Announcement

Collapse
No announcement yet.

4th Axis head for gear hobber

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • 4th Axis head for gear hobber

    I have a converted gearbox as a 4th axis on my gear hobber but when doing large gears I get unacceptable vibration markings on the teeth which i put down to the head not being robust enough.



    This is how it stands as of now.

    Somewhere in one of the sheds / parts warehouses / junk trap / lower 40 I have a big universal dividing head, not sure what make, it's ages since I have seen it.

    Off to find a rope to put round my waist and go looking .............post later.
    .

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




  • #2
    Why not send your apprentice?
    Best of luck John.
    Dave

    Comment


    • #3
      Originally posted by DFMiller
      Why not send your apprentice?
      Best of luck John.
      Dave
      "I lose more apprentices that way.."
      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

      Comment


      • #4
        Doing it now, not Monday.
        Well found a big Cincinnati dividing head weighs about two tonnes, Got some slop in it that won't adjust out so trying to strip it at the moment, what a brute of a badly designed unit, it's like doing keyhole surgery.

        Just come in for a coffee and if I can't get the worm out in the next 20 minutes it's going in the skip, I gave jigsaws up when I was 12
        .

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



        Comment


        • #5
          Originally posted by John Stevenson
          I have a converted gearbox as a 4th axis on my gear hobber but when doing large gears I get unacceptable vibration markings on the teeth which i put down to the head not being robust enough.



          This is how it stands as of now.

          Somewhere in one of the sheds / parts warehouses / junk trap / lower 40 I have a big universal dividing head, not sure what make, it's ages since I have seen it.

          Off to find a rope to put round my waist and go looking .............post later.
          John, "not robust enough" is probably not the cause. It is probably back lash in the gears that is causing the problem. A gear box is built to transmit power and motion in one direction only and there was probably a very generous spec on back lash.

          I have frequently wondered about back lash in any and all of the indexing heads or RTs used for making gears. It seems to me that if you want the best quality gears, you have to lock the rotation down for each tooth. A PITA, yes, but how else would you prevent the vibration and forces of cutting the teeth from allowing/forcing the indexing device from turning backwards while cutting? My RT has two locks placed at 180 degreese from each other. I would like to motorize them to allow faster manual operation and also operation via CNC.

          You might consider adding locks to your present gear box if you weren't hobbing. Hobbing seems to require a gearing system with low back lash or some other means of preventing counter rotation.
          Paul A.
          SE Texas

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

          Comment


          • #6
            Paul,
            It has as little backlash as possible.
            When I got the box I stripped it and bored the worm bearing housing out about 5mm oversize then fitted eccentric sleeves so I could get the worm as tight in as possible.

            Short of spending some serious cash on a zero backlash unit, read about 5K I have to put up with what i can get to use.

            Anyway got the big Cincinnati stripped and found the fault, the inner worm lock nut had come loose and allows the worm to float.
            As is it's a crap design as there is no way to adjust backlash between the central input gear and the worm gear.
            It relies on two deep addendum mangle gears to be jack of all trades.
            Knocked off for tonight, be having a rebuild tomorrow and see how I can get the drive straight onto the worm shaft and throw all the central shaft with division plates etc away. Already thrown the lock mechanism away so it can't rub or foul.

            It's got to do one job and one job only, it will never go back to a manual dividing head, sound cruel but it's a 12" head and they have no real value, too big for the home shop and industry doesn't want to know, I think I paid £20 for this one, can't remember it was so long ago.
            .

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



            Comment


            • #7
              John,
              So how does this story end?
              Did you beat it into submission?
              Thanks
              Dave

              Comment


              • #8
                Sorry took some pics but forgot to upload them.

                This is the head as it started out



                It's the full head, 5 1/2" centres with the tail attachment, just visible for helical gearing and the compound bit where you can move the plate.

                Seems a shame to gut it but these are not worth a lot here, too big for a home shop and industry doesn't want them, had it ages and think I paid £20 for it.

                Anyway managed to get it to pieces and checked out, very good inside, repacked the bearings and assembled it to the stage it's at now.




                All the bits not fitted are being thrown but it's a horrible design.
                because they want to use a central shaft for the input, they take the drive down to the worm by two mangle gears.
                Unfortunately because the worm housing is on an eccentric there is no adjustment and it's only correct in one position.

                In that position the worm could have backlash or if the worm is tight in mesh the gears could have backlash.

                State of play at the moment is I have got a piece of alloy 8" diameter x 3" long to make a cover plate so i can drive the end of the worm shaft via big stepper motor.
                The gear will probably go, it's just on there to hold everything together. It's gone onto back burner whilst I was waiting for the alloy but now other things have got in the way.

                I'd sooner get the slotter fitted to the POS Bridgy and I have the CVA lathe [ 10EE clone ] in bits for a set of spindle bearings.
                .

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



                Comment


                • #9
                  Originally posted by John Stevenson
                  Sorry took some pics but forgot to upload them.

                  All the bits not fitted are being thrown but it's a horrible design.
                  because they want to use a central shaft for the input, they take the drive down to the worm by two mangle gears.
                  Unfortunately because the worm housing is on an eccentric there is no adjustment and it's only correct in one position.

                  In that position the worm could have backlash or if the worm is tight in mesh the gears could have backlash.
                  Simple solution, Put your new input shaft on an eccentric bushing too.
                  (And the rest of the drive train! weee. j/k)
                  Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

                  Comment


                  • #10
                    John,
                    That is looking good.
                    I see you have your new lad perfecting his sweeping technique. I see far too much clean floor in the picture. ;-)
                    Thanks for the pictures John.
                    Dave

                    Comment


                    • #11
                      Originally posted by Paul Alciatore
                      John, "not robust enough" is probably not the cause. It is probably back lash in the gears that is causing the problem. A gear box is built to transmit power and motion in one direction only and there was probably a very generous spec on back lash.

                      I have frequently wondered about back lash in any and all of the indexing heads or RTs used for making gears. It seems to me that if you want the best quality gears, you have to lock the rotation down for each tooth. A PITA, yes, but how else would you prevent the vibration and forces of cutting the teeth from allowing/forcing the indexing device from turning backwards while cutting? My RT has two locks placed at 180 degreese from each other. I would like to motorize them to allow faster manual operation and also operation via CNC.

                      You might consider adding locks to your present gear box if you weren't hobbing. Hobbing seems to require a gearing system with low back lash or some other means of preventing counter rotation.
                      The solution to backlash I considered using consists of fitting a servo on the output shaft of the gearbox and running it in "torque" mode. Using the parameters in its controller, the torque applied to the shaft can be adjusted from 0 to 100% of full servo motor torque and in either direction - so it could be aiding or opposing the desired rotation direction.

                      The torque applied would tend to keep the gear teeth in contact on adjacent sides and should not have to be large, as the finishing cut can be small.

                      I have not persued this yet, as I was happy with the gears as cut with the gearbox I am using, rated for +/- 6 arc-min.
                      TexasTurnado

                      Comment


                      • #12
                        Originally posted by John Stevenson

                        Seems a shame to gut it but these are not worth a lot here, too big for a home shop and industry doesn't want them, had it ages and think I paid £20 for it.
                        Maybe you should send it on the slow boat west then. It seems to me prices for a US made mid sized dividing head here start ~$150 and go up from there. Why so many think they are so valuable I will never know, but they do.
                        "I am, and ever will be, a white-socks, pocket-protector, nerdy engineer -- born under the second law of thermodynamics, steeped in the steam tables, in love with free-body diagrams, transformed by Laplace, and propelled by compressible flow."

                        Comment

                        Working...
                        X