How many hits for a hole?

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  • How many hits for a hole?

    Guys,

    I have a question. How many hits are needed to measure a hole?
    At my company some people are starting a gage R & R test. I said that a minimum amount of hits are 3. With 3 hits it is not possoble to measure the roundness, so more hits are needed.
    I then mentioned that I had heard somewhere that Prime numbers (ie 3,5,7,11,13,17 etc) are the best to use.
    Is this so? If yes why?

    Cheers

    John
    sigpic If only all problems were so simple

  • #2
    There have been numerous discussions on this. Here is one.
    When in doubt, post code. A second set of eyes might see something you missed.
    sigpic

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    • #3
      Well, I can't tell you what is best. I can only tell you what I use and why.

      I almost always use 11 because I have found that on my machine with my probe, that gives me the diameter values which are closest to those measured by a bore mic. I don't do a lot of roundesses. I am also not in a business which requires high speed and throughput.

      What I have heard about using prime numbers. On machined parts often times vibration will cause a regular patern of irregularities. By using a prime number for the number of hits you will ensure that the hits will not always fall on the same part of the irregularities.

      Imagine a triangular part. 3 hits or any multiple of 3 hits will give a distinct pattern. With a square any multiple of 4 hits will give a distinct pattern. A prime number of hits will ensure that some hits will fall off of the patern.

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      • #4
        The more the better. I usually use 24 hits (Every 15 degrees).
        I used to be high on life but I built up a tolerance.

        Brown & Sharpe Global Advantage
        PCDMIS CAD++ v2011mr2
        PH10MQ/SP600M


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        • #5
          Just 4 hits, but I am automotive sheet metal, not NASA.
          sigpic
          Originally posted by AndersI
          I've got one from September 2006 (bug ticket) which has finally been fixed in 2013.

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          • #6
            I start out with 6 or 8 if there are no form requirements. Some processes and call-outs require more hits and/or strategic placement of the hits as Goodluck mentioned in regards to process induced form issues (usually, but not always workholding induced).

            Craig
            <internet bumper sticker goes here>

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            • #7
              Because of the algorithm used for diametral measurement, I tend to vary the amount of hits with the size of the bore / shaft, more surface - more hits.

              I also use even numbers to simulate 2 opposing point method but I do run in situations where primes are more suitable.

              Keep in mind that CMMs are not the best tool for diametral size / form measurement - but if you're looking for locations, it's pretty reliable.
              PC-DMIS CAD++ 3.7 from 4.2 MR1

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              • #8
                Originally posted by Underspec View Post
                The more the better. I usually use 24 hits (Every 15 degrees).

                Depending on what industry and what you are dimensioning. I'm in plastics and I dimension form alot. Yes the CMM is not very accurate for these types of things but the graphical analysis helps alot. For hole locations I agree with the 3-4 points.

                A scanning CMM would be nicer if management would dish out the fund$. But, I have to work with what I got.
                I used to be high on life but I built up a tolerance.

                Brown & Sharpe Global Advantage
                PCDMIS CAD++ v2011mr2
                PH10MQ/SP600M


                sigpic

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                • #9
                  According to the National Physical Laboratory (NPL) - Measurement Good Practice Guide No. 41 by David Flack, the recommended number of points for a circle is 7 (to detect up to six lobes).
                  sigpic
                  Xcel 15-20-10 - PFXcel 7-6-5 - Merlin 11-11-7 - Romer Absolute 7525SI
                  PCDMIS 2012
                  Windows Office XP

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