Metrology: Thread Gage Out of Spec

<...the probable causes of this defect...>

One possibility is a difference in temperature between the two measurement events.

Nominal dimension for the gage? Material of the gage?

Temperature is maintained (or should be maintained) within <±0.5°C. You have to check if this condition was respected, if not the references could have been different. You should ask for this too.

Hi,

I'm not in mechanical metrology, but I do know that those things are checked at 20°C. There would not be measurement results from 9 years ago surely. My guess is it was out of specs when it was stored. You probably could have bought a new one for the price of two calibrations.(?) I don't see any choice but to replace it now.

-S

Size of a steel gage can change over time due to latent metalurical effects. The slow conversion of Austinite to Martinsite or perhaps vis versa, it has been a couple decades since I knew this information well.

guest is correct that metallugical effects can cause distortion. I'm not real familiar with the steels used for gauges, but I've seen some duplex stainlesses move by orders of magnitude more than what we are discussing here, so it is conceivable that metallurgical effects could be in play here.

Another possibility could be that the gage has been inspected to a higher class in the same thread size which narrows the acceptable dimension parameters. Gage materials, like any others, can move over time. Even slight amounts of corrosion will grow a dimension, and when removed, will leave it lower.

The last possibility is that the 9 year old cert was done on 25 year old equipment and just wasn't as reliable as what we have availible today. Also, some labs only verify pitch, and not form.

first of all it has to be checked with what care it was stored.whether the right rust preventive was applied . a closed atmosphere calls for accumulation of moisture and more stresses on the part being stored hence this variation.pl check the position of part being stored.