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 operation and use of hob lead checkers and contour testing fixtures for gear manufacturing.
| Identifier | ExFiles\Box 136\5\ scan0338 | |
| Date | 15th January 1940 guessed | |
| - 4 - Only two simple operations are required to set this machine up for a given load, the proper change gears for driving the spindle and the Sine-Bar setting which is determined by the axial lead of the hob. The Indicator arrangement on this machine also has a 5 to 1 ratio, and will, therefore, register ten thousandths of an inch. The contact point of the indicator is brought against the cutting edge of the hob tooth and the indicator dial is set at zero. As the table of the machine is advanced the hob will start to rotate, and the slide carrying the indicator arrangement will move transversely away from the table. The amount of movement of the slide in one convolution of the hob is controlled by the setting of the sine-bar, and is equal to the lead of the hob being checked. This setting is determined by the helix angle of the hob, and is positive. Figure #9 gives a general view of the Hob Lead Checker. If the hob has been made correctly, the indicator point will register zero on all cutting edges. If there is a deviation of the hob tooth from the true helical path due to grinding, or if the hole in the hob is large and does not fit the arbor, or if the gashes in the hob are not properly spaced, the indicator will register a great variation in the cutting edges as they pass by the indicating points. Figure #10 is a lead chart showing how the load of the hob can be plotted graphically so that it can be seen at a glance just where and how great the errors are. HOB CONTOUR Next of importance is the angle of inclination, or the pressure anglo of the hob, and its modification if there be any. The bearing produced on the gear to be cut is largely controlled by the contour of the hob, and may be varied for different applications. Figure #11 shows a hob on the contour testing fixture, the indicator arrangement is constructed so that a movement of .001" on the contact point is registered as.005" on the dial indicator. Therefore, a movement of .0002" on the point of contact will register .001" on the dial indicator. By this amplification it is possible to check the contour of hob tooth to ten thousandths of an inch. This is a very sensitive instrument and having a keen edged contact finger it will actually register such depressions and elevations as are caused by rough grinding. It will also show any deviation from the true pressure angle by means of a sine bar setting. The indicator slide can be moved inward in increments of .010 or .015" and readings taken at the different points and plotted on a graph chart, thus showing any variation in the contour of the hob tooth. Figure #12 is a contour chart showing a modified hob tooth form; the center line of modification being the master form, the other two lines being tolerance lines beyond which the form must not go. However, if the hob should check to either extreme the hob will still cut gears without interference. Figure #13 is a formula sheet which shows the method of figuring correction in pressure angles for both straight and spiral gash hobs for extreme accuracy. | ||