Patent GB-513239-A (GB)
Title: Improvements in or relating to the manufacture of gears
IPC: B23F1/08
Filed: 1938-04-04
Abstract: 513,239. Gear-cutting. GLEASON WORKS. April 4, 1938, No. 10306. Convention date, April 17, 1937. [Class 83 (iii)] Milling-cutters.-Tapered gears having longitudinally curved teeth are generated by a continuously rotating tool having an operative finishing portion arranged only part round the cutter periphery and having a varying effective shape, different parts of the operative portion being arranged to operate at different definite points along the tooth length of a gear, there being a circumferential gap in the cutter to permit indexing of the work without withdrawal of the cutter. In order that a cutter 51 may represent simultaneously two sides of the basic crown gear of correct tooth taper, the tooth sides should be equally inclined to radii 55, 56, Fig. 1. It is shown that this condition may be closely approximated if the inclination i of the cutter axis to the basic crown gear axis is obtained from the formula:- where- Ï is the pressure angle, # the spiral angle at the mean point 66, A the mean cone distance, P the diametral pitch at 66 and r the mean cutter radius. The cutter should be placed so that the points of convergence of its teeth sides lie on a line 75.inclined to the crown gear pitch plane 74 at an angle i. This arrangement would not give the required dedendum angle da and to avoid this disadvantage the finishing edges of the cutter operate at only one point of the tooth space the blades being of such varying height that the tooth bottom 82 is obtained. All the blades have straight side cutting edges. The cutter is preferably of the single cycle type having blades only partway round the periphery with a gap to permit indexing the blank without withdrawal. The invention may be applied to generating one member of a pair whereof the other member is form cut. The rotation of the cutter is timed in relation to the generating roll. Thus a cutter representing a crown wheel with centre at 91 may be rolled from 90 feet to 90 inches while its rotation in direction 94 is similar to that of circle 95 rolling on a larger circle 96. The top edges of the cutter may be on a helix of constant lead and a formula is derived in the Specification for finding the lead of such helix. The shape of the surface in which the cutter tips should lie may also be obtained experimentally by using a tapered milling- cutter embodying the desired root angle to mill the correct top surface in a soft cutter blank which is rotated and given the correct rolling movement. The shape of the cutter blades adapted to 'operate at the centre and small end of the blank are shown in Figs. 6 and 7 respectively and a blade shorter than that shown. in Fig. 6 operates at the large end. It is shown that owing to the rolling motion the cutter 126 which operates at the small end of the tooth space will clear the large end of the blank and not affect the finishing cut at the large end. Similarly the cutter which operates at the large end does not affect the finishing cut at the small end. Each blade round the cutter thus cuts at a definite point along the length of the tooth space. For cutting gears from solid .the cutter has both roughing and finishing blades. In Figs. 12 and 13 the teeth 140-146 are roughing teeth of increasing height and are followed by finishing teeth 150 arranged on a helix 151 the first of which takes a finish cut at the small end and the last a finish cut at the' large end. When the gap 155 is reached the blank may be indexed and the cutter rolled reversely relative to the blank. The roughing blades are preferably somewhat narrower than the finishing blades. For cutting on the down roll the cutter is arranged to rotate in the opposite direction and to start at the large end of the tooth space. In this case the helically arranged finishing blades are of increasing height. With this form of cutter, the end of the operation is at the small end and it may therefore be speeded up. In a modification, the cutter may operate during the roll in both directions, being provided with a set of roughing blades arranged on a "rising." helix which cut from the large to the small end on the up-roll, and then a set of finishing blades arranged as a " falling " helix which cut from the small to the large end on the return roll. In some cases the points of convergence of the side cutting edges of the finishing blades do not lie in a plane but in a helical surface in order to obviate bias bearing, i.e., diagonal disposition of the tooth contact, in making gears: U.S.A. Specification 1,980,365 is referred to in this connection. A cutter of this kind, adapted to cut during anticlockwise rotation is shown in Fig. 21. The opposite side edges of the roughing blade 273 converge on to the helix 285 and the roughing blades are of increasing height leading to the first finishing blade 275. The finishing blades are adapted to cut at different points along the tooth space and are of increasing point width, their sides all converging on the helix 285. Anindexing gap is located at 288. A similar cutter with oppositely-facing blades may be adapted to rotate in the opposite direction for cutting on the down-roll. In this case the first finishing blade cuts the large end and the last finishing blade the small end of the gear. Directions are given for determining the lead of the tooth convergence" helix 285 in order to obtain complete elimination of bias bearing. The helical cutter process may also be used for generating hypoid gears conjugate to a basic helicoidal segment represented by the cutter without axial motion of the cutter. Where a spiral bevel or hypoid gear is cut by the above-described method, the conjugate pinion may be cut by the tool shown in Fig. 27 having teeth 320 complementary to the gear cutting tool 321. Generation is effected by rolling between the pinion blank and the crown gear represented by the cutter. The desired taper in depth is obtained by adapting the heights of the tool teeth to the point along the tooth length where each tooth cuts. Thus in Fig. 31 the blades 326, 326' cut at the small end of the pinion and the blades 328, 328<1> cut at the large end, intermediate cutters of a single cycle cutter being of shapes which vary progressively between these limits. Such a ,cutter may also be used for generating a pinion for mating with a non - generated gear. The teeth may be either of constant point width as in Fig. 31 or the point width may be increased progressivelywith 'the height of the teeth so that the wider cutters cut at the large end. Where pinions are to be cut from the solid, the cutter, Fig. 34, has roughing teeth 345, 346 of increasing height arranged in front of the finishing blades 347, 348. The opposite side cutting edges of the finishing blades may converge either into a circle or a helix. The blades of the outer row 337, are slightly higher than those of the inner row 338 since they operate on the root surface at a point' somewhat near the large end of the gear. The cutter may either rotate cantinuously or it may be oscillated, one complete oscillation for each generating roll, by gearing it to the machine cradle. Where an oscillating cutter is used indexing is effective at one end of the roll and a smaller gap between the first and last blades is required. For finish cutting large gears or for shaving and lapping gears the work may be indexed after a number of revolutions or oscillations of the tool, the cutting edges being readjusted by rotating the cutter slightly between each " pass." Either lapping, shaving or burnishing tools of. male or female type may be used. A shaving tool is shown developed in Fig. 36. The tool is grooved either straight (as shown) or diagonally. An indexing gap is provided at 355 and the tops of the teeth are helically arranged as explained above. For lapping or burnishing the tools are made without cutting edges. In another modification, particularly for roughing by generating, the blades may be spirally arranged. In a modification of this form, Figs. 42 and 43, three rows of blades are arranged in two roughing spirals 383, 382 and a circle or helix 381, this row being similar to a single cycle cutter described above. The heights of the rows are graded as shown in Fig. 43. The first blades of each row cut at the small end and the last blades at the large end. The pressure angles of the three may also be different (Fig. 43); the form being determined experimentally by gashing a pinion of the required shape and using it for forming the cutter tooth shapes in a lead blank. The normal pinion roll is from 398-398<1> but the first roll is preferably from 399-399<1> to reduce the cutter load when the first cuts are taken. The blank must then be given one more indexing movement than the number of teeth in order that the first tooth may be recut to full depth. In, a further modification two coaxial and independently driven cutters are employed, one being an ordinary face mill cutter for roughing, and the other a single-cycle spiral cutter. The invention is also applicable to tools with curved cutting edges. In all cases the blades may be sharpened so that alternate blades have oppositely disposed side cutting edges.