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).
The process and importance of hob setting for precision gears.
| Identifier | ExFiles\Box 136\5\ scan0341 | |
| Date | 15th January 1940 guessed | |
| - 7 - HOB SETTING FOR PRECISION GEARS Many users of hobbing machines fail to realize the full importance of proper hob setting. The necessity of having the hob running true, and centralizing it on a tooth or a space when low numbers of teeth are cut, is common knowledge; but the selection of the best cutting zone in a given hob to produce tooth forms of the highest possible accuracy is not so widely understood nor appreciated. It is not recommended that all of the refinements in hob setting herein-after described should be indiscriminately employed in the set-up for small lots of "commercial" accuracy, because of the added expense involved. However, for gears requiring the utmost quietness and smoothness of operation, as in turbine reduction sets, automobile transmissions, timing gears, and printing press machinery, the proper hob setting is often responsible for the satisfactory performance of gears that would otherwise have been rejected. HOB RUN-OUT Obviously the first consideration in setting a hob is its fit on the arbor, which it should fit without play, otherwise it may shift under cut and the most careful set-up will be of no avail. The next step is to clamp the hob on the arbor, using a minimum number of spacing collars. The outer bearing should be in place before applying the wrench to the clamping nut, otherwise the arbor may be damaged by springing it out of line. The hub or proof diameters should be chooked by means of an indicator graduated in ten-thousandths of an inch. An indicator graduated in half-thousandths or thousandths cannot be relied upon for this class of work. Run-out should be within .0002", and the high points of the run-out on each end of the hob should be in line. Any eccentricity will produce an error in tooth form, particularly when the high points of the run-out are opposite on the two ends of the hob. If the run-out of the hub diamoters exceeds the limit stated, it can be readily corrected by loosening the clamping nut, rotating the spacing collars, and re-clamping. The slight errors in parallelism of the faces of the collars are thereby utilized to bring the hob into correct alignment. If may be necessary to repeat this process a few times before the hob runs satisfactorily. With a little practice the operator will be able to true up the hob in a few minutes' time. This is one of the most important elements in hob setting, and it will be found that the extra time required will be amply justified by results. It is very necessary in the first place to make hobs accurately, when precision gears are required, and it is equally important to see that the hob is sharpened properly and mounted properly on the machine. Otherwise, all the good that has been put into the hob is destroyed when a hob is mounted inaccurately. In other words, if a hob runs out on a hobbing machine, it has the same effect as a drunken lead. COMPOSITE EFFECT OF HOB CUTTING TEETH The composite effect of errors in the hob teeth is another important element influencing tooth form. Even though the most accurate ground hob obtainable is used, the errors of which may be within .0002" for form, tooth spacing, and weave, the combined effect of these errors may effect the form several times the amount of line. | ||