Patent GB-817461-A (GB)
Title: Improvements in or relating to gear teeth finishing
IPC: B23F21/03
Filed: 1955-07-11
Abstract: 817,461. Gear - finishing; gear - cutting. SCHLICHTHORLEIN, J. July 11, 1955, No. 19950/55. Class 83 (3). [Also in Group XXIII] In a method of shaving, lapping, hobbing, or shaping involute gears, even-numbered contactpath sections succeed each other when the rolling-point is at, or near, the point of transition between two of said contact-path sections, or when said rolling-point is at, or near, one end or outside of said contact-paths, while alternately even-numbered and odd-numbered contact-path sections succeed each other within said contact-paths when said rolling-point is situated within an even-numbered contact-path section and when at the same time an oddnumbered contact-path section, on which acts the greatest average tooth load, is directly contiguous to that part of an even-numbered contact-path section on which acts the smallest average tooth load as a result of the location of said rolling-point. Spur or helical gears are finished by toothed tools, such as shaving or shaping-cutters, lapping-tools, hobs or racks, of which, in general, only one is positively driven. In shaving gears, the work may be rocked to produce crowned or conical teeth. Fig. 1 shows the contact paths KF crossing at the rolling- point W. Fig. 2 represents the contact path e bounded by the addendum-end K and dedendum-end F and divided up into sections e2 when two tooth-sides are in contact, e31, e32, e33 when three sides are in contact, and e41, e42 when four sides are in contact. If the work lags slightly relative to tooth-rotation, projections and recesses will be produced on the teeth at corresponding points. The effect of the main forces on the work-teeth, i.e. frictional and static forces, is discussed for a particular example in the Specification. The tooth-load attains a maximum on the three-side contactsection e32, and a minimum on the three-side contact-sections e31, e33, and varies unsymmetrically over the contact-path so that grooves are formed in the teeth. If the three-side contact-sections are eliminated, e.g. by reducing the work-diameter and thus shortening the contact-path, the load variation is symmetrical and no groove is formed. Such a contact-path is shown in Fig. 6d, and has two-side-contact and four-side contact sections e2, e41 and e42 respectively. Symmetrical load-variation is also obtained if the rolling-point W is situated between the sections e41 and e2, at the end K or F, or outside the path e. The production of a perfect involute when the load-variation is symmetrical can occur only once in the life of a tool, as the relationship is destroyed by sharpening. If a tool produces a groove on the tooth, it may be modified by altering the length ratio c # = -, where c=the distance between the e work root end of the contact path and the middle of an even-numbered contact-path section, and e=the length of the contact path. The length ratio can be varied by giving tip-relief to the rough-cut work gear tooth section, or by undercutting, by altering the tool addendum or the tool-tooth profile displacement, by changing the number of tool teeth, and so on. The tool teeth may be of modified involute form to crown or relieve the teeth of the work-gear. The tool may, itself, be a gear to be lapped with the work-gear. Specification 817,462 is referred to.