Patent GB-474227-A (GB)
Title: Improvements in or relating to rotary gear cutters
IPC: B23F21/28
Filed: 1936-01-25
Abstract: 474,227. Gear-cutting. NATIONAL BROACH & MACHINE CO. Jan. 25, 1936, Nos. 2414 and 2415. Convention dates, Jan. 26, 1935 and Dec. 2, 1935. [Class 83 (iii)] A rotary gear-cutter of the type adapted to be rotated in meshing engagement with the gear to be finished as used for instance in the machines described in Specifications 415,875 and 474,281, has one or more grooves provided in the flanks of the teeth to form cutting edges, the spaces between the roots of adjacent teeth being enlarged to form relief channels extending longitudinally of.the teeth. The axes of the gear 21, Fig. 2, to be finished and the cutter 10 are relatively inclined at an angle between 3‹ and 30‹ and during the finishing operation the work and tool gears are fed relatively in the direction of the work axis. The tooth 23 of a gear being finished is carried by axial movement past a number of cutting edges formed by the grooves 13 while at the same time the tooth is guided on opposite sides by the land portions 14b, 14c, left between the grooves. The lands and grooves may be each about 1/32" wide or the bands may be between À02" and À04" and the gashes between À025" and À06" wide. The grooves are preferably formed in planes perpendicular to the tool axis and run into the clearance channels 12 formed at the roots of the teeth and serving to assist the flow of oil and the removal of cuttings. The grooves are formed before hardening and are preferably left untouched by the final grinding operation of the profiles after hardening. The end lands are preferably of double width for strengthening purposes. The serrations on opposite flanks may be either opposite each other or in staggered relation. The width of the cutter is from ¢" to 1", generally about ¥". A 7" diameter cutter with a helix angle of 10" may be used to finish a 4" diameter gear 1¥" wide with a helix angle of 25‹, the cutter and gear axes being crossed at 15‹. The direction of rotation of the cutter, which is preferably the driving member, is preferably such that the gear during its axial feed moves against the rotation of the cutter, the rotation being reversed when the axial feed is reversed. The axes of gear and cutter are preferably fed towards each other automatically until the desired amount of metal has been removed. The instantaneous band of contact between the gear blank and the cutter is in the case of a helical gear located as shown at 34, Fig. 9, the inclination of the band varying according to the helical angle and the depth of cut being greatest at the central zone of the band. The width of the band depends on the differences of helix angle of the gear and cutter, being narrower the greater the angularity. The lines of cut on the blank are as shown in Fig. 10 there being a reversal of direction at the pitch line. There is thus a tendency to over-cut the blank near the pitch line and to correct this the cutter profile is preferably modified to a curve 38, Fig. 11, differing from the involute 37, the pitch line difference 39 being from À0003" to À0015'. This may be effected by modification of the band mechanism, Fig. 12, used to finish grinding the cutter profiles, the base circle 40 being made undersize and stems being inserted at 42. The ends of the tape are also lowered below their normal tangent position as at 43. In cutting a gear 52 near to a shoulder, Fig. 15, the point of intersection of the tool and work axes must be near one edge of the tool and in this case only one end of the cutter teeth is effective and the cutter tends to produce a tapered gear. To correct this the cutter tooth 78 may have different helical angles or opposite sides, Fig. 23. In cutting gapped gears, Fig. 20, the cutter 82 may be preceded by a guide gear 83 of greater diameter and modified helical angle, which engages the part 81 and maintains the guidance while the cutter passes over the gap 80. In another modification, Fig. 18, only the central part 70 of the cutter is serrated while the end portions 69 are relieved. In another modification, the serration may be inclined to the plane of rotation of the cutter. In another modification, the cutter may be profiled so as to cut gears which are relieved from involute shape at the pitch line. In some cases the cutter may be designed to remove additional material from the tops of the teeth to allow for distortion produced by heat treatment. The cutter profile may initially have a large addendum, Fig. 22, and as the profile is set back by successive sharpenings from a to a<5> the addendum is correspondingly reduced from b<1> to b<5> and the pitch circle diameter from c<1> to c<5>. The bottom of the grooves in the cutter may in some cases be straight or they may be cut so that the grooves taper in depth from top to bottom of the teeth. The dimensions of the bands and grooves may also be of different proportions from those given above. The Specification as open to inspection under Sect. 91 describes also a machine, Fig. 1 (Cancelled), for finishing gears by means of cutters such as those described above. This machine is similar to that described in Specification 415,875. A modified machine for the same purpose and forming the subject-matter of Specification 474,281 is also described. This subject-matter does not appear in the Specification as accepted.