Can anyone explain "the demon" to me?

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  • Can anyone explain "the demon" to me?

    The other day, I had to run a program that a predecessor had written that checked the location of threaded holes. What they had done was to drop a probe into the center of the threaded hole, touch off N,S,E,&W, then raise the probe to a second level and do the same, creating the cylinder.

    I instantly wrote a new program. Number one, there's no way to get an accurate hole location off threads in that manner. Instead, I'll either scan the cylinder, using the appropriate pitch to match the thread, or thread a plug gage insert into the hole and scan the male shaft that extends above the hole. (I know there are gage sets that accomplish the same thing, but with the tolerances I'm generally looking at, my methods should suffice). Number two is "the demon". I've read in multiple places about measuring a cylinder with two planes of 4 touches each, and why it's frowned upon, as the consequences can be strange. I just wasn't too good at explaining to my boss why this is something to avoid, and why I didn't use an established program.

    Can one of you gurus lay it down in words they'll understand?

    I'm on a Global Performance 12.15.10, using PC-DMIS 2018 R2.

  • #2
    If the distance between the two levels is close to the diameter of the cylinder (minus tip diam), you can compare it to taking the eight hits on the corners of a cube - three equally possible cylinders at 90 degrees to each other!

    Three levels, or at least rotate the two levels relative to each other (CIR, CIR, construct CYL) will make it better.

    But, as you write, a threaded hole should be measured with pitch (or a thread gage)!
    AndersI
    SW support - Hexagon Metrology Nordic AB

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    • #3
      Can anyone explain to me why this thread's content is asking about how to explain a technical coordinate metrology issue (common to all CMM software) to a pointy-haired boss, but it's title is asking about explaining the PC-DMIS user community's love/hate nickname for this particular software?

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      • #4
        Originally posted by Ego Murphy View Post
        Can anyone explain to me why this thread's content is asking about how to explain a technical coordinate metrology issue (common to all CMM software) to a pointy-haired boss, but it's title is asking about explaining the PC-DMIS user community's love/hate nickname for this particular software?
        Why do you ask?

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        • #5
          Originally posted by AndersI View Post
          If the distance between the two levels is close to the diameter of the cylinder (minus tip diam), you can compare it to taking the eight hits on the corners of a cube - three equally possible cylinders at 90 degrees to each other!

          Three levels, or at least rotate the two levels relative to each other (CIR, CIR, construct CYL) will make it better.
          Thank you. That's a reasonable explanation. I think they might just understand that.

          Originally posted by AndersI View Post
          But, as you write, a threaded hole should be measured with pitch (or a thread gage)!
          God, I love vindication. Been arguing that with them for months...
          Last edited by Quality ish; 05-14-2019, 05:49 PM.

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          • #6
            But, as you write, a threaded hole should be measured with pitch (or a thread gage)!
            But a thread plug is at MMC and will move around. You'll get no repeatability with that.
            I prefer the tapered Tru-Pos plugs.

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            • Quality ish
              Quality ish commented
              Editing a comment
              If the gage insert seats, it should center itself within the threaded hole. Yes, the Tru-Pos plugs would be optimal, but I can't even get them to send me to school, let alone buy me gaging like that.

              And... As I said: With the tolerances I'm working with, this methodology should suffice. It's certainly more accurate than probing the inside of a 5/8"-11 thread with a 3mm ruby...

          • #7
            I Don't know why you would say DEMON?, If you look real close to the Hexagon Emblem, You can...........Oh wait, I think I see what you are talking about.

            th.jpg
            (In Memory of my Loving wife, "Ronda" who I lost March 7, 2016. I love you baby.)
            They say "Nobody's Perfect." I must be Nobody.

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            • louisd
              louisd commented
              Editing a comment
              that's just awesome! you made my day kirbs!

            • KIRBSTER269
              KIRBSTER269 commented
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              louisd

            • Kp61dude!
              Kp61dude! commented
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              Thank you Kirby!!

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