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Minor diameter - what is proper way to measure ?

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  • Minor diameter - what is proper way to measure ?

    What is the proper tool/method for measuring the minor diameter of a screw sample ? Putting any measuring device into the threads ( trough to trough) results in skewed position, as threads are never 180 degrees apart. Is that good enough? Is it better to measure from trough to opposed crest and then subtracting from the major diameter ?

    Thanks,
    Gary


    Appearance is Everything...

  • #2
    Screw thread micrometer with the proper anvils or thread wires. See: http://bbs.homeshopmachinist.net/thr...t=thread+wires

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    • #3
      Not to denigrate the question how to do it, but I never have; the first thing I always do is measure the OD and the pitch using a pitch gauge if necessary to determine 55 or 60 deg then refer to my thread tables.

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      • #4
        When I am cutting external threads on the lathe I cut a thread relief about .005 over the minor diameter. When the tip starts to scratch it I am close enough for what I do. Remember the threading tool is not completely sharp pointed.
        Byron Boucher
        Burnet, TX

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        • #5
          Originally posted by goose View Post
          Putting any measuring device into the threads ( trough to trough) results in skewed position, as threads are never 180 degrees apart. Is that good enough?
          I would think so. I'd have to check, but I seem to remember the tolerances for minor diameter (thread root diameter) are wide open in comparison to, for example, pitch diameter.
          Originally posted by goose View Post
          Is it better to measure from trough to opposed crest and then subtracting from the major diameter ?
          It would certainly be more accurate. Especially so if you made a number of measurements at various angles of rotation and averaged the result. Is it necessary? I would expect not unless you're involved a very high tolerance job in a professional capacity. Then, I expect, other more exotic instrumentation would be available to you. FWIW, an optical comparator is very versatile in making thread measurements but is out of the range of the average home machinist. I always cringe when I see people suggest their use --- do you really expect me to have one of those just sitting in the corner of my home shop?! C'mon...
          Last edited by Arthur.Marks; 10-11-2013, 03:08 PM.

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          • #6
            You could use a Toolmakers microscope or optical projector if you happen to have one. I can't really see why you would want to measure the minor diameter? The 3 wire method is a really accurate method of checking threads and not difficult to master.

            Tony

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            • #7
              Even though a measurement from trough to trough is not at 180 degrees, if you need an accurate number and can measure adjacent troughs accurately, you can calculate what the real number would be using trig. I'm also wondering why that would be needed in a real situation.
              .
              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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              • #8
                Originally posted by goose View Post
                What is the proper tool/method for measuring the minor diameter of a screw sample ? Putting any measuring device into the threads ( trough to trough) results in skewed position, as threads are never 180 degrees apart. Is that good enough? Is it better to measure from trough to opposed crest and then subtracting from the major diameter ?

                Thanks,
                Without "special" anvils for any mechanical measurment device, Optical methods will be the only way to measure the minor diameter.

                Wires and commercial thread anvils will only confirm pitch diameter.

                A thread could have a perfict pitch diameter, and if the threading tool had a misground tip geometry, the root, or minor diameter could be completely out of standards and specification.

                Visualize a "sharp V point" on a 8 tpi screw thread. The minor diameter would be much smaller than prescribed. (and still measure just fine over wires!)

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