From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Guide on the correct procedure and importance of sharpening hobs, page 5.
| Identifier | ExFiles\Box 136\5\ scan0339 | |
| Date | 15th January 1940 guessed | |
| -5- SHARPENING OF HOBS QUENTLY. The most important rule for the maintenance of hobs is SHARPEN FRE- It is always, and especially in the case of hobbing, more economical to resharpen more often, removing only a little from the face of the tooth than the reverse, as after the cutting edges become blunt, deep pitted portions of the tooth flanks very soon become effected by the wear. The time apparently gained by running the hob longer is turned into loss by the more rapid wear and by the longer grinding time, consequently less tool life. It is very important that a hob be sharpened properly, as a hob sharpener can spoil a good job by not sharpening it concentric. It is, therefore, necessary that the hob user provide himself with the means of checking hobs so that these errors can be detected. We might mention here that, in sharpening hobs, every care should be taken not to try to remove too much metal at a time with the grinding wheel. If too much metal is removed at once, it will cause excessive heat and cause check marks in the hob teeth as shown in Figure #14. There are two very important reasons why hobs should be kept properly sharpened. (1) Economy: A sharp hob will cut much faster, use less power and wear much longer. (2) Better Work: A sharp hob will produce more accurate work, with a finish far superior to that obtainable from a hob not in first class condition. A correctly sharpened hob tooth (A) has neither of the common errors shown at (B) and at (C) in Figure #15. When a tooth is ground with a hook (B) it will cut a gear with a decreased pressure angle. When ground ahead of center, as at (C), the gear tooth will have an increased pressure angle. This is better demonstrated in the enlarged views shown in Figure #16. Either of these errors will produce an incorrect form and should be carefully avoided. Care must be used to maintain correct flute spacing. If some of the hob teeth are ground farther back than others (d), causing a reduction in tooth height (E), only the remaining teeth will do all the work. This means faster wear and irregularity of cutting action and theform of the gear teeth will not be the same on both sides. Hobs made with a hook, (for special work), can be sharpened satisfactorily provided the amount of hook, marked on the hub, is carefully maintained. For grinding, the beveled side of the grinding wheel should be used, medium grain and of soft grade - yet hard enough to prevent flying grit. The wheel should be kept clean; a glazed wheel may also crack the hob teeth. The flat side of a grinding wheel cuts away the face of the tooth as it touches the spiral surface far from the center point, (in the illustration the crossing point of work axis and wheel axis Figure #17) and grind there. The larger the spiral angle of the grinding flute, the larger must be the bevel angle of the grinding wheel in order to provide sufficient curvature away from the surface to be ground. | ||