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  • Gear pressure angles

    I know that the universal advice is not to run gears of 14.5 deg and 20 deg pressure angles together. I've got some 20 degree gears, 18DP and need to cut a 127T/135T compound gear to make a gear set to cut imperial threads on a metric lathe, but I've only got a 14.5 degree cutter. In all honesty, given the speeds involved (low), and the relatively light duty, and occasional use, am I going to have a problem running these gears together?
    '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

  • #2
    i had the same problem and gave it a lot of thought
    the teeth will get a sliding component in the contact and therefore some wear eventually
    the gears i had did mesh but did not run very smoothly

    shouldnt the combination be 127/100 tooth?
    a 47/37 combination will give you 127.027027

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    • #3
      No, for the lathe in question (A UK made Denford 280) it is definitely a 127T/135T in accordance with the original manufacturers details.
      '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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      • #4
        Richard,
        Not a problem in this case.
        I have seen special for hobbing machines flycut with a 60 degree triangular insert so the hobbing machine can make a proper one.
        .

        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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        • #5
          Thank you John. I was hoping you would reply, not many people with as much experience in gearing as you have.
          '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

          Comment


          • #6
            I have a set of Britannia changewheels that have a peglike profile, as cast, not machined. Seem to work ok for that lathe. Involute gears even of the same angle slide, that's their thing, and why you need oil in you gearbox plus are constant velocity so your car doesn't judder along the road. Clock gears are cycloidal and roll with low friction despite no oil but are not constant velocity which suits a clock just fine.

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            • #7
              Generally speaking, when cutting metric threads with an English (inch) lead screw, you use a compound gear which has 127 teeth on one side and 50, 100, 135, or some round number of teeth on the other. These two gears are combined into a single compound gear which sits on a single hub.

              But this is not the only way to do it. Notice that one of those gears on the compound is a driver gear and the other is a driven gear. Thus, using an example from my South Bend 9, to cut a 1mm thread, I would use:

              A Stud gear: 16 . . . driver
              B Compound: 127 . . . driven
              C Compound: 100 . . . driver
              D Screw gear: 40 . . . driven

              The Stud gear (A) meshes with the 127 tooth side of the compound (B). And the 100 tooth side of the compound (C) meshes with the Screw gear (D). The sequence is A, B, C, and D. In this scheme both the 100 tooth and the 127 tooth must mesh with the existing change gears.

              BUT, this gear train can be rearranged in any way that preserves the two driver gears and the two driven gears and the overall gear ratio will be exactly the same. Thus any of the following sequences would work:

              A, D, C, B

              C, B, A, D

              C, D, A, B

              All four of these sequences would produce the exact same, 1mm thread on my lathe. Keeping the 100 and 127 tooth gears as the compound is only for convenience. Notice that the first two of them have the 100 and 127 tooth gears (B and C) meshing with each other.

              SO, you could make your 127 and 135 tooth gears using ANY pitch and ANY pressure angle as long as you use the same specs for both of them. Then arrange them to mesh with each other in your gear train instead of combining them as a compound. One of these odd pitch gears would be on either the driving stud position or on the screw gear position. The other one would be combined with one of your existing gears in a compound in the middle position. You do need to preserve the roles of each of these gears as driver or driven. I would assume that your gear train, like mine, has the 127 tooth gear as a driven gear. So the 135 tooth would have to be in a driver position in the gear chain.

              In my example above, an additional, idler gear was needed to allow all the gears to mesh. You may also have to add an idler to your chain, but I suspect that it would be between the stock gears for your lathe and not on the 127/135 pair unless you use a really small pitch for them. Idler gears do not change the overall ratio so they can have any tooth count. That brings up an additional advantage of doing it this way; since these two gears only need to mesh with themselves, you can use a smaller pitch for these large tooth count gears and they will have a smaller diameter. This may help in the use of the limited space in the change gear mounting area.

              And this would run a lot smoother than if you mix pressure angles.
              Last edited by Paul Alciatore; 04-24-2015, 04:22 PM.
              Paul A.
              SE Texas

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

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              • #8
                All very true Paul, but I'm copying the OM's set up, and this uses a 127/135 compound gear, so thats what I'm stuck with I'm afraid.
                '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

                Comment


                • #9
                  Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....

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                  • #10
                    More like threading a 55 degree Whitworth nut on a 60 degree UN bolt, which certainly can be done. I have seen pictures of working change gears made from wooden circles with nails driven in for teeth. Also a wooden circle with teeth made from a metal strip corrugated by running it between gears and epoxied to the circle.
                    Originally posted by Video Man View Post
                    Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....
                    Don Young

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                    • #11
                      ?????

                      But you are NOT stuck with it. That was my whole point.



                      Originally posted by Richard P Wilson View Post
                      All very true Paul, but I'm copying the OM's set up, and this uses a 127/135 compound gear, so thats what I'm stuck with I'm afraid.
                      Paul A.
                      SE Texas

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

                      Comment


                      • #12
                        Originally posted by Video Man View Post
                        Short answer is that running 20 degree gear with 14.5 degree gear is sort of like threading a 20 tpi nut on a 14.5 tpi bolt.....
                        Bollocks.
                        .

                        Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



                        Comment


                        • #13
                          Originally posted by Paul Alciatore View Post
                          ?????

                          But you are NOT stuck with it. That was my whole point.
                          Yes I am. I'm following the OM's original system, and anyway there isn't room to mesh the 127 with the 135.
                          '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

                          Comment


                          • #14
                            Originally posted by John Stevenson View Post
                            Bollocks.
                            Thats what I thought John, but I thought I'd let you say it first, you do it so much better.
                            '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

                            Comment


                            • #15
                              Originally posted by Richard P Wilson View Post
                              Thats what I thought John, but I thought I'd let you say it first, you do it so much better.
                              Thanks. I was hoping someone else would take that one. :-)
                              ...lew...

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