Quality Digest

I've never checked NPT thread gages before, and I assume the 3-wire method can still be used...anyway I have a chart listing the various major and pitch diameters (small, at notch, large)

Never mind the PD's, I can't get any type of reading on the major diameter to come out. For instance I have a 1/2-14 NPT gage here, and on my chart it says at the large end the diameter should be .83936". I get nowhere near that. If I set my mic to .83935", the gage passes through with plenty of room, it's a lot smaller.

Do I have the wrong chart, am I measuring something wrong, or is there a different method to calibration than a standard thread work plug?

I suppose the possibility exists that the gage is out of spec, but I doubt by that much, and I have a few similar gages of different size, and get the same result.