Calibration of Master Thread Plug Gages

C

cklapperich

#1
Conflicting information. When calibrating master set thread plugs, everyone seems to be insisting on super accurate tools like bench mics or super mics (which we don't have). But according to unified thread tables I found online and ASME documents listing min and max pitch diameters show ranges of a thousandth of an inch or more - so I shouldn't need a tool capable of reading down to the ten millionths.

How much accuracy is truly necessary when measuring pitch diameter?
Can a digital mic be used? If not, why not?
We're not ISO 17025 compliant if that helps.
 
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S

smkeilman

#3
I'd suggest looking into the rules and philosophy regarding gage tolerancing. Most gaging is premised on the gage tolerance using up no more than 10% of the available product tolerance. For example:

1/4-20 UNC 2A product thread
PD tolerance ~0.004

1/4-20 UNC 2A X GO/NO GO ring gage
PD tolerance 0.0003

1/4-20 UNC 2A W Setting plug
PD tolerance 0.0001

Any device that checks something must be more accurate than the feature it checks. It does you no good to verify a gage with 0.0003" tolerance with a device that is certified to 0.001".

There are several reasons why a hand mic is not a great tool for this. First, even if your mic can resolve 0.000025", it will not have a quoted accuracy better than +/- 0.000025". This means half your tolerance is being burned in your measurement device, before anything else such as operator effects are considered. Also, at the precision we're considering, measurement force starts to factor into consideration. Thread gages are meant to be measured with a specific measuring force, dependent on the pitch of the thread gage.

The supermic recommendation was mostly for rear-covering during an audit. No one will question the applicability of that machine, but a competent auditor will likely take issue with a hand mic to measure setting plugs.
 
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