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Hobbing Question

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  • Hobbing Question


    I am trying to hobb the "teeth" on a bolt for a pinch wheel drive and I am currently failing miserably.

    Here is a link to what I am trying to do. The section on the bolt starts about 1/3 of the way down the page.

    I cut a groove in the bolt, without a problem, on a lathe. The problem is that I can't get the teeth to be cut correctly.

    I machined a holder up to hold the bolt on the table of a small horizontal milling machine. The tap is installed in the spindle.

    What is happening is that most of the teeth are cut fine. But, there is always a short area where the teeth are completely missing. I guess you could say that they are milled off by the tap.

    Any ideas?

    Thanks in advance,

    There are only 10 types of people in the world: Those who understand binary and those who don't.

  • #2
    If it's not a concentricity issue where the tap is not touching in one area, it sounds like the bolt isn't able to rotate freely so that the drag in one spot compromises the timing of rotation between the tap and bolt. (If I'm understanding the process at all)
    "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill


    • #3
      I guess maybe I don't understand what you're doing. You need to get the diameter of the bolt correct to cut an integer number of teeth. For instance, if you want 13 teeth cut with a 1/2-13 tap, then you need a circumfrence of 1" or a diameter of ~.318, otherwise you'll "mill" off the existing teeth on the second rotation of the blank.