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).
Page detailing gear hobbing techniques, machine condition, and hob positioning.
Identifier | ExFiles\Box 136\5\ scan0342 | |
Date | 15th January 1940 guessed | |
- 8 - of the individual hob tooth errors. For example, suppose a hob has 60 teeth, 25 of which comprise the cutting zone when producing a certain gear; of the 25 teeth, perhaps 10 serve as roughin teeth only, and the remaining 15 actually generate. (See Fig. 18) Each of the 15 teeth will form a portion of the involute curve. See Fig. 4. It will be readily seen that although each tooth may be individually accurate, errors in spacing will cause certain teeth to cut too much on one side and not enough on the other, thus modifying the involute considerably. It may appear that with 15 teeth in the generating zone, a wide departure from the true involute is possible; in practice, however, the errors cancel each other to a large extent. By shifting a hob axially, different composite sets of teeth of the hob comprise the generating zone; by this means it is possible to select one or more best positions of a given hob when cutting a gear of a given number of teeth. RECORDING THE HOB POSITIONS When satisfactory positions of a hob have been determined, they should be recorded by means of a special micrometer setting device attached to the machine. The high points and amount of run-out may be marked on the hob with blue vitrol solution. Thus, on future jobs the hob positioning can be duplicated without resorting to trial or dummy gears. Frequently several good positions will be found on one hob, all of which can then be utilized before sharpening is required. Sharpening the hob may affect the form slightly; it is therefore best to repeat the trial cuts and check for form after each sharpening. Another important point in hobbing accurate gears is to be sure that the hobbing machine is in good mechanical condition. By good condition, we do not only mean good ways and bearings on the machine, but also accurate index and feed gears. This is very important, as the accumulated errors in the index and feed gears and also the lead screw will show up in the gears being cut. It would be a good policy every so often to check the angular velocity of the hobbing machine with instruments provided for this purpose. It is the practice at times when cutting gears of heavy pitches to take a rough and finish cut, thus obtaining better accuracy. In roughin, single, double, and triple thread unground hobs are used. It is not a good practice to use greater than a triple thread hob for roughin because there will be no economy effected. The additional cost and variation of hobs greater than triple thread will more than offset the time saved in cutting, and the quality of work will not be as good. Figure #19 is a chart for use when estimating cutting time required for hobbing a load of gears, and is useful when making a time study of a job. Figure #20 is a very useful chart when setting up a new job. We might add here that it is just as important to run hobs at the proper speeds as it is to mount the hobs properly. To run hobs too fast or too slow, the highest efficiency cannot be obtained. The determining factors for hob speeds are the type of material being cut, its hardness, physical properties, and the quality of finish required. Figure #21 is a graph chart showing the recommended length of worm gear jobs. The length can easily be determined when the axial circular pitch and the number of teeth in the worm gear to be cut are known. | ||