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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).
Hob centering gages and the causes and remedies for visible flats on gear teeth.

Identifier  ExFiles\Box 136\5\  scan0340
Date  15th January 1940 guessed
  
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HOB CENTERING GAGES

The function of hob centering gages is supposedly to make the gear teeth symmetrical or "balanced". Centralizing either a hob tooth or space with the common center line between hob and blank will cause the series of flats comprising the tooth forms to be theoretically even on each side. In practice, these flats, caused by the successive hob teeth generating the involutes, are so close together that they merge into smooth curves. Usually they cannot be detected by the most sensitive profile measuring devices, and cannot ordinarily be seen when a gear is rolled with its mate to observe the bearing.

When visible flats do appear on hobbed work, the remedy is not to resort to centralizing gages, as is commonly supposed. Such a procedure may occassionally result in better balanced forms, but will not eliminate the flats. This practice is carried over from the time when most gear teeth were produced by milling with disc cutters, the latter requiring accurate centralization to secure balanced forms.

It is far better to eliminate the cause of such visible flats, rather than attempt to balance the flats. The presence of flats is commonly caused by a combination of too few a number of gashes in the hob, and a low number of teeth in the blank, particularly on coarse pitches. In this case the obvious remedy is to increase the number of gashes in the hob.

If the number of teeth to be cut is extremely low, as in automobile starter pinions having five or six teeth, it may not be practical to increase the hob gashes sufficiently to eliminate all visible flats, but such pinions are not used at high speeds and therefore accuracy of forms is not so important.

Likewise when infeeding a worm gear of a low number of teeth, where frequently a small worm diameter limits the number of gashes that it is practical to provide in the hob, appreciable flats may be present. These can be eliminated by tangential feeding rather than infeeding.

Inaccurate hobs, particularly unground hobs, will sometimes generate visible flats because of high lead errors. The remedy is obvious.

Faulty sharpening of the hobs, resulting in hob runout or in erratic spacing of the flutes, may cause the same effect.

Faulty mounting of the hob in the hobbing machine is another possible cause of flats.

For the general run of spur and helical gears, therefore, a hob centering gage is not necessary. It has been our experience that it is much more important to have the hob run true than to centralize on any particular tooth or space, and for very precise work, to select the most accurate zone on a given hob by axially shifting it to various positions. The errors produced in tooth forms from eccentrically running hobs, and from lead, spacing and form errors inherent in the hobs, sometimes amount to many times the error from generating flats, for it should be remembered that even if a hob is set central, it does not necessarily follow that accurate tooth forms will be produced. The combined effect of the errors in the hob, and particularly the hob setting, must still be considered.
  
  


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