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Free hobbing - a rogue method?

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  • #46
    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?
    2730

    Keep eye on ball.
    Hashim Khan

    Everything not impossible is compulsory

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    • #47
      OP has deleted this post
      Last edited by Robint; 10-15-2021, 07:34 AM.

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      • #48
        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, 09:36 PM.
        2730

        Keep eye on ball.
        Hashim Khan

        Everything not impossible is compulsory

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        • #49
          OP has deleted this post
          Last edited by Robint; 10-15-2021, 07:34 AM.

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          • #50
            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.

            2730

            Keep eye on ball.
            Hashim Khan

            Everything not impossible is compulsory

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            • #51
              OP has deleted this post
              Last edited by Robint; 10-15-2021, 07:35 AM.

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              • #52
                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.
                Cayuga, Ontario, Canada

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                • #53
                  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.
                  'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                  • #54
                    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.




                    VitŮŽria, Brazil

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                    • #55
                      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, 07:26 PM.
                      2730

                      Keep eye on ball.
                      Hashim Khan

                      Everything not impossible is compulsory

                      Comment


                      • #56
                        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, 07:50 PM.
                        25 miles north of Buffalo NY, USA

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                        • #57
                          thanks for the pointer ncf, I just bought a used copy of the newer version for not much money.

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                          • #58
                            OP has deleted this post
                            Last edited by Robint; 10-15-2021, 07:36 AM.

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                            • #59
                              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.
                              25 miles north of Buffalo NY, USA

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                              • #60
                                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
                                There are no stupid questions. But there are lots of stupid answers. This is the internet.

                                Location: SF Bay Area

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