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  • Hobbing a worm gear with a tap

    Would you use the minor or major diameter of the tap to determine the number of teeth to gash? I'm planning on using an M14x3 tap and the gear blank's radius is approximately 59mm with a circumference around 371.5mm. The minor diameter would give 118 teeth, while the major diameter would give 123 teeth.

  • #2

    Neither. You need to use the pitch of the tap.
    Last edited by Arcane; 09-29-2020, 05:43 AM.
    Location: Saskatoon, Saskatchewan, Canada

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    • #3
      I think you mean the major or minor diameter of the resulting worm wheel. Dividing 371.5mm by the 3mm pitch gives 123.8 teeth. If you just plough a free to spin blank into a rotating tap, it would probably give you 124 teeth - or maybe 123, however it was feeling on the day. It might give you 123.8 teeth, which wouldn't make you happy!

      If you need an exact number of teeth, then it's going to need a dividing head and a 14mm diameter narrow cutter, ideally set at a bit of an angle to the blank's axis to rough out the required number of teeth..This will give the teeth of the tap something to follow and let it finish the profile more accurately.

      Ian
      All of the gear, no idea...

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      • #4
        Originally posted by Ian B View Post
        I think you mean the major or minor diameter of the resulting worm wheel. Dividing 371.5mm by the 3mm pitch gives 123.8 teeth. If you just plough a free to spin blank into a rotating tap, it would probably give you 124 teeth - or maybe 123, however it was feeling on the day. It might give you 123.8 teeth, which wouldn't make you happy!

        If you need an exact number of teeth, then it's going to need a dividing head and a 14mm diameter narrow cutter, ideally set at a bit of an angle to the blank's axis to rough out the required number of teeth..This will give the teeth of the tap something to follow and let it finish the profile more accurately.

        Ian
        Absolutely my experience when I tried this maybe 15 years ago.
        I cut it off twice; it's still too short
        Oregon, USA

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        • #5
          Well for one thing , your blank should be a calculated diameter, for a specific number of teeth.. as far as I know..
          anything else would be just playing around.. guesswork..
          as in most machining operations, figuring things out before hand, gives quickest results..

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          • #6
            So you should be looking at the pitch diameter of the worm wheel which would be pi x diameter minus .877 x pitch. My calculation for 2*59*pi equals 370.7. When you say 59mm approximate radius you may have a more accurate number. Subtracting 2.631 (.877 x 3mm) from 370.7mm gives me 368.1 mm. Dividing 3mm circular pitch into that then gives me 122.69 teeth. It has to be a whole number, so figure 123 teeth. This would leave a little at the tip of the wheel teeth uncut, but that's probably to your advantage since a gear would have a rounded tip and root.
            .
            "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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            • #7
              Thanks everyone. Pitch diameter and gash at an angle is the answer.

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              • #8
                Please share your progress and video if possible
                Thanks
                olf20 / Bob

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                • #9
                  Originally posted by elf View Post
                  Thanks everyone. Pitch diameter and gash at an angle is the answer.
                  Gashing to make sure the tap synchronizes with the workpiece is the most important. In fact the desired angle may not be that important either if you're doing most of the cutting with the tap. It will establish the correct angle from the tap pitch. There is a theoretically ideal pitch diameter but you may have a hard time cutting it accurately to size. That may not be that important if you have control of the spacing of the worm to worm wheel. Again, in theory you can establish it if you can measure across the tap and the workpiece OD and know what you're shooting for.

                  Good luck with it.
                  .
                  "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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                  • #10
                    The number of teeth +2, times pitch of tap divided by pi should give the diameter. John b.
                    John b. SW Chicago burbs.

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                    • #11
                      There was a small bit of misinformation in the original post. The tap is a TR14x3mm instead of M14x3. The pitch diameter is between 12.415mm and 12.191mm, minor diameter is 10.5-10.135, and the major diameter is 14-13.82. I'll probably take a little of the diameter of the blank to make a nice even 120 tooth worm gear.

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                      • #12
                        After reading this post tonight, it sparked a memory of my attempt at "Hobbing a worm gear using a tap" It's been a long time ago, and I must have found an writeup about it because of the photo on the paper on my work bench. I made a fixture using some bearings I had and machined a few parts and had a go of it. I don't think I ever perfected the procedure. I tried a tap and then made a hob from a piece of threaded acme rod. I would bet the whole setup is in a drawer out in the shop.
                        Attached Files
                        _____________________________________________

                        I would rather have tools that I never use, than not have a tool I need.
                        Oregon Coast

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                        • #13
                          That formula is based on a standard gear tooth form. But an acme tap will not be making that. It will be closer to a stub tooth profile. But it will not meet all specifications for that, either.

                          I believe the formula for stub teeth is (teeth + 1.6). I am working from memory so someone please correct me if I am incorrect.

                          But even with the correct formula, doing a free rotating hobbing is not going to be an exact science. In lugnut's posts he appears to have succeeded in doing this, but he may have had some failures along the way. And I would bet that he tried to plunge the cutter in to the full depth before the blank had a chance to rotate by more than a few teeth. If he had allowed it to rotate a complete revolution with a shallow cut, I think the tooth count would have been different, larger. Personally, I would go full depth, but approach the cut from one side so that the cutter first makes some small gashes in the edge of the blank and those gashes would allow it to synchronize as the blank moves by additional rotations. That is probably not possible with his set up and difficult on a lathe. A milling machine would be much better.



                          Originally posted by john b View Post
                          The number of teeth +2, times pitch of tap divided by pi should give the diameter. John b.
                          Paul A.
                          SE Texas

                          Make it fit.
                          You can't win and there IS a penalty for trying!

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                          • #14
                            OK, today I made a small search in my parts draw and found the fixture I posted photos of in my #12 post. It still has the same disk on it thats in the phots. My main objective was to see if a worm gear and worm could be made using that method. I was not worried about correctness or formula, I was seeing if I ever needed a small worm setup that I could do it. If the teeth come out even with out messing up the previous teeth, who cares.
                            The funny part of all this is: that same gear blank was still on the fixture. I must have decided to leave it as a reference.
                            By the way that gear ended up with 102 pretty good teeth.
                            Last edited by lugnut; 09-30-2020, 09:30 PM.
                            _____________________________________________

                            I would rather have tools that I never use, than not have a tool I need.
                            Oregon Coast

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