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  • Gear Cutting Hobs

    I always understood that one of the advantages of hobs for gear cutting was that you only needed one to do the whole range from pinion to rack, instead of a set of 8 involute cutters. Is this correct?
    The reason I ask is that I've got a number of 10mm bore hobs for .5 module and 1.0 module, but they are marked variously '21-25T', 26-34T,35-54T, 55 -134T, 135-RT. They are a quality make, not Far Eastern, unused and in their original packaging marked 'Horstmann Precision Gear Hob' The teeth do seem to vary in profile between hobs.

    As my project to construct a small hobber along the lines of the 'Jacobs' self build hobber progresses really slowly, if I mounted these on a suitable arbor, and set the vertical head of my mill over to the marked helix angle, is there any reason why I should not use these with my dividing head to cut modular gears in the 'normal' way, one tooth at a time?

    Richard
    '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
    A mix up in terminology ?

    This is a hob.



    This is a gear cutter. Top right


    ??
    .

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



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    • #3
      And a real hob will have

      1) a helical shape to the toothspace....like a thread

      2) tooth shape will be "rack" type, or "modified rack".

      The "hobs" you have might be the type cutters which have parallel rings of teeth, basically a multiple involute cutter. Never seen one, but I am told they exist.
      CNC machines only go through the motions

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      • #4
        John
        Very possibly a mix up in terminology, I know nothing about hobs, I've only ever cut gears with the 'normal' cutters. Anyway, what I have are as shown in your top left hand picture, 5 for 1.0 mod, and 5 for 0.5 mod. Do I need all 5 to hob gears from 22T to rack? Can I use them as conventional cutters mounted on an arbour if I set the vertical head to the helix angle?

        Richard
        '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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        • #5
          The tooth space is helical, the teeth are not in parallel rings, and to the naked eye the tooth shape looks rack shaped.

          Richard
          '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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          • #6
            Originally posted by Richard P Wilson View Post
            is there any reason why I should not use these with my dividing head to cut modular gears in the 'normal' way, one tooth at a time?

            Richard
            If the teeth are rack, you will not get the right gear tooth shape unless you use them in the normal way to "generate" the shape.

            "Generates" means that the cutter tooth takes all positions relative to the gear tooth as it engages and disengages, not just one single position. That way it cuts a tooth that will engage with a rack as a gear should, not just a toothspace that looks like a rack.
            CNC machines only go through the motions

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            • #7
              here is a good demo of conventional hobbing with a mini-mill

              http://www.youtube.com/watch?v=ZhICrb0Tbn4

              sam

              Originally posted by J Tiers View Post
              If the teeth are rack, you will not get the right gear tooth shape unless you use them in the normal way to "generate" the shape.

              "Generates" means that the cutter tooth takes all positions relative to the gear tooth as it engages and disengages, not just one single position. That way it cuts a tooth that will engage with a rack as a gear should, not just a toothspace that looks like a rack.

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              • #8
                Originally posted by Richard P Wilson View Post
                The tooth space is helical, the teeth are not in parallel rings, and to the naked eye the tooth shape looks rack shaped.

                Richard
                If the teeth are spiral then I think the cutters were designed for cutting worm wheels.

                Mike

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                • #9
                  Cycloid gear hobs.

                  Or?

                  Additional info & pics: http://bbs.homeshopmachinist.net/thr...Hobbing-a-gear

                  .
                  Last edited by Dr. Rob; 01-06-2014, 11:46 PM.

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