Our lab has adjustable thread rings on its scope of accreditation. When we were initially accredited, thestatistician the company used as a consultant (before my time) set the uncertainty for the rings at .0003". There was no budget made, per se. The value is derived from the X tolerance of the pitch diameter for the set plug used to set the ring.
In a recent assessment, the assessor asked to see the budget. No one had questioned it before... and whatever documents we had that explained it are lost to history.
Does anyone know of a reference that discusses this uncertainty? I don't mind leaving it at three tenths, but I have nothing to technically justify it beyond my own words.
TIA

I would have to question why the consultant did not provide a budget. Having an expanded uncertainty digit to reference is all well and good, but truly the meat and potatoes of MU IS the budget, the budget tells you where your uncertainty comes from, these values can then be analysed and improvements made accordingly...i.e. if your highest contributor is the reference standard you calibrate against you can then decide if it is valuable to your company to decrease measurement uncertainty for this device by using a more accurate reference standard, etc...

It sounds to me like you will need to re-evaluate your adjustable thread ring gages for uncertainty...trying to backtrack this consultants' work to validate his MU value would be quite difficult with nothing to go on. Perhaps you could contact him/her to get some information. Sounds as if you'll need to start at square 1 though.

As for a reference document, I cannot provide something specific for adjustable thread rings...TN 1297 and ISO GUM will give you valuable guidance however.

Off the top of my head, you'll want to have the following in your budget:

Type-A
Repeatability & Reproducibility

Type-B
Instrument Resolution
Instrument component uncertainty contributors
Reference standard uncertainties (/2 to return to std unc)
Thermal effects (different temps between part/gage)& (different thermal coefficients between part/gage)

These would be basics to start with.
Hopefully you find some of this helpful, good luck ;)

I can appreciate this.....300 microinches is reasonable - perhaps - if all main issues are under control.....but the budget will list the "assumptions", that is, the conditions documented.....

The budget provides the detail necessary to understand the uncertainty assumptions.....

Hope this helps.

Hershal

One of the best places to find information on uncertainty budgets is to look at other people's scopes. The best scope I have found for Threaded rings is from Glastonbury Southern Gage.

They show their set plug uncertainty and the comment says "In accordance with ASME B1.2, para 5.1.1: the ring is sized to a plug, with the plugs uncertainty given"

To me this says that you do not have to have a separate thread ring uncertainty if your only method of verification is to use a calibrated plug. Some other poepl use different methids with various measuring machines. In that case, they would need a separate budget.

Glastonbury's scope can be found here:

http://www.a2la.org/scopepdf/1553-02.pdf

If it is good enough for a manufacturer who is accredited, it should be good enough for a calibration lab!

I hope this helps.

Joe

Thanks jfgunn!

I had done what you advise... only I looked for "adjustable thread rings" trying to avoid plundering through scopes for just solid thread rings. I see some people used the plug tolerance as we have been doing.

But I really like this idea of using the plug's uncertainty. Funny, the scope you mention doesn't note if they mean solid or adjustable rings.

Thanks again.