Trusting Gage Manufacturer Calibration Certifications

We are not AS9100 certified yet (working towards it), but we do work for aerospace:

A little over 1 year ago we got a part that has Cadmium plate (.0002/.0005 thick). Per our standard method of manufacturing, we buy pre-plate thread ring gages to check the thread acceptance during the manufacture of the part changing the pitch diameters of the go and no-go based on the stated plate thickness. We also verify the pitch diameter with a thread micrometer a few times during the run (as a reference).

Now, 1 year later, I send our gages out to be calibrated and they come back with the pitch and minor diameter measuring per machinist hand-book as a normal thread. The gages themselves list the pre-plate pitch diameter and that they are pre-plate gages. The gage certifications I got with the gage show they are 'pre-plate'.

This brings up an issue. How far can I trust gage manufacture certifications? I realized this is most likely an isolated incident but..... If we had gotten a rejection on the parts, I most likely would not have sent the gages out to verify size, I would verify the certifications (which show good).

If this happened to you, what would you do. Has this happened to you, if so, what did you do. I am sending the gages back to the manufacture to review, should I do something else?

Personally, I generally do not accept a manufacturer's calibration, except when issued under an accreditation, so that there is sufficient oversight.

Otherwise, send the gages out to get calibrated by an accredited calibration lab, with that in their scope, and importantly, make sure they give you the uncertainty. Uncertainty is error, which you need to know.

Hope this helps.

Michael, as a general rule for me, not very far.

Manufacture companies are really good at making their product; most of the time calibration is a secondary activity. Some of the worst certificates I've seen come from mfg.; they use some internal (proprietary) procedure, and it's not very clear what they verify.

Too, I have to watch mfg. Many times they provide some certificate of conformance or something; and did not ever verify the instrument that I have. They just did some lot acceptance test.

I'm in the same camp with Hershal. If they are not accredited, I will only use a mfg. calibration service if there is no other vendor that will calibrate it. Otherwise, send it to a competent calibration vendor.

Did the calibration cert you received indicate how the rings were calibrated? Unless you sent them to the manufacturer or verified that the cal house had the correct setting plug- they were probably not done on a setting plug.

If that is the case, I recommend you purchase a setting plug. A set plug is much easier to verify and then set rings to.

On the topic, I never trust a mfr's certificate. Compliance certificates mean nothing, and as a technician who sees new gages come in all the time, the amount of gages I send back for rework would probably make your head hurt. I always urge our customers to let us check the gages when they purchase. It's not to make extra $$- it's to verify that the gages are correct for their own protection.