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 discussing the design, calculation, and testing of hobs for generating gear teeth.
| Identifier | ExFiles\Box 136\5\ scan0337 | |
| Date | 15th January 1940 guessed | |
| - 3 - fixtures will also be of unlimited value to the users of hobs to control the quality of tools coming into their plant. To design and use a hob intelligently, it is necessary to understand the geometrical laws involved. Figure #4 shows the increments of cuts as a hob generates a gear tooth. Figure #5 shows a method of calculating the last point of contact that a gear tooth has with a hob tooth, also the last cutting position that a hob may be set and still obtain full generating action. In designing the hob it is necessary to know what these contacts are, as the involute gear as it is made today, especially in the automobile field, has a modified tooth, and the hob has to be designed to form these modifications, or approach curves (as they are sometimes called) without breaking the arc of contact with the mating gears. Figure #6 is a formula showing how to find the start of modification on a hob for a known modification on the gear, which is predetermined by an analysis of the gear combination in question and by use of the formula shown in Figure #7. After making these calculations, a hob must then be made that will generate the involute and modify it correctly; therefore, it must have the correct contour on all of its cutting teeth. It must have all of its cutting teeth in the true helical path and all of its cutting edges concentric. In modifying gear teeth, care must be taken not to modify them too excessively, because any modification of the involute form reduces the amount of involute over-lap, or the number of teeth in contact. To check hobs for these fundamental requirements of correct contour of teeth, proper location of contour on the helical path of the thread and uniform distance of contour from the axis of the hob, it is essential to have checking equipment that is designed for this purpose and is theoretically correct. The testing instruments that have been developed will indicate how close the finished hob approaches the ideal conditions. Figure #8 shows a hob on the lead testing machine which uses a sine bar that can be set to check any lead from .144" to 2.6" by using the sine bar method of checking, the possibility of lead screw errors showing up in the hob are eliminated. DESIGN PRINCIPLE The main components of the machine are shown in Figure #8. These are the Hob Spindle, the Indicator Slide, and the Sine-Bar Table. In order to check the lead of a given hob, the spindle must make one complete turn while the indicator slide travels a distance equal to the axial lead of the hob to be checked. This we obtain in the following manner: The rotation of the spindle and the movement of the indicator slide are both controlled by movement of the Sine-Bar table. This table clearly shown in Figure #8 carries the Sine-Bar and also a precision master rack. Driven by this master rack is a pinion which through precision ground change gears causes the spindle to rotate, at the same time the Sine-Bar actuates the movement of the indicator slide. | ||