Calibration of Thread Gages with Metric Standards with Conversion to Imperial Units

[Bill Ragsdale]

Hello,

Long time observer first time poster here. I had a situation come up where the previous quality manager at my new position decided they could save money by outsourcing our thread gages overseas. Looking at the cal reports from the manufacturer it shows that all the gages were calibrated in mm and then stated as equivalent to imperial units ie inches.

I was wondering if this procedure is acceptable from and ISO 17025 calibration lab? We are starting to see some of our gages fail and I'm thinking that it has to do with the conversion and the stacking of the number rounding as we increase the thread counts. Does anyone have any experience with this? Is this an acceptable practice?

Thank you in advance!

 

[Jen Kirley]

Welcome Bill!

I would say that conversion to inches seems okay as long as it is within the needed discrimination. Rounding is problematic though, certainly as is "stacking the number of rounding" if your tolerances are very small.

That said, your having some troubles with these gauges certainly indicates trouble that I wonder about. I am not sure the rounding can be the root cause by itself. This rounding would be less of an issue if the calibration service provider was using standards with a significantly smaller discrimination. The industry habit is at least 10:1 but I have never found the absolute requirement for that.

Before I post this I want to be sure I understand you: are the gauges purchased from overseas or are the calibration services overseas?

 

[John Broomfield]

Also, you and your customers may find this info from NIST useful:

Busting Myths about the Metric System

It's a hot topic as you can see from the comments!

 

[dwperron]

I have had to use metric wires for english plugs and vice versa in times when one of my sets of wires was out for calibration. I don't remember having an increase in out of tolerance results.
For example, look at a 1/4-20 thread. The pitch diameter tolerance for this thread is ±0.00015". The tolerance of the thread wires used is ±0.00002". Any rounding errors on the conversion from metric to english on the wire size would not be significant to the thread tolerance.