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Gears hobbed with ISO bolts

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  • #31
    Originally posted by DrMike View Post

    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.
    Would it be fair to say that this would be a similar process to gear shaping, but done with a rotary motion? A gear shaper uses a round cutter that is indexed simultaneously with the gear blank, but in your method it's more like rolling the gear along the pitch circle along a rack? You eliminate the need to drive the gear and the cutter simultaneously through the use of careful indexing and cutter positioning to still give a correct involute profile?
    Cayuga, Ontario, Canada

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    • #32
      Originally posted by DrMike View Post
      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.
      Given that you need to gash the blank for free hobbing and I have a dividing head, I might as well cut the worm gear with the dividing head. That project is a fair ways off though

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      • #33
        Originally posted by Tom S View Post
        Would it be fair to say that this would be a similar process to gear shaping, but done with a rotary motion? A gear shaper uses a round cutter that is indexed simultaneously with the gear blank, but in your method it's more like rolling the gear along the pitch circle along a rack? You eliminate the need to drive the gear and the cutter simultaneously through the use of careful indexing and cutter positioning to still give a correct involute profile?
        Yes.
        I didn't come up with this method, but yes.
        Yes.

        SE MI, USA

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        • #34
          Originally posted by Tom S View Post

          Would it be fair to say that this would be a similar process to gear shaping, but done with a rotary motion? A gear shaper uses a round cutter that is indexed simultaneously with the gear blank, but in your method it's more like rolling the gear along the pitch circle along a rack? You eliminate the need to drive the gear and the cutter simultaneously through the use of careful indexing and cutter positioning to still give a correct involute profile?
          It is the same principle as the Sunderland gear shaper, but turned into a milling cutter.

          You do the indexing. You index a fractional tooth at a time, but you have to index both ways, longitudinally and rotationally, to maintain the relation that the hobbing machine does automatically.

          It's actually a lot more trouble than using a formed cutter, but if you do everything perfectly, you MAY make a better gear. And you do not need all 8 cutters.

          The indexing generates flats, and the succession of flats approximates the tooth form, "generating" it just as the hob does. The number of indexing steps per tooth controls the number of flats, and the resulting gear accuracy.

          A hobbing machine does that automatically, as successive teeth pass down the toothspace and the gear train continuously "indexes" the blank. There are all sorts of details to an actual hob, such as multiple passes, "hunting teeth", etc to reduce the number of flats, etc.

          With the "milling shaper cutter" you have to handle everything, including the very precise indexing that the hobbing machine does through gearing.

          "Hobbing" is actually just "roughing" the gear to shape. Hobbyists tend to think of hobbing as making good gears, and it makes "OK " gears. But if better gears are wanted, then grinding, crossed axis gear shaving, or the like are used after hobbing to rough shape.
          2730

          Keep eye on ball.
          Hashim Khan

          Everything not impossible is compulsory

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          • #35
            I did some zero lead hobbing and posted photos here: https://bbs.homeshopmachinist.net/fo...-making-a-gear
            There’s a short video of the gears all running here: https://twitter.com/solutionsbydave/...533014017?s=21

            Dave
            Just south of Sudspumpwater UK

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            • #36
              Looks like the knives have come out over on PM. Probably the last we'll hear of Robint.
              It's all mind over matter.
              If you don't mind, it don't matter.

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              • #37
                It seems to me that if you can make your own homemade hob and if you could get your mill to sink with a homemade indexer this would be a great way to make your own gears cheaply and be alot of fun. Thats what homeshop machining is all about ,isnt it.
                How hard is it too sink your mill to a homemade indexer or rotary table. ?
                This guy seems to know what he is doing.
                Im not clever enough to do it. But I would love to make one.
                https://www.youtube.com/watch?v=7WleHVtIc1c

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                • #38
                  Originally posted by plunger View Post
                  It seems to me that if you can make your own homemade hob and if you could get your mill to sink with a homemade indexer this would be a great way to make your own gears cheaply and be alot of fun. Thats what homeshop machining is all about ,isnt it.
                  How hard is it too sink your mill to a homemade indexer or rotary table. ?
                  This guy seems to know what he is doing.
                  Im not clever enough to do it. But I would love to make one.
                  https://www.youtube.com/watch?v=7WleHVtIc1c
                  Its been done by a couple of members here, but sadly it seems to have a have high mortality rate...the two I recall doing so are the late John Stevenson and Texas Tornado. You can make your own hobs, but imo its hard to justify in they aren't that much money on ebay (given the complexity of making them) and it would be hard to get a DIY version as nice as a commercial one. Even if you went to the time and trouble of making a relieving tool (e.g. Eureka) its not HSS and its not ground after hardening. otoh, given where you are, shipping might make rolling your own more appealing, so for sure many people have made their own hobs.

