recomended minimum hits for holes?

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  • recomended minimum hits for holes?

    I'm wondering how many hits I should be taking? I'm sure there are many things that one should consider, but a basic guideline is what I'm looking for.

    I've been measuring holes, usually as a 2 or 3 level cylinder, always with 5 hits per level. These holes range from 1mm to about 15mm.
    Most of our customers don't include any GD&T. One in particular has a general form and position of .2 on all their drawings, and tighter tolerances are usually TP 0.05 - 0.1. Once in a while I'll see a TP 0.01.
    2012 CAD++ MR1
    Brown & Sharpe ONE
    PH10T

  • #2
    3!

    Now that I've got that out of my system, this is doomed to turn in to a mix up of all kinds of bad and good info. This is like one of those automotive forum posts about "what synthetic oil should I use in my car". Everyone has an opinion, not a lot of is based on real facts.

    What is most important is that you can verify that the CMM data matches some other measurement technique. Pin gauges are the best way "in my opinion" if it's possible.

    There are a lot of great threads about this already, and one of them even links to a PDF of a book called "Good measurement Practices" which "in my opinion" is a great read for anyone who does layout work.

    Personally, I like to use prime numbers and the larger the better and I know that other people here will also state the same. My reasoning for this is it will reduce the amount of tri-lobe error induced from a tp2 or similar type of probe.

    But don't take it from me, do a search on this forum and read up, this topic has been hashed out at least 20 times here and there are a lot more involved and intelligent responses that I have given you here.
    Did this post solve your problem? Let everyone know and help future users out!

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    • #3
      Thanks for the tip about previous threads. I'll delve into those!
      2012 CAD++ MR1
      Brown & Sharpe ONE
      PH10T

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      • #4
        I do 6, and this is because when calibrating the probe hits the ball 13 times, one hit on top and 6 hits per level in 2 levels. This way calibration and inspection are done with the same number of hits and at the same location which should be a little more accurate than if the number of hits differs form calibration to inspection.

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        • #5
          Originally posted by Kaji Ryoji View Post
          There are a lot of great threads about this already, and one of them even links to a PDF of a book called "Good measurement Practices" which "in my opinion" is a great read for anyone who does layout work.
          That answered all my questions. Just in case someone else does a search and this thread turns up, here is a link: http://publications.npl.co.uk/npl_web/pdf/mgpg41.pdf
          2012 CAD++ MR1
          Brown & Sharpe ONE
          PH10T

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          • #6
            With a scanning probe, if the ball diameter is upper than the hole diameter, you vcan find the location of a hole with one point ! (auto center scan)... and without any accuracy !!!!!!!!

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            • #7
              Originally posted by Nano Vujkovic View Post
              I do 6, and this is because when calibrating the probe hits the ball 13 times, one hit on top and 6 hits per level in 2 levels. This way calibration and inspection are done with the same number of hits and at the same location which should be a little more accurate than if the number of hits differs form calibration to inspection.
              Have you ever actually watched the calibration sequence?
              13 hits for calibration will measure 7 hits around the equator 5 hits half way out and 1 hit on the pole.
              I never use even numbers for hits. ever.

              (If you use a TTP, scanning probes are very different)
              Roses are red
              Violets are glorious
              Don't try to sneak up
              On Oscar Pistorius.

              MIL-TFP-41C

              3.7mr3, 2012mr1, 2013mr1

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              • #8
                Originally posted by pwhitsa View Post
                13 hits for calibration will measure 7 hits around the equator 5 hits half way out and 1 hit on the pole.
                I never use even numbers for hits. ever.
                I guess i never really paid attention to this, just took it for granted. Guy that trained me told me this. Thanks for pointing it out.

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