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Indexing a large prime number, 127 for example.

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  • Indexing a large prime number, 127 for example.

    I have never tried to index 127 on a dividing head but I understand it is a compound process requiring considerable concentration to do the job however for most home shop jobs there are surely easier ways and not all home shops have dividing heads.

    For example fit a stepper motor to your dividing head and learn new skills which is always a good thing.

    You could carefully plot the 127 steps on a plywood disk and work from that.

    Another way might be to borrow a 127 gear and latch index directly from it.

    But easiest of all may be to make a plastic gear and use that to latch index.

    http://madmodder.net/index.php/topic....html#msg37410
    Last edited by The Artful Bodger; 10-28-2017, 03:30 PM.

  • #2
    Originally posted by The Artful Bodger View Post
    I have never tried to index 127 on a dividing head but I understand it is a compound process requiring considerable concentration to do the job however for most home shop jobs there are surely easier ways and not all home shops have dividing heads.

    For example fit a stepper motor to your dividing head and learn new skills which is always a good thing.

    You could carefully plot the 127 steps on a plywood disk and work from that.

    Another way might be to borrow a 127 gear and latch index directly from it.

    But easiest of all may be to make a plastic gear and use that to latch index.

    http://madmodder.net/index.php/topic....html#msg37410
    Just use a dividing head with a graduated handwheel in place of the plate. Print a table of cumulative positions to work from, and index manually to each position. This avoids any compounding error, limiting your error to the precision allowed by your particular dividing head. My last dividing head didn't even have plates, I used this process every time. It was 120:1 ratio and had 90 graduations on the handwheel. It was dead easy to approximate half way between the graduations too. If the rest of your setup is capable of greater accuracy than you can approximate this way (doubtful for many of us, I think) then add a vernier scale to the handwheel.

    Sent from my BLN-L24 using Tapatalk
    Max
    http://joyofprecision.com/

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    • #3
      True, true, but not all home shops have a dividing head. Nonetheless I now have a graduated disk for my dividing head on the to do list!

      Comment


      • #4
        Does the fancy method involve taking the degrees per step and multiplying it up until it forms a whole number? Or a number that is close enough to a whole number to make do?

        For example the degrees per step for a 127 tooth gear would be 2.8346456*. If we multiply that by 7 we get 17.007874. That's pretty darn close, no? So if we index by 17 degrees per step and go around the gear cutting teeth for a total of 127 times we should spin the blank by a total of 7 turns and end up with 127 teeth.

        What about the 0.00787 per tooth we are ignoring? It's going to add up over all the cutting by a total of 127 times. Which makes it.... wait for it.... ONE whole tooth ! ! ! ! So that would suggest that we can do this 17* stepping and we should end up with just one tooth that needs a little extra help. Like form the last but one tooth then before we unlock the table arrange an indexing pin and engage a formed tooth. Then lift it away, unlock the table from the handwheel and index it over by one of the already formed teeth for cutting the last tooth.

        Or did I miss something in all the math?
        Chilliwack BC, Canada

        Comment


        • #5
          Got a DRO with the bolt circle feature? Just drill out a new plate on the mill.

          Comment


          • #6
            Or you could just buy one for less than a $100 bill. I know you guys spent more than that on a QC tool post. If it's a permanent fixture to your lathe and makes life easier, why diddle around with plastic? What's going to happen when that plastic gear is running in the drive train?

            Four reasons to just buy it.
            It's less than a Universal Dividing Head.
            It's less than a Dividing Head with controller and stepper.
            It's less than buying a plastic printer and software.
            It's made out of cast iron.

            https://www.amazon.com/Boston-Gear-G.../dp/B004N62UT6

            Comment


            • #7
              A piece of wood, plus paper and pencil will make an indexer.

              Wood cut as a wheel as much larger than your gear as you can make it.

              Measure around the wood wheel. Divide that distance by 127, and put marks at the resulting distances on the paper. Wrap paper around big wood wheel.

              Mount wheel and gear blank on a shaft, set up to turn in place on the mill. Put on a pointer and some means of holding the wheel/shaft locked.

              Turn wheel to a mark. lock and cut toothspace. Move to next mark, lock and cut.... repeat all around until all are cut.
              4357 2773 5647 3671 3645 0087 1276

              CNC machines only go through the motions

              "There's no pleasing these serpents"......Lewis Carroll

              Comment


              • #8
                My thinking was that he would use the plastic item with some sort of 127 divisions as the master for making whatever it is he needed. Also the 127 divisions wasn't identified as a gear but just an example of a prime number that would not go at all easily into a rotary table without dividing plates or a head.

