Problems with Three Wire Method on No Go plug gage

Greetings everyone, newcomer here that's just gotten into the world of QC at my company, and almost immediately upon arriving I was handed the task of doing our monthly gage calibrations. I've picked up everything reasonably well so far, but my stumbling block seems to be using the Three Wire Method on threaded plug gages. My company uses a Pratt and Whitney Model C Super Micrometer, which we verified to still be in good working order last week. It reads gage blocks and Deltronics pins accurately, and I've used it to calibrate several pins without any issues.

However, thread gages still baffle me. I can get measurements in tolerance on the Go side of our thread gages without any issue. But when I move to the No Go side, my measurements are either on the very low end of the tolerance (i.e. if I should be reading .1872 I get .1869), our out of tolerance entirely, and this is consistently happening across numerous gages of varying thread pitches. We have checked the Super Micrometer, the thread wires were calibrated back in May, and everyone who's looked at my technique and how the wires are placed in the threads seems to think it's okay. I've even tried with a brand new No Go gauge that has never been put in a part, and I read that as .0005" lower than nominal.

Does anyone here have any advice? Is this a common problem people have with thread gage calibrations?

 

Those kind of gages are pretty stable - I suspect your answer will turn out to be they're supposed to read like that.

(Your mileage may vary, not a real doctor, seek a trained calibrationist before using my advice, side effects may include weird measurements)

 

I suggest you can send out several plug gages to professional calibration lab to get measurement result, you can compare the result, then you also get some suggestions from workers of calibration lab.

 

If you have a new plug with a long form cert., check the new plug against the Cert. results... I have done this in the past