Patent GB-558947-A (GB)
Title: Improvements in or relating to method and means for forming worm gearing
IPC: B23F13/08
Filed: 1941-10-13
Abstract: 558,947. Gear-cutting. SCOTT, G. R. Oct. 13, 1941, No. 13171. Convention date, Oct. 24, 1940. [Class 83 (iii)] (Also in Group XXIV] In a method of and apparatus for relieving the teeth at the ends of a globoidal worm the worm is rotated and the corresponding wormwheel is obtained by imparting bodily rotative movement to a milling or grinding tool 4 engaging the worm teeth, the relative rotative positions of the tool and the worm being varied so as to relieve the end portions of the worm to improve load distribution under heavy stresses. The tool 4 of frusto-conical form is mounted on a rotary table 1 and is driven from a motor through belt gearing 6. In axial section the sides of the tool are straight lines tangential to a basic circle concentric with the table 1 and with the wormwheel with which the globoid worm 24 is designed to mesh. The table is driven to give the desired worm-wheel movement to the tool from a motor 10 through belt gearing 13, shaft 15, gears 16, worm gearing 18, shaft 20, gears 26, spiral gearing 28, 29, shaft 30, and worm 32. The shaft 20 is also connected through worm gearing 22 to the shaft 23 carrying the worm 24 to be relieved. The ends of the worm teeth are relieved on both flanks as shown in Fig. 3, the primary slight relief 52 merging into a greater secondary relief 51. For a worm operating at a centre distance of 6 inches from the worm wheel the relief 52 may extend over one inch of flank and may amount to about .005 inches at its junction with the steeper relief 51 which may taper to about .030 - .040 inches at the tooth end. The surface 51 does not actually engage the worm wheel in operation but this part of the worm adds strength to the ends of the teeth. The surface 52 will come into engagement at high loadings. The unrelieved part 50 of the worm and the relieved part 52 together extend over a length closely approaching the diameter of the basic circle referred to above. To produce the relief a branch train from the shaft 20 drives a shaft 39 connected through clutch 40 to a worm 37 driving a cam 34 adapted to impart axial movement to the spiral gear 28 and so modify the movement of the table 1 and the tool. For hand operation the clutch 40 is disengaged and as the tool approaches the end of the thread the cam 34 is rotated by means of a handle 35 so as to cut first the shallow relief 52 and then the steeper relief 51. For automatic operation the clutch 40 is engaged and the cam driven by the worm gearing 37 at such speed that it makes one revolution while the reliefs 51 and 52 are being formed. In a modification the cam member 34 is replaced by a part circular cam 60, Fig. 4, which rotates at the same speed as the table. As the tool is about to enter the worm, a rise 61 on the cam displaces the tool to cut the extreme end of relief 52 and then the cam surfaces 63, 65 successively come into action while reliefs 51, 52 are formed. The central concentric cam part causes zero relief of the central part of the worm thread and then cam parts 67, 68 produce the two reliefs at the other end of the worm.