                But picking THAT number out of thin air does make one wonder, doesn't it?
                Chilliwack BC, Canada

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                • #9
                  I think I did indicate that the plastic gear would be used as an index.

                  Comment


                  • #10
                    Originally posted by J Tiers View Post
                    A piece of wood, plus paper and pencil will make an indexer.

                    Wood cut as a wheel as much larger than your gear as you can make it.

                    Measure around the wood wheel. Divide that distance by 127, and put marks at the resulting distances on the paper. Wrap paper around big wood wheel.

                    Mount wheel and gear blank on a shaft, set up to turn in place on the mill. Put on a pointer and some means of holding the wheel/shaft locked.

                    Turn wheel to a mark. lock and cut toothspace. Move to next mark, lock and cut.... repeat all around until all are cut.
                    I like this idea -- in fact I do it this way occasionally. This may be the rare instance where circumference is your friend.

                    You could even mark out 127 equal divisions on a piece of elastic band from a fabric shop and stretch it to fit around the waistline of whatever object to be divided.
                    25 miles north of Buffalo NY, USA

                    Comment


                    • #11
                      Originally posted by Illinoyance View Post
                      Got a DRO with the bolt circle feature? Just drill out a new plate on the mill.

                      I did that several years ago. Worked just fine.
                      Gary Davison
                      Tarkio, Mo.

                      Comment


                      • #12
                        This falls under "shade tree mechanics."
                        Get an old wood cutting band saw blade. A 1/2" x 3 tpi is a good size. Cut a section having either 128 or 255 teeth. Now cut a disc out of 3/4" plywood slightly larger than the calculated size that will accept the strip of saw blade around the perimeter. Chuck the disc on a mandrel and adjust its size until the blade JUST overlaps by one tooth. Fasten the blade with many little brads set in the gullets of the teeth. Cobble up an indexing "finger" to follow the circumference of the disc. I THINK that you now have your indexer and it will be accurate enough for "government work."
                        Duffy, Gatineau, Quebec

                        Comment


                        • #13
                          Originally posted by Duffy View Post
                          This falls under "shade tree mechanics."
                          Get an old wood cutting band saw blade. A 1/2" x 3 tpi is a good size. Cut a section having either 128 or 255 teeth. Now cut a disc out of 3/4" plywood slightly larger than the calculated size that will accept the strip of saw blade around the perimeter. Chuck the disc on a mandrel and adjust its size until the blade JUST overlaps by one tooth. Fasten the blade with many little brads set in the gullets of the teeth. Cobble up an indexing "finger" to follow the circumference of the disc. I THINK that you now have your indexer and it will be accurate enough for "government work."
                          You beat me to it. I have the same suggestion except using perforated steel strap, the kind you earthquake-proof your hot water heater with. Might not be perfect but I bet it's plenty "close enough".

                          metalmagpie

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                          • #14
                            The name escapes me at the moment, but several years ago a contributor to HSM magazine who had submitted several articles on gear cutting described a method of making temporary change gears of any tooth count.

                            He ran a length of strapping steel through the lathe's gear train, producing a corrugated ribbon of the approximate tooth form. He wrapped this around a plywood blank of the diameter needed for the desired tooth count and fastened it in place. The voids in the resulting "teeth" were filled with bondo. While not for sustained use, I see no reason it would not work for a few uses, or as a master for simple indexing.
                            Last edited by JCHannum; 10-28-2017, 07:30 PM.
                            Jim H.

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                            • #15
                              Try compound indexing
                              Simple to do with BS-0 this will indexing any number to 250

                              Dave


                              Originally posted by The Artful Bodger View Post
                              I have never tried to index 127 on a dividing head but I understand it is a compound process requiring considerable concentration to do the job however for most home shop jobs there are surely easier ways and not all home shops have dividing heads.

                              For example fit a stepper motor to your dividing head and learn new skills which is always a good thing.

                              You could carefully plot the 127 steps on a plywood disk and work from that.

                              Another way might be to borrow a 127 gear and latch index directly from it.

                              But easiest of all may be to make a plastic gear and use that to latch index.

                              http://madmodder.net/index.php/topic....html#msg37410

                              Comment

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