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to be confident or not on a result...

JEFMAN
I start this new thread because of another one which talk about the confidence on cmms.
It make me think about different side of this subject.
A friend of mine said, long time ago: "in metrology, the only thing that I'm certain is the existence of the uncertainties !"
This subject is as close to accuracy than it's to phylosophy !
When a part or a dimension is bad, who doesn't begin by re-measuring it ?
When a part or a dimension is good, who begins by re-measuring it ?
When a tech calibrates the cmm, he measures (ISO10360-2) 5 lengthes in 7 locations three times.
If you can, just take a look at a location measured with a Koba-step, by measuring all the steps.
If you choose the same lengthes than the tech ,you will find few deviations.
If you choose different lengthes than him, ou will see more important deviations.
In both cases, the cmm is generally "in the tol". but not at the same place !

What can I say as conclusion ?
Still be confident in your cmm, according just the right place to uncertainties, and don't talk about this to your boss, you avoid some questions Wink !

Ah !
It's friday, happy week-end !
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  • So how would one go about making a validation program, per the specifications described in ISO 10360-2?


    If you can, use a Koba-step twice a year, 19 locations mini, all the steps, 3 reps, and write the result in the same file.
    Calculates the average of deviations, the standard deviations and use it as cmm part of uncertainties.
    Use a "standard part", which represents what you measure usually, and measure it, and here again, look at the average of dimensions and std dev.


    I'm specifically interested in ISO 10360-3, regarding CMM validation when a rotary table is the 4th axis.
    I've never trusted the rotary.

    You can do the same with a "standard part" !



    One more thought, the uncertainty can be divided in three parts, the cmm, the part and the method.
    Here, we used to say that the cmm is for 1%, the part for 10% and the method takes the rest ! (algo, number of hits, repartition, reading and understanding the blue print.....).
    I would add that the designer is responsibble of 80% of them !
  • in reply to JEFMAN
    So according to your calculations Mr. Calculus what % do you trust your CMM?
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