this roll. The cutter movement is continued to a position 90<SP>1</SP> so that in the return roll the work moves through the same distance backwards but the.tool moves through a greater distance 98 than before. On the return roll the principal cutting is done at the large end of the gear. At the end of the return roll the blank is indexed. The machine used for cutting gears by the above method has its cutter axis parallel to the cradle axis and a work spindle adjustment which allows the offset required for cutting hypoid gears. A suitable machine is a modification of that described in Specification 543,408. The roll-ratio variations are obtained by axial movement of the worm 115 driving the cutter cradle 105. The cutter 100 may be adjusted as described in the above mentioned Specification. A bevel gear 118 and the gear train which drives the work spindle are driven alternately in opposite directions and the gear 118 is connected through gears 121, shaft 123, and change gears 126 to a worm 130 driving a laterally fixed cam 120 coacting with one of two abutting surfaces 135, 1351 on a slide 136 connected to worm 115. The cam is held against one abutment or the other by a hydraulic cylinder 140 the change-over being made at the end of each roll. Thus during a rolling movement from the position shown in Fig. 10 the cam 120 moves the worm 115 to the left in the same direction as the cradle movement. At the end of the roll the cam is reversed and the slide shifted to engage abutment 135<SP>1</SP> with the cam. The worm 115 thus still moves to the left but is now moving oppositely to the cradle so that a different ratio of roll is obtained. At the end of the return roll the abutment 135 is returned to the cam while the blank is indexed. The cam 120 may be of involute form to give uniform motion to the slide 136. Alternatively the cam may be formed so that the change in roll ratio is effective in one direction of the roll only. In another modification, the roll ratio variation is effected by a roller mounted on a swinging arm which may be adjusted to vary the roll ratio to different extents during actual rolling movement. The mechanism for actuating the slide 136 may also be modified in various ways according to the type of the gear to be produced. In another modification, Fig. 22, the cam may be replaced by a pair of rollers 200, 201 which are mounted so that they may be adjusted on a plate 204 along the line 203 joining the centres and also perpendicularly to this line, one or both of the abutments 205, 2051 being adjustable. The roller settings are varied in conjunction with the change gears 126 to cut gears of various types and they may be set not only to give variation in ratio between up and down rolls but a variation in roll ratio during the actual cutting. In one case the rollers may be set so that no modification in roll ratio is produced and a pinion having opposite tooth sides curved about centres which are equidistant from the basic crown gear axis may then be cut with a cutter having equal pressure angles on opposite sides. A construction of roller type control member is shown in Fig. 29, the rollcarrying part 218 having intersecting rack teeth 222, 223 coacting with any of a number of intersecting rack spaces in the head 215 to which the member 218 is secured by a yoke. In another modification, Fig. 32, a cam 230 is used to impart the required movement to the cradle worm. The cutters are in most cases of straight profile but one or both sides of the teeth may be curved as shown in Fig. 34. The method of the invention may also be used in combination with a set-over of the cutter axis in cutting opposite tooth faces. Thus, Fig. 35, if 264 is the cutter axis position at a mid point of the uproll, the position is changed at the end of the up-roll and occupies a position 2641 at the middle of the down-roll the amount of shift being determined by the taper desired and the change in the roll ratio serving mainly to control the tooth profile shape. In another modification the pinion instead of the cutter is set over between the up and down rolls. By using a cutter having a smaller pressure angle than the gears to be cut the point width of the cutter may be increased. The teeth may be undercut near their roots to reduce the work of the points of the finishing cutters.