                  Here's a video of John's set up and some photos I kept. Note he's using a universal mill (means the table swivels). In the photos he's cutting helical gears so has the table set over the to hob's helix, and then the work on the table at the helical gear's angle

                  You can do the same on a vertical mill, albeit less rigid, by tiling the head to the hob's helix angle.

                  https://www.youtube.com/watch?v=PvSa6BO9wGQ





                  Last edited by Mcgyver; 10-17-2021, 01:14 PM.
                  in Toronto Ontario - where are you?

                  Comment


                  • #39
                    Originally posted by plunger View Post
                    It seems to me that if you can make your own homemade hob and if you could get your mill to sink with a homemade indexer this would be a great way to make your own gears cheaply and be alot of fun. Thats what homeshop machining is all about ,isnt it.
                    How hard is it too sink your mill to a homemade indexer or rotary table. ?
                    This guy seems to know what he is doing.
                    Im not clever enough to do it. But I would love to make one.
                    https://www.youtube.com/watch?v=7WleHVtIc1c
                    As far as I can see there is no need for an indexer or rotary table for use with a hob as the action is continuous so you just need a ratio between hob to gear blank spindle. This ratio would be the same as the number of teeth on the gear being created. So, for a 32 tooth gear you need the hob to turn 32 times for one rotation of the gear blank which may be difficult if you do not have a selection of gears on hand!

                    Meanwhile, I am trying to think of an easier way.

                    John

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                    • #40
                      Originally posted by The Artful Bodger View Post
                      which may be difficult if you do not have a selection of gears on hand!

                      Meanwhile, I am trying to think of an easier way.
                      Of course I knew what I meant, but how would you if I don't say? with the JS and TT examples, what I was getting at but neglected to note was that sync'ing is done electronically. Encoder on the spindle, stepper on the 4th axix.

                      EDIT: In days of old sync'ing was done between the spindle and X axis for cutting spirals, but it would a be a challenge to gear the spindle and 4th axis. For one thing they moving (relatively) and are at an angle. I went looking in some ancient machining books, but all I find is spirals, no hobbing via mechanical synchronization. With electronics all that bother and complexity is done away with. This, plus the ability to tilt the mill head is what makes hobbing so much more accessible now for home shops, vs needing a hobbing machine

                      A walk in the park for a skilled program such as yourself!
                      Last edited by Mcgyver; 10-17-2021, 05:36 PM.
                      in Toronto Ontario - where are you?

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                      • #41
                        Originally posted by The Artful Bodger View Post

                        As far as I can see there is no need for an indexer or rotary table for use with a hob as the action is continuous so you just need a ratio between hob to gear blank spindle. This ratio would be the same as the number of teeth on the gear being created. So, for a 32 tooth gear you need the hob to turn 32 times for one rotation of the gear blank which may be difficult if you do not have a selection of gears on hand!

                        Meanwhile, I am trying to think of an easier way.

                        John
                        So if one could attach an encoder to the mill spindle and use it to drive the gear holding device, be it an indexer ,rotary table or purpose made gear holder then this is pretty doable for a home shop environment. ? And if one makes a hob or buys one then you could cut the entire range of gear tooth counts for that specific pressure angle and dp. ? Am I right in saying this. I'm referring to spur gears only?

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                        • #42
                          Originally posted by The Artful Bodger View Post

                          As far as I can see there is no need for an indexer or rotary table for use with a hob as the action is continuous so you just need a ratio between hob to gear blank spindle. This ratio would be the same as the number of teeth on the gear being created. So, for a 32 tooth gear you need the hob to turn 32 times for one rotation of the gear blank which may be difficult if you do not have a selection of gears on hand!

                          Meanwhile, I am trying to think of an easier way.

                          John
                          As far as I know, that is how the commercial machines actually work. The older (Fellowes Gear Shaper) machines used something like a shaper but they still indexed the gear blank and the cutter together
                          25 miles north of Buffalo NY, USA

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                          • #43
                            John Stevenson’s hobber was (iirc) an old victoria horizontal mill with an encoder on the spindle which drove a pulse train into a set of simple divide logic gates that powered a stepper to rotate a dividing head.
                            There is a write up somewhere on the internet in one of the many model engineering forums - not sure which one.
                            Dave
                            Just south of Sudspumpwater UK

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                            • #44
                              Originally posted by small.planes View Post
                              John Stevenson’s hobber was (iirc) an old victoria horizontal mill with an encoder on the spindle which drove a pulse train into a set of simple divide logic gates that powered a stepper to rotate a dividing head.
                              There is a write up somewhere on the internet in one of the many model engineering forums - not sure which one.
                              Dave
                              Sr John described (one of) his system over on MadModders https://www.madmodder.net/index.php/...html#msg131908

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                              • #45
                                Do you have old Fisher and Paykel washing machines in South Africa? You should collect a few with 'smartdrive' motors.

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