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Question: HSM Article: A Micrometer Height Gauge

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  • Question: HSM Article: A Micrometer Height Gauge

    I am in the process of reading this article in the May/June 2018, Home Shop Machinist and have come on a statement that I can not understand. While describing the machining of what the author calls a "Quick Acting Nut" he is talking about making a threaded hole which goes from side to side in a cylindrical shaped part. At this point there is already a 6mm, drilled hole in the part and this 6mm hole is centered in the lathe, being held with a four jaw chuck prior to boring it out to 10mm which will be the minor diameter of the Acme thread.

    After centering it, boring it to 10mm, and counterboring both ends of the hole to the OD of the threads with the (Acme?) threading tool he says,

    "Because the tool used to form the counterbore has the flank angle of the thread it will be found less likely for a burr to be thrown up at the start of the thread, but if this does occur then the counterbore index readings can quickly remove them." (Italics are mine.)

    Apparently this is not a strict counterbore, with a flat bottom: the Acme threading tool would leave a 30 degree angle that matches the flank of the thread it will cut next in the process.

    What I do not understand is the phrase "counterbore index readings". Just what is a counterbore index reading and how does it remove a burr? Is this a typo? Or what?
    Paul A.
    SE Texas

    Make it fit.
    You can't win and there is a penalty for trying!

  • #2
    Paul,

    The answer to your question lies in the preceding paragraph.

    Because the tapped hole is perpendicular to the cylinder, the author has carved a shallow counterbore, or spotface. He cut it with the threading tool, so, yes, it won’t have a flat bottom, but instead will have an angled bottom matching the Acme thread form. It’s because the floor of this counterbore matches the angle of the thread that there is less chance of the burr being left from the threading process.

    Graham cut the counterbore deep enough to cut fully into the cylinder and cut to an OD slightly larger than the OD of the internal threads. In the preceding paragraph he says to pay attention to the distance traveled in making the cut so you can repeat it exactly on the other side.

    When he mentions that the counterbore index readings can correct a problem, he means that adjustments to the final depth and OD positions used for the c’bore initially can fix things. While not necessarily a typo, the wording is a little muddy and I should have caught and fixed this section. A few words would have clarified things.
    George
    Traverse City, MI

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    • #3
      Thanks George.
      At the end of the project, there is a profound difference between spare parts and extra parts.

      Comment


      • #4
        Yes, thanks George.

        See, some people really do read your magazine.
        Paul A.
        SE Texas

        Make it fit.
        You can't win and there is a penalty for trying!

        Comment

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