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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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  • , :
    a few hikes later (Slight smile), I would answer that a % is not the right unit for an uncertainty.
    Saying the uncertainty is 1% would be 0.01 mm for a length of 1mm, and 10 mm for a length of 1 m... I hope the cmm is better than this Wink !
    In my case, i believe that measurements uncertainties are within ±10µm for parts size around (0.5 m)^3, taking into account the cmm, the part and the method, and describing them in the report (number of hits, algorithm....)
    To give an example, measurind a Koba step in 19 locations, 52 steps, 3 repetitions give a standard deviation on deviations around 1.2 µm.
    If you measure the distance between a point and a least square plane without tacking into account the flatness in the uncertainty, you can be wrong.
    An easy way to see the uncertainty is changing the algorithm evaluation on the same COP.
    Another solution is repeating the same measurement at the same location, and in differnet locations on the cmm, and each time, looking at the average and the std dev... It can be very surprising !
    If you have a large standard ring (around Ø 250 mm), just measure it 10 times horizontaly and 10 times verticaly and look at the average circularity, you should see a little difference, not a lot µm, but the gravity is here, so the uncertainty also Slight smile.
    Sorry for the µm sellers...
  • in reply to JEFMAN
    I would expect nothing less from you Jeffery
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