Patent GB-362448-A (GB)
Title: Improved manufacture of globoidal gears
IPC: B23F11/00
Filed: 1930-08-27
Abstract: 362,448. Gear-cutting. WADE, H., 111, Hatton Garden, London.-(Cone Worm Corporation; Norfolk, Virginia, U.S.A.) Aug. 27, 1930, No. 25630. [Class 83 (iii).] In a method of cutting a globoidal worm wheel 3, Fig. 1, the teeth are finished by means of a hob located at the same centre distance from the wheel as the mating globoid worm 2 and having cutting edges which move so that they represent the thread flanks of the mating worm. The wheel is preferably generated in two stages, the hob being advanced perpendicularly to the wheel axis during the roughing stage and maintained at the theoretical centre distance during the finishing stage. The circle 1 concentric with the wheel axis, determines the length of the worm which is contained within the parallel tangents 21, the diameter of this circle being determined by the tooth angle o taken in the midplane of the worm. Thus in Fig. 7 the hob 4 and blank 3 are rotated in timed relation and during the roughing operation, the hob, as shown on the left of the figure, is advanced through the distance n towards the blank until the full depth is attained. In order to permit this in-feed of the hob without interfering with the desired finished profile of the worm wheel teeth, the hob teeth are made thinner than the teeth of the theoretical mating worm, this result being achieved by making the flanks of the hob teeth tangential to the circle 1 but arranged at a smaller angle c than the theoretical angle o. For the finishing operation the blank is first angularly adjusted relatively to the hob by an amount v, Fig. 9, in a clockwise direction while one set of tooth flanks is finished and is then adjusted by a similar amount in an anticlockwise direction while the opposite flanks are finished. In both cases the hob and blank are rotated at their correct relative speeds. These angular adjustments are effected by means of screws 16 coacting with an arm 14 secured to a collar which supports the blank. The angle v through which the blank is adjusted is equal to the half difference between the angles c and o. In a modification the angular shifts for the finishing operations on respective sets of flanks may be imparted to the hob instead of to the wheel blank, the amount of the shift being equal to the angle w, Fig. 2, which shows the difference between the positions of the hob teeth (double-shaded lines) and the theoretical mating worm (single shaded lines). The amount of thinning of the hob thread is such as to avoid the removal during the roughing operation of material at the parts f, g of the wheel profile. The thinning must als obe arranged so that sufficiently wide top faces are left for the outer cutting edges of the hob. The hob is also made of slightly larger diameter than the mating worm to provide root clearance of the worm wheel teeth. The top clearance may be obtained by slightly reducing the outer diameter of the wheel blank. Several types of hob are described. In one form 4, Fig. 4, used preferably only for the roughing operation, the hob is provided with parallel flutes which provide an obtuse cutting edge 22 and an acute cutting edge 24. The usual top and side relief is provided at b. In another form the flutes are approximately perpendicular to the hob threads. In another form 5 straight flutes 20 are provided but only the acute edges as at 28, 29 are employed for cutting, the backing-off operation being in each case towards the obtuse edges 30 which are non- cutting. This hob, which is intended for the finishing operation, is rotated first in one direction to finish one set of tooth flanks and then in the opposite direction to finish the opposed flanks. The hobs preferably have at least one cutting edge approximately in the plane of the parallel tangents 21 and any material such as 34 beyond these edges is cut away so as to be non-interfering.