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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, 06:33 AM.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:33 AM.

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  • nickel-city-fab
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    FWIW, 6TPI Acme screws mesh beautifully with 18 DP, 14.5 PA gears, and should do a nice job of hobbing the same. Either free hobbing or regular. I recommend investing in a Acme thread pitch gauge. The most difficult part is making an acme thread that is hard enough to cut something, and making a definite number of teeth.

    Some math might have4 to occur.

    One can also make one's own division plates without too much trouble, using only common layout tools. One plate for 15 holes, and one plate for 16 holes. The product of the two plates is 240 positions, and any factor of 240. This would make it possible to generate minutely accurate plates over a couple of generations.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:32 AM.

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  • Jim Stewart
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    Once again I find myself wishing that George would enable the "dislike" button that's part of vBulletin. (Yeah, never in a million years would he self-inflict that headache.)

    -js

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  • nickel-city-fab
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    Originally posted by mattthemuppet View Post
    thanks for the pointer ncf, I just bought a used copy of the newer version for not much money.
    It'll come in handy if you ever decide to do some fancy weird stuff with your lead screw for example. I was considering a SB model B instead of the A that I got, just because of this book. I got the A because it was in better shape and already tooled up.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:36 AM.

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  • mattthemuppet
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    thanks for the pointer ncf, I just bought a used copy of the newer version for not much money.

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  • nickel-city-fab
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    Originally posted by J Tiers View Post

    This aligns with the idea of the hob flanks doing the guiding, and makes sense in general. The gashes would be needed to start the blank off.

    AS for the Laws book, I can find it if I need to. Thanks for the offer. I realized that I have a couple other references that may have info as well, and I will take a look at them.
    I highly recommend Ivan Law's book. https://www.amazon.com/Gears-Gear-Cu.../dp/0852429118

    His book can boggle the mind a bit when you start to digest it.... very densely packed but not too many pages. Incredibly useful charts and formulae, etc. I've gone through it 3 or 4 times in the last few years and I'm still absorbing it.
    Last edited by nickel-city-fab; 10-12-2021, 06:50 PM.

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  • J Tiers
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    Originally posted by Richard P Wilson View Post

    As an aside, and something I hadn't thought of, he says that the hob must be in contact with the gear at all times, so don't use a hob with wide flutes on a narrow gear!..................
    This aligns with the idea of the hob flanks doing the guiding, and makes sense in general. The gashes would be needed to start the blank off.

    AS for the Laws book, I can find it if I need to. Thanks for the offer. I realized that I have a couple other references that may have info as well, and I will take a look at them.
    Last edited by J Tiers; 10-12-2021, 06:26 PM.

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  • davidwdyer
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    I spent the afternoon trying out some new free hobbing techniques.

    As I suspected, turning the mill manually and slowly worked like a charm for "gashing". It was just too easy and quick.

    I had to stand on a chair with the ratchet and socket but the hob was sharp and it easily cut "gouges" in the gear face to guide the future milling.

    I did not gash across the entire face of the gear, but just at the edge to provide a hold for the cutter to grab when turning on the machine.

    The good news is that it worked well and produced the expected number of teeth.

    AND the teeth ran straight across the face of the gear. Great!

    The BAD news is that the tooth form was not anywhere near involute. I will try to send some pictures tomorrow.

    Now I'm going to have to go back and try something else, like tilting the head. If this is the solution, I'll have to eat my words but that's OK.

    I've been wrong before. And I know how to apologize.

    I tried a hob with larger teeth to I could see the gear form better.

    I plan to try an even larger teeth so that the form will be easy to see. Well, the saga goes on.




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  • Richard P Wilson
    replied
    Originally posted by J Tiers View Post

    Nothing at all has been dispelled, or even discussed, unless I have a LOT of folks on "ignore" (I have nobody on ignore). All I have seen is "you should not", and "you have to". No convincing arguments, just the statements, amplified by "trust me, it won't work".

    The Laws book is not common to find. And don't say "ebay".... ebay and Walmart are places I just don't go.
    Ivan Law's book probably isn't very common in the USA, but its readily available in the UK. Its a useful, practical guide to gear cutting in the homeworkshop. Ivan does discuss hobbing of worm wheels, and only briefly mentions how to free hob a worm wheel. As an aside, and something I hadn't thought of, he says that the hob must be in contact with the gear at all times, so don't use a hob with wide flutes on a narrow gear!

    What I did find, tucked away in my copy of Ivan's book is a couple of articles from 'Model Engineer', the first on17th March 1995 and the second and concluding article on 21st April 1995, by a chap called Jock Smith, entitled 'Gears from scratch', describing how he cuts gears (spur and helical) using the free hobbing method. Its nice and clear, and when I get time I'm going to. give it a try. He makes his own hobs
    Both Jock Smith and Ivan Law say you need to gash the blank first, gash can be any shape within reason, and the indexing doesn't have to be perfect providing it produces the right number of teeth. What is clear, reading both of them, is that to cut a spur gear, you should set over the hob by the pitch angle, so it cuts a straight tooth, and for a worm wheel (providing the pitch of your hob matches the pitch of the worm thats going to mesh with the worm wheel), you don't!
    So the parties who say 'skew the hob' are right if they are cutting spur gears, and the parties who say 'don't skew the hob' are right if they are cutting a worm wheel. Otherwise, both are wrong!

    Anyway, JT, I don't know how to link the book or the articles on to here, but if you are interested in a nice clear explanation of free hobbing, I could scan the articles and send them via a PM.

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  • Tom S
    replied
    Originally posted by Robint View Post
    Guess you need to source your own text book then I cant help you, maybe another member can help you out there in LA as to whats available, but youve got to do your own homework. Its not my brief to be a Gear educator here

    My OP is addressed to those who know something about gears, have made them, know the issues and might be interested and help develop the method

    Ive had some success with it which Im prepared to share with skilled members

    And don't say "ebay".... ebay and Walmart are places I just don't go.

    Holy Bat guano is this guy hobbled

    Robin
    If you're not here to educate and show people the method that you are pushing, then what are you doing here? You claim to have worked out the kinks in a 'functional' (and I say that because from what I understand it doesn't produce a true involute profile, but a rough approximate that functions) method, but all you're doing is cryptically telling people that they won't like it because they're gear snobs? Who's the real snob in that situation? If you're interested in engaging in meaningful conversation and having people acknowledge that what you are doing works, why not fully explain what you're doing and work through criticism and questions from interested people? Instead you've got some sort of messiah complex going on and won't bring yourself down to the level of explaining yourself. The reason J Tiers is the only one engaging is because it appears he's got the most patience, everyone else left after the first page of diatribe.

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  • Robint
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    OP has deleted this post
    Last edited by Robint; 10-15-2021, 06:35 AM.

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  • J Tiers
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    Originally posted by Robint View Post
    .............................


    Hope we have finally dispelled the myth about not matching the work piece with the helix angle of the Hob tool

    Affirmative Assertions - make it so

    Robin
    Nothing at all has been dispelled, or even discussed, unless I have a LOT of folks on "ignore" (I have nobody on ignore). All I have seen is "you should not", and "you have to". No convincing arguments, just the statements, amplified by "trust me, it won't work".

    The Laws book is not common to find. And don't say "ebay".... ebay and Walmart are places I just don't go.

    Leave a comment:

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