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Gears hobbed with ISO bolts

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  • #16
    it's funny, that Robint thread lead to ncf posting about Ivan Law's Gears and Gear Cutting book which I then bought off Amazon for not much money and is now resting on the couch beside me. So I can actually now understand a little of what you're all saying So far it's very well written and understandable, but I haven't yet gotten to the meaty bits.

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    • #17
      Originally posted by mattthemuppet View Post
      it's funny, that Robint thread lead to ncf posting about Ivan Law's Gears and Gear Cutting book which I then bought off Amazon for not much money and is now resting on the couch beside me. So I can actually now understand a little of what you're all saying So far it's very well written and understandable, but I haven't yet gotten to the meaty bits.
      It'll take a while to all sink in, trust me. I've been through it a few times already and its a bit humbling to read. VERY informative, though.

      Spoiler alert: after all the calculation, he walks you through how to make your own involute form cutters, and back off the relief angles. Looks just like the commercial cutters. One watchmaker demonstrates a bit on youtube: https://www.youtube.com/results?sear...=eureka+device
      Last edited by nickel-city-fab; 10-15-2021, 08:50 PM.
      25 miles north of Buffalo NY, USA

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      • #18
        Have you read the chapter on how to make the 'eureka' device for form relieving your own home made cutters? I'd love to see one working. Its a marvellous book, I successfully made a number of bevel gears following his instructions.
        I bought stuff for a Dore Westbury milling machine off him when he ran MES near Sheffield, must be 30 years ago now. He is a genuinely nice guy.
        '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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        • #19
          Involute teeth do not rub? I had thought that, but when I expressed it on this very BB some years ago, I was told that they do rub.

          But now you have me thinking about that again.

          Starting with two involute gears with the same number of teeth. And they are perfectly spaced. Yes, a pair of teeth do only contact each other at a single point (OK a line) on each tooth. But is there no rubbing? With the same number of teeth, the involute profile on each of the teeth should be the same. And contact starts with a point near the OD on one tooth in contact with a point below the pitch circle on the other. When the point of contact is at the line between the two axes of the gears, the points of contact will be at the pitch circles of both gears. Then as they continue to rotate, the points of contact will be the reverse of the initial points when they are at the position of last contact. So, the distance from initial contact to final contact for both gears will be the same.

          But that does not mean that they did not rub. The involute curve is not one of constant curvature (radius). The curvature changes from one point on the involute to the next. The high side of the tooth (above the pitch circle) will have longer radii than the low side (below the pitch circle). And this implies that the high side of the tooth will be shorter than the low side, which will be longer. So, with different distances traveled by the point of contact for the first half of the contact path, there MUST be some rubbing. And the same argument also applies for the second half of the contact path, but the two teeth change roles as the points of contact also change from the upper to lower halves of the tooth face respectively.

          Overall, the total, relative travel is zero, but one gear gets ahead for the first half of the travel path and then slows down, relatively, during the second half to be back to a relative zero by the time contact is lost. And the second gear experiences the opposite; first slower and then faster.

          If this is true even for the exact same involute curve of gears with the same tooth count, then it is even more true (more rubbing) when different tooth counts produce involutes that are not equal to begin. The involute on a gear with a small tooth count will have a collection of radii that are completely obvious to the naked eye while the involute on a gear with a large number of teeth will be almost impossible to be distinguished from a straight line even with instruments. Those curved involutes are quite obviously longer than the almost straight ones and so there must be a lot of rubbing when they are in mesh.

          NO, involute gear teeth do not just roll, without any rubbing. With involute tooth form gears, there is always some rubbing except when two racks are in mesh with each other. And this is why a good lubricant is important in any gear train that uses involute gears.



          Originally posted by skunkworks View Post
          There is efficiency and there is rubbing... With and involute gear profile - the teeth roll through one another and there shouldn't be any actual rubbing.. Just point contact. Non involute profiles will wear faster. (if I understand it right.)
          Paul A.
          SE Texas

          And if you look REAL close at an analog signal,
          You will find that it has discrete steps.

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          • #20
            So there was drama on this forum that I had absolutely no part in? Darn, I missed it...
            So who is the sicko that suggested to him to post over on PM? Your sick sense of humor deserves praise.
            Last edited by RB211; 10-16-2021, 12:55 AM.

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            • #21
              I've thought about a relieving device, but then, I thought "why make that?

              You do not need to make all the motion work that generates the relief. All you really need is to finid ONE relieved cutter, and use a follower arm type mechanism to duplicate that relief on other parts. Superimpose the form on the relieving motion, and there you are. You have to do that part anyway.

