# of Hits per Hole Diam

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  • # of Hits per Hole Diam

    Just one of those questions every one is wanting to know.
    Can someone tell me what would be a basic number of hits for a circl. The diam is .1285". I have been using 6 hits. I have just changed it to 4 hits. Is the more hits the better or just the basic hits enough?
    Thanks ahead of time.
    B. Jacobs
    B&S Global 12.15.10
    2014.1

  • #2
    As with any feature of size the more hits the better.
    James Temmen

    There is no job so simple that it can't be done wrong.

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    • #3
      cmm's are not the best tool for checking hole size. .1285 should be checked with pins. for location 4 or 6 hits are fine. as i previously stated: form is everything. that's why when you check a master ring gage the size is consistant whether you use 4 or 400 hits.
      sigpic
      Southern Man don't need him around anyhow!

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      • #4
        Circle Hits required

        If you are only after a simple diameter, 4 is a good value to start with, but dfefinitely the minium. If you are going to check the form/roundness of the hole, then you should consider a minimum of 8.

        Already stated, more is better. As with anything else, there are tradeoffs. In this case, you are balancing time required to measure against quantity and quality of data from the feature.

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        • #5
          In my opinion you need three points to define a circle,but if you need the Roundness you will take more points,If I'm not mistaken it all depends on the tolerance you're dealing with if it's a very tight tolerance you will need more points,also by having a good roundness on your hole it will tell you that you don't have a bad probe hit in the hole.

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          • #6
            I always use 5 hits. To me, its not overdoing it, and yet, not underdoing it. I think it mainly depends on the Critical Tolerance of the hole. Like Gearge said, many times if the hole is critical, use gage pins. I actually put it in the program to stop and move out of the way, prompt to check critical hole with a gage pin, the program enters the value I type in, into the feature so that it may be calculated into the true position as needed to be.
            sigpic

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            • #7
              I don't remember the reason exactly but I heard that the number of hits should be a prime number ( 3, 5, 7, 11, etc...) Never understood quite why - something about the way the math works. I couldn't prove it wrong so what the heck... I usually use 7 hits. Also, CMMs are NOT bore gages.
              Links to my utilities for PCDMIS

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              • #8
                CMMGUY,

                Some early versions (mainly DOS software) of Mitutoyo GeoPak, Sheffield and B&S (ValMeasure) software used to recommend prime numbers mainly for improved calculations for form.
                Xcel & MicroVal Pfx & Global 37mr4 thru 2012mr1sp3
                Contura Calypso 5.4

                Lord, keep Your arm around my shoulder and Your hand over my mouth. Amen.

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                • #9
                  I have always used five hits in most holes. For me it just makes sense to not divide the hole into halves as a round number will do, so I stick to the same advice that cmmguy stated, 5, 7, 9, 11 etc.

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                  • #10
                    I always use prime number hits, 3, 5 etc...

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                    • #11
                      I use 5 hits but then again my tol is .015in mostly
                      sigpic
                      if you had soap on a rope it would be tied to yer ankle

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                      • #12
                        I choose to use 9, hasn't let me down for many years. I will agree that criticals should be checked with a gage pin to find the 3 min points in the hole. You'll find the best fit CMM result usually be slightly larger simply because your probing seldom lands on those 3 points.

                        TK
                        sigpicHave a homebrew

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                        • #13
                          Good response tking~

                          I also use 9 mostly.

                          I read somewhere that you should use 7 to pick up up to 6 lobes on a diameter.

                          Kev
                          RFS Means Really Fussy Stuff

                          When all you have is a hammer - everything looks like a nail....
                          sigpic

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                          • #14
                            Thanks, Kev

                            I thought you folks were going to kick my skinny old butt on the concentricity thingy.

                            TK
                            sigpicHave a homebrew

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                            • #15
                              Never 3 unless its a machined hole. 4 or five on sheet metal and 5 min on plastic. If you only use 3 holes not only can you get a very wrong Diameter size you will get a false reading on your location.
                              sigpicSummer Time. Gotta Love it!

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