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Primitive worm gear hobbing

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  • Primitive worm gear hobbing

    I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

    https://www.youtube.com/watch?v=J0o3W4_LRBw

  • #2
    That's pretty awesome but why not use a tap? It's harder & sharper.

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    • #3
      Originally posted by flylo View Post
      That's pretty awesome but why not use a tap? It's harder & sharper.
      ????? He used a tap to make the gear. Or what were you thinking he should use the tap for?

      It looks like it should work for light duty.

      Mike

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      • #4
        Originally posted by flylo View Post
        That's pretty awesome but why not use a tap? It's harder & sharper.
        A tap would really shorten the life of the worm wheel.

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        • #5
          Originally posted by Horst View Post
          Does anyone know if this would actually work?
          you hear about it often enough. What I don't understand is how the change in pitch is accounted for or works it self out......i.e. when you start the taps pitch is at the addendum and when done at the dedendum.
          Last edited by Mcgyver; 01-24-2018, 08:26 PM.
          .

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          • #6
            If the teeth look a bit mashed just keep going and when they straighten out you are at the finished diameter.

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            • #7
              Originally posted by Horst View Post
              I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

              https://www.youtube.com/watch?v=J0o3W4_LRBw
              Works best with a spiral flute tap and then machine cut thread on the worm wheel as threaded rod lacks precise fit.

              http://www.model-engineer.co.uk/forums/latest_posts.asp

              search for worm wheel plenty of reading here
              Last edited by velocette; 01-24-2018, 08:15 PM. Reason: access to more info

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              • #8
                My recollection of reports is that using a tap on a plain free-wheeling blank isn't reliable for a particular number of teeth on the wheel. It will find a place somewhere and work, and many times that may be sufficient. If you just needed a quick and dirty gear reduction it's great. If you needed a specific pitch that you've calculated for, it helps to gash the wheel before addressing it with the tap so it registers the desired number of teeth.
                .
                "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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                • #9
                  Originally posted by Horst View Post
                  I saw a video where a fellow cut a worm gear wheel by advancing a free spinning blank against a tap being rotated in a lathe. I inferred that the worm then consisted of a lenght of threaded rod equal to the tap. Does anyone know if this would actually work?

                  https://www.youtube.com/watch?v=J0o3W4_LRBw
                  It's my understanding that it works well enough for light use. But if you wanted something more durable and which needs to be able to generate more torque in the output that you want an Acme form of thread. Otherwise the angled teeth cause more separation force and wear more quickly. And in fact it may be that square cut teeth on the worm and hob are the better option.

                  I also think I've seen writeups where the blank was given a rounded groove in the periphery so the tap would engage more fully right from the get go. Saves a lot of the initial start up crunching and gets the blank turning more positively. Might be an aid to achieving the proper number of teeth too.
                  Chilliwack BC, Canada

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                  • #10
                    Nothing says that the tap must be a v-form tap. You could use an Acme tap.

                    Then, if you do not do as is shown, but instead you set the tap to the correct depth relative to the desired worm wheel, and then advance it from the small end toward the deepest cut, it should do a good job of cutting, just as if it were actually cutting a thread in a nut.

                    It will first cut shallow, and as you advance it the cut will fill out until you get the correct depth and form.

                    You should be able to get any number of teeth that you want, set only by your accuracy in making the depth. You could set a little shallow, and adjust until it cuts perfectly, perhaps.
                    Last edited by J Tiers; 01-25-2018, 02:21 PM.
                    1601

                    Keep eye on ball.
                    Hashim Khan

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                    • #11
                      Originally posted by Mcgyver View Post
                      you hear about it often enough. What I don't understand is how the change in pitch is accounted for or works it self out......i.e. when you start the taps pitch is at the addendum and when done at the dedendum.
                      I've done it a few times and best I can see because the wheel is free to move it can centralize in the cut.I think this allows it to behave like a gear hobb and correct any small errors as the cut progresses.

                      I do know it works better with a spiral flute tap and on larger tooth counts.

                      I have been itching to try it with a spiral flute Acme tap and see what happens for obvious reasons
                      I just need one more tool,just one!

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                      • #12
                        I found that after the first revolution the teeth are somewhat deformed but as cutting progresses the diameter of the wheel decreases and the teeth take on a more perfect shape.

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                        • #13
                          If you start with the "front end" of the tap, where the partly ground away teeth are, and advance the tap slowly by one tooth for each revolution of the blank, it should cut progressively deeper teeth in the blank, without deformation. It sould behave much as if it were cutting a thread inside a hole.

                          I think that was actually shown here years ago, but it might have been somewhere else.
                          1601

                          Keep eye on ball.
                          Hashim Khan

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                          • #14
                            The past couple of issues of HSM have revisited the Phil Duclos dividing head. I built the original version over twenty years ago. Too frugal to purchase the worm & gear, I free hobbed my own, turning an approximation of an Acme thread for the worm long enough to cut off a section to case harden and use to free hob the gear with.

                            I don't recall all the details of calculating the blank dimensions at this point, but ended up with a 40 tooth gear on the first pass. I might have gashed the blank, but with no means of indexing, I can't recall how I managed to accomplish that.
                            Jim H.

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                            • #15
                              Mebbe with a 40 or 80 tooth gear?

                              Gears make really good index references.
                              1601

                              Keep eye on ball.
                              Hashim Khan

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