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  • #16
    Originally posted by Richard P Wilson View Post

    Indeed, form cutters are only correct in the middle of their marked range, and deviate increasingly towards the end of their range. I agree that a hob, used in the situation where both the hob and the blank are driven can give a better profile than a form cutter used towards the end of its range. The problem is that free hobbing, due to drag might not give such good results, probably not as good as the form cutter at its worst.
    Quite true. The problem with free hobbing is the lack of consistent positive drive, which is sort of inherent, since there is a "no-drive" point as the cutter tooth starts a new cut.

    That brings up the question of whether a hob with staggered cutting edges might work better. That way, there might be better stabilization of the blank as the cut starts. I think overall that there are some issues with that due to relief on the cutting teeth.

    A "hob" without relief would have the best guidance of the blank, if you really want to have the best possible performance. Staggered cutting teeth would help.

    I have not looked at the details of hobs and hobbing for a while, and there is something nagging at me with respect to tooth position. I forget what the issue is, but it has to do with details of the tooth form, the facets produced, and their positions. Someone with more specialized knowledge may be able to explain. Zahnrad would know, but I think he left in disgust.

    At the end of the day, it may be more productive to set up a proper drive gear train than to attempt to figure out how to do hobbing, using an already expensive hob, but trying to do the rest of the process on the cheap.


    Originally posted by Richard P Wilson View Post
    ......................................., and, as I said before, if I've got to use the dividing head to gash the blank prior to free hobbing, I might just as well put a form cutter on, and cut the gear in one operation rather than 2
    Yep. I've done both, actually. When making bevel gears, gashing is a good plan if your holder is not exceedingly stiff and solid. Or if you prefer to have less wear and tear on the cutters. Since you usually make 3 passes anyway, the gashing step is not that terrible an added step.

    It does reduce the work the cutter has to do, if it happens that you do not have proper facilities to sharpen them.



    2730

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    Hashim Khan

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