Here is the situation that I am facing.

I have three NPT tapered tubing. The factory manufacture them and checked "pass" with NPT gauge. However, one have sent them to laboratory to check the thread dimension of them using CMM and found that the thread is out of tolerance.

That made me confused on which result should be trusted.

The following is the dimension and data after checking by CMM.

(1)3/4 - 14 NPT +Tol -Tol Actual Size Result
END 0.96768 0.00446 0.00446 0.96931 ok
L1 0.98887 0.00446 0.00446 0.98857 ok
L3 1.00179 0.00446 0.00446 0.99849 ok

(1)1-11 1/2 NPT +Tol -Tol Actual Size Result
END 1.21363 0.00543 0.00543 1.19005 -0.01815
L1 1.23863 0.00543 0.00543 1.21637 -0.01683
L3 1.25630 0.00543 0.00543 1.23348 -0.01739

(1)1 1/4-11 1/2 NPT +Tol -Tol Actual Size Result
END 1.55713 0.00543 0.00543 1.53555 -0.01615
L1 1.58338 0.00543 0.00543 1.56261 -0.01534
L3 1.60130 0.00543 0.00543 1.57940 -0.01647

Please extend your help on this. We still did not know whether the tolerance provided by the laboratory is correct or not. They are refering the tolerance spec. from ANSI/NCSL Z540-1 & ISO 10012-1.

Please help.

Thanks,
Eddy