GB240002A - Improvements in gearing and method of generating the same - Google Patents

Abstract

240,002. Trbojevich, N. Sept. 22, 1924. Gear-cutting; millling-cutters.-Toothed gears are generated by means of a conical hob having a helical cutting-thread which in axial section corresponds to a constant-pitch rack-element engageable with the gear to be cut and a cone angle substantially equal to the pressure angle of the rack element. The hob is positioned so that its pitch cone is tangential to the blank pitch cylinder and a tangential plane to the hob thread at the point of contact coincides with the rack plane from which the teeth are developed, the hob and blank being rotated in timed relation and the hob having a, relative bodily movement. The relative positions of the hob and blank are shown in Fig. 1, where a is the cone angle of the hob W and also the pressure angle of the teeth. It is shown that an involute lying in the plane of a lamina G of the gear is odontically conjugate with a helix h described about the hob axis, when the hob and gear are rotated in unison. The principle may be used for generating either straight or helical gears. In an application to worm gearing, Fig. 2, two similar tapered worms 12, 13, of the same pitch and hand are mounted on a common arbor and separated by a spacing collar 17, the right hand worm being angularly adjustable by means of a driving-collar 21 provided with a plurality of threaded holes. The thickness of the worm threads is less than the interdental spaces between the wheel teeth so that each worm engages on one side only and backlash is determined by the relative adjustment of the worms. The wheel is cut by radial feed of the cutters one of which is shown in Fig. 7. The cutter is relieved by a tool which reciprocates in a direction parallel to the sides of the pitch cone. The cone angle B is preferably slightly less than the pressure angle a of the corresponding rack element to avoid interference. The radial feed of a conical hob of this form can be considerably greater than the usual cylindrical hob thus increasing the speed of production. If only one. conical hob is used the teeth may be first cut on one side only and the blank then reversed on its arbor to cut the opposite sides. In a modified form of worm rearing only one hob is used and the worm produced has non-sym. metrical teeth adapted to engage a single conical worm. The hobbing of a helical gear 34 by means of two conical hobs 32, 33 is performed as shown in Fig. 10. In a further modification, Fig. 11, spur gears are produced by a large conical hob 35 having less than one thread convolution, the hob being moved tangentially of the blank and the movement along the blank axis being dispensed with. In this method grinding-tools are preferably employed, and the cutter is trimmed by a diamond point 38 mounted on a rotary shaft 39, Fig. 13. The cutter is rotated and fed forward from a hand-wheel 41, the pitch of the feed screw 44 being equal to p cos a where p is the pitch of the teeth and a the cone angle of the hob. A modified method of forming the cutter is also described. In a machine for carrving out the process the cutter 37, Fig. 16, is driven from a pulley 46 and the blank 36 is rotated in unison through gearing 47 and worm gearing 48. The blank is moved tangentially from a hand wheel 50 through change gears 51 and differential table screws 52, 53. In another modification, Fig. 17, two grinders 54, 55 may simultaneously finish opposite side, of the teeth in a single movement of the blank along the tangent q. Specification 220,027 is referred to.