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Free hobbing - a rogue method?

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:34 AM.

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  • J Tiers
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    I saw earlier the comment from the "never tilt the cutter" group that you will get angled teeth if you do compensate for the hob angle. The "you gotta tilt it" group seem to contradict that.

    That suggests to me that there IS some "special sauce" here that allows the "no tilt" and "gotta tilt" groups to both get good results, they must be doing different things. So what do they do different to get the good results with "the wrong method"? (They each say the other is wrong).

    I'm thinking it is a little hard to get a very wrong pitch, the hob flanks should prevent that. The depth and OD are easy to get right.

    Hobs differ. Some have straight fluting, some helical. You'd think the helical might work a little better, although they are not as easy to find.

    How about pics from each group so we can see what they are using and the setup?

    If you gash, then the gashing should be like what you want. Straight if spur, angles if helical gear is to be made.

    I'm suspecting that the requirement to gash is not as big a deal as it is made out to be.
    Last edited by J Tiers; 10-11-2021, 08:36 PM.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:34 AM.

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  • J Tiers
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    Hobbing is a very standard way of properly generating a gear. Gear cutters are the "desperation" method to "cut" a "working" gear. So is free hobbing, to an extent. It generates a good gear of whatever pitch etc you end up with. If you can accurately predict that, then you get "standard" gears.

    Hopefully, you at least get a gear that fits the last one you made. And there is really the only issue. Can you control well enough to get a bunch that fit each other?

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  • Bob Engelhardt
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    This is a weird thread. Free hobbing has been around forever - we're not discussing some revolutionary discovery & how it works. Nor are we addressing a member's questions about how to free hob. Mostly the thread seems to be a pissing contest.

    Hobbing's been around forever and yet people still buy gear cutters and cut gears the old way. Must be that hobbing isn't all that great and just an alternative for the desperate without another option. I was once desperate and hobbed a gear - I have other options now.

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  • J Tiers
    replied
    Originally posted by Robint View Post
    Gears should have the right pitch, the right pitch diameter, and the right form.
    Unquote

    But its not STATE LAW as I keep saying
    You are, of course correct, it is not state law.

    In actual fact, it's FEDERAL law, if anything.

    You can deviate at your own risk, risk of them fitting nothing else in the world.... or even each other.

    But they all should have whatever you choose or they do not fit and work together. So some sort of fairly precise knowledge of what will result is needed, or it will not go well. Consistency is king.

    What the standard is, well, that's your problem..
    Last edited by J Tiers; 10-11-2021, 04:12 PM.

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  • Robint
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    OBTW anyone wanting to make one of these cutters should go for a narrower slot. Mine were 2.5mm . ( suggest 1/16" or 1,5mm) for my 3mm pitch

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:38 AM.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:38 AM.

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  • skunkworks
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    well - with an encoder and a computer running linuxcnc - you can create real gears with the correct shape...



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  • J Tiers
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    Not sure what the fuss is about.

    Gears should have the right pitch, the right pitch diameter, and the right form.

    The argument here is as to how "free hobbing" maintains the right form and pitch. Show how that happens and it all disappears.

    I say the flanks of the hob will drive the work, so it should be close. Not perfect. Likely good enough. The hob does the form.

    The errors may occur as the cutting edge cuts, because guiding quits during that time.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:37 AM.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:37 AM.

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  • Doozer
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    Forgive me if this was mentioned before, as I only read some of this thread...

    So for free hobbing to work without gashing, consider this.
    For the spacing of a given number of teeth to be correct,
    this is based on the pitch diameter, not the addendum.
    So make a gear blank thicker than needed, make 2 stepped
    diameters, one bring the pitch diameter and one being the addendum diameter.
    Start the hob on the pitch diameter step and get it spinning, then feed the cutter
    into the larger addendum diameter. That is why people have success with
    quickly plunging in a knurling tool. To get it deep to the pitch diameter quicky,
    so the knurl spacing comes out. Apply the same concept to gear hobbing,
    only create step for the pitch diameter, instead pf plunging quickly in the knurling example.

    --Doozer

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:37 AM.

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