              I have not done either method, but I can see that using a follower is bound to be a lot simpler than all the monkey motion to generate the same thing that is already present on existing cutters, waiting to be used.
              2730

              Keep eye on ball.
              Hashim Khan

              Everything not impossible is compulsory

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              • #22
                I will openly admit my ADHD and mathematical lack of ability gets in my way to wrap my head around all of this. But like that Helicron example if you made a hob (looks like a series of plunged vees as opposed to a thread ,or am I mistaken) and you had indexing capability can one produce gears over the entire range to generate a workable gear like an 18 tpi for example that could work in a southbend.
                The gears he makes seems to be aimed for much smaller use like clockwork. It is expensive in a third world country to buy gearcutters ,especially eight of them just for one range. Its also difficult to grind a flycutting bit without good eyes and some decent grinding equipment. It would really solve alot of problems .

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                • #23
                  Unfortunately, the hob is a thread, not as you hoped, a series of plunged vees. To make matters worse, though not insurmounable, if you want to produce a gear of a known DP, such as 18tpi, the pitch of the hob thread will be a very odd one, not a nice round number. The method shown, using bolts is OK if you only want a pair of gears that will mesh with each other, but don't have to meash with any existing ones.
                  '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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                  • #24
                    Originally posted by Richard P Wilson View Post
                    ...I'd love to see one working.
                    https://www.youtube.com/watch?v=F_w92CEMlT4
                    https://www.youtube.com/watch?v=U_kqi3dqr50

                    Concept extended to relieve a hob: https://www.youtube.com/watch?v=kJ8kyC_bpHs

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                    • #25
                      Originally posted by Paul Alciatore View Post
                      Involute teeth do not rub? I had thought that, but when I expressed it on this very BB some years ago, I was told that they do rub.

                      But now you have me thinking about that again.
                      ...

                      But that does not mean that they did not rub. The involute curve is not one of constant curvature (radius). The curvature changes from one point on the involute to the next. The high side of the tooth (above the pitch circle) will have longer radii than the low side (below the pitch circle). And this implies that the high side of the tooth will be shorter than the low side, which will be longer. So, with different distances traveled by the point of contact for the first half of the contact path, there MUST be some rubbing. And the same argument also applies for the second half of the contact path, but the two teeth change roles as the points of contact also change from the upper to lower halves of the tooth face respectively.
                      It is true that involute curves do not have constant curvature. And it is also true when two gears mesh the location of the contact point changes quicker above and below the pitch circle in order to keep that contact point - which is always perpendicular to the surface of the tooth - along a fixed pressure line. Also true is that the two involutes are rotating about different points, and the speed of the contact points along the involute curves change as the contact point moves along the radius - just enough to keep up with the changing differences in length.

                      This is the entire point of using the involute for gear teeth. One of the advantages (there are others) of using this specific mathematical curve for the flanks of gear teeth is that the location and speed along the tooth flanks are exactly the same for any two mating gears made with the same pressure angle (which determines the specific starting point and shape of the involute) and module (or diametral pitch). Mathematically, there is no relative motion along the involute tooth flanks of two mating gears, all rolling, no sliding.
                      ...

                      Originally posted by Paul Alciatore View Post
                      NO, involute gear teeth do not just roll, without any rubbing. With involute tooth form gears, there is always some rubbing except when two racks are in mesh with each other. And this is why a good lubricant is important in any gear train that uses involute gears.
                      Mathematically, involute teeth roll on each other without sliding. This means the gears must have perfect involutes and be mounted precisely so their pitch circles roll on each other. However, this is an ideal and difficult, if not impossible, to pull off in practice. First of all, there has to be some backlash (radial clearance between the pitch circles) in a gear set for it to function. The more precisely the teeth and gears are formed, the less backlash is needed. Involute curves are not particularly difficult to generate, but generating many identical copies of a specific involute shape located regularly and precisely along a curved surface (especially the pitch circle of a gear, which is clearly defined but cannot be seen or measured) is a challenge. Fortunately, another property of involute gear teeth is that they are remarkably tolerant of small deviations from the exact shape (but not spacing) and increases in center-center distance while still maintaining most of their rolling-without-sliding and constant pressure line properties.

                      So yes... since no actual gear set can be made absolutely perfectly, all actual gear teeth, regardless of shape, slide with respect to each other to some extent and could use some lubrication. That said, practical, real-world involute gears, depending on their quality and mounting details, slide on each other far less than other tooth forms.

