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6Mar/12Off

Measuring Plain External Diameters – American vs European

A case related to pin gages being rejected by our European Customer

The ASME B89.1.5-1998 is the correct document to review. Reference para. 6.1, pg. 5 & 6 as well as Table 4 on pg. 7. The values were computed using the equation in Puttock and Thwaite’s CSIRO Technical Paper #25. This statement is also on page 6. The question that should be asked is, what is their measurement uncertainty? Remember that the Europeans add measurement uncertainty into their measurements. We do not do that in the US. Depending on the uncertainty budget, they may not have the accuracy to measure a .000020” gage. Our measurement uncertainty is .000012” based on our latest accredited calculations . This is not good enough to accurately measure a Class XX pin. Our customers accept our measurements as long as our uncertainty is stated on the calibration certificates. According to ANSI/NCSL Z540 & ISO 17025 the total uncertainty of the measurement should not exceed 4:1. That would require a total measurement uncertainty of .000005”. The only lab that I know of that can measure this accurately w/in the 4:1 Rule, and prove it, is NIST.

See the link to the NIST web site that has a calculator to assist in the measuring force calculation.

http://emtoolbox.nist.gov/Elastic/Case10.asp

Note: The measuring force changes with the diameter of the part being inspected.

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