Help with MU and ISO17025 basics - Hired as a CMM programmer

T

True Position

#11
Almost got my Type A estimates done for each category of tool, figure I'll finish up the CMM and depth micrometer today. Leave the Type B stuff which I'm still somewhat lost, but figure I can work my way through tomorrow.

Just recieved a copy of the standard an hour ago, I'll read it tonight with a strong drink, or three.
 
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M

micron master

#12
Hawat,

Believe me when I say this:
I do not often recommend A2LA for anything.

That being said, go to their main page and find the
"document finder" link. When there, find the "A2LA
guide to expression of uncertainty". They actually have
some examples in there that will help you. IIRC they have
an example that covers what you need, and if not, there is one that will be close.
 
T

True Position

#13
For the more experienced out there

I just checked a bore 5 times, re-alligning each time.

My results were:
52.6805
52.6803
52.6803
52.6801
52.6802

Standard deviation of that set is .00014832

For a type A uncertainty of .000066mm?

While that does make me happy, I'm worried my math is screwed up somewhere since it's such a small number. Of course, I'm going to have a lot of type B uncertainty due to temperature fluxuation multiplied by the potentially large lengths.

Just wanted a quick check if that looks reasonable to people who see these things more often then I.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#14
I come up with a Type A pretty close to yours so you likely are OK.

As for the brekdown in ranges of measurements, it is a good practice unless you have a need to do uncertainty for every discreet measurement. Just make the breakdown real, in that the ranges should reflect reasonable change points. Calipers/micrometers have some built-in points with gage blocks, but you have to establish yours based on the measurements.

As for the temp, the GUM method is to go by the readings at the time, but taking worst case over the week will work just as well for your use I suspect.

Hope this helps.

Hershal
 
T

True Position

#15
Sorry to be bumping my own thread.

Sorry to be bumping my own thread, but I just had one of my uncertainty calculations called into question for a particular measurement.

If you were measuring a hole's position in X or Y with a height gage w/o gage pins, you first measure the bottom of the hole, reset your height gage/indicator, and then measure the top.

Now, for the uncertainty, would it be reasonable to double the uncertainty for the height gage? I gave just the straight uncertainty I had for the tool and got comments about being worried. If they want a bigger number, I can give them one, but I want it to be a reasonable assumption I would be making.
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#16
For the height gage itself, take the uncertainty listed on your accredited cal cert, divide by two to get it back to standard uncertainty and that number is ready for use.

However the top and bottom readings will have some uncertainty value which is determined by the Type A calculations.....each of those two readings will have a Type A contribution to your uncertainty.

Hope this helps.

Hershal
 
W

waderl

#17
Hawat,

We are a small independent Biotech/Pharmaceutical calibration lab and went through our initial ISO/IEC 17025 accreditation 3 days ago. We took 1 major and 3 minors, the major being our uncertainties for our scope. It looks like the post 3 weeks ago by Hershal is a good start. I’m feeling your pain and having trouble too. The formula in the two manuals (Measurement and Good Practice Guide No.11 by Stephanie Bell and EA-4/02) are really hard to follow with my little experience. Our auditor only wants the best measurement (type B) in each area (pressure, temp, speed …..) for our Scope of Accreditation. Can anyone direct me to a good step by step example of figuring out a best measurement?

Thanks for your help.
 
T

True Position

#18
In that regard, I did luck out. Micron Master's suggestion to check A2LA's site for information had two quite good documents on uncertainty of measurement, one specific to dimensionals, which was even more helpful. I'd recommend checking the thing they have first, it's more readable then most things out there.
 
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