                      SE MI, USA

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                      • #26
                        Originally posted by mattthemuppet View Post
                        it's funny, that Robint thread lead to ncf posting about Ivan Law's Gears and Gear Cutting book which I then bought off Amazon for not much money and is now resting on the couch beside me. So I can actually now understand a little of what you're all saying So far it's very well written and understandable, but I haven't yet gotten to the meaty bits.
                        When you get through Law's book for the basics, read things like this before you try free-hobbing.
                        https://gearsolutions.com/features/a...-gears-part-i/

                        I'm guessing Robint didn't bother.

                        SE MI, USA

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                        • #27
                          Originally posted by plunger View Post
                          I will openly admit my ADHD and mathematical lack of ability gets in my way to wrap my head around all of this. But like that Helicron example if you made a hob (looks like a series of plunged vees as opposed to a thread ,or am I mistaken)
                          What the helicon site shows isn't a hob - a hob is the rack profile wrapped into a helix. A thread of sorts, except the pitch is the DP or module of the gear. you can however use the Helicon technique to make a gear that will work for light duty, low speed applications.

                          in Toronto Ontario - where are you?

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                          • #28
                            Originally posted by plunger View Post
                            I will openly admit my ADHD and mathematical lack of ability gets in my way to wrap my head around all of this. But like that Helicron example if you made a hob (looks like a series of plunged vees as opposed to a thread ,or am I mistaken) and you had indexing capability can one produce gears over the entire range to generate a workable gear like an 18 tpi for example that could work in a southbend.
                            Yes, this can work, but the cutter isn't a "hob." Hobs specifically have helical cutting teeth that cut the spaces between multiple gear teeth at a time.

                            One way to make involute gear teeth is with a rack. The rack has straight-sided teeth, and if they are at the proper (pressure) angle and have the proper dimensions, one can move the rack across the face of the gear to make a cut. The rack is then advanced and the gear blank rotated in successive increments to generate the teeth. The teeth made thus will have facets; the smaller the increments the more precisely the involutes are formed. If the gear is rotated X degrees between cuts, the rack has to be advanced a corresponding distance so that its pitch line rolls on the gear's pitch circle:

                            rack move = X * (pi/180) * (Dp/2)

                            where Dp is the diameter of the pitch circle of the gear that is being generated. Naturally this takes a while for small steps (and many small facets on the gear faces), but the geometry of a rack is simple and can be used for any size gear with the same module (or diametral pitch) and pressure angle.

                            (This is exactly how I generated the ISO-bolt teeth in the original post in this thread, but in CAD instead of on a shaper)

                            Click image for larger version  Name:	Gear1.JPG Views:	2 Size:	13.8 KB ID:	1966192

                            From here, it's not hard to replace the rack with a milling cutter. (This is the method described here: http://www.helicron.net/workshop/gearcutting/) The parallel cutting edges of the milling cutter are exactly the same shape as the rack. Such a milling cutter is straightforward to make in a home shop, and involves turning the cutter blank, milling and relieving the cutting teeth, and hardening the cutter. If properly made, it is good for gears of all numbers of teeth given the same module (or diametral pitch) and pressure angle.

                            In use, cutter is passed along the face of the gear to make a cut, then the gear blank is rotated and the cutter advanced along its length for the next cut, exactly like the rack above. Since the cutter will have a finite length, it's necessary to move it backwards once in a while to stay on the teeth. For example, on every 5th rotation of the gear blank one would the milling cutter backwards 4 times the "rack move" above.

                            Click image for larger version  Name:	Gear2.JPG Views:	2 Size:	17.9 KB ID:	1966193

                            This takes some time, but this method can and has given excellent results, and can produce gears that to AGMA standards in every way, to those who are patient and careful, with tools and techniques that are found in home shops.
                            Last edited by DrMike; 10-16-2021, 09:41 AM.
                            SE MI, USA

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                            • #29
                              Originally posted by Richard P Wilson View Post
                              Have you read the chapter on how to make the 'eureka' device for form relieving your own home made cutters? I'd love to see one working. Its a marvellous book, I successfully made a number of bevel gears following his instructions.
                              I bought stuff for a Dore Westbury milling machine off him when he ran MES near Sheffield, must be 30 years ago now. He is a genuinely nice guy.
                              Check the videos that I linked above, one fellow made it and shows it in operation.
                              25 miles north of Buffalo NY, USA

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                              • #30
                                Originally posted by RB211 View Post
                                So there was drama on this forum that I had absolutely no part in? Darn, I missed it...
                                So who is the sicko that suggested to him to post over on PM? Your sick sense of humor deserves praise.
                                That was me
                                I was hoping that Zahnrad Kopf would set him straight over there, since he *owns* a gear manufactory.
                                25 miles north of Buffalo NY, USA

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