Thread Gauge Calibration Failure for Pitch Diameter being too large

[jdm2008]

After many years of passing calibration results, we recieved a thread gauge back that failed for pitch diameter being too large. Unless I am drawing a blank on the pitch diameter definition, this seems impossible. How could the plug gauge pitch get larger from wear? The pitch diameter is where the thread thickness is the same as the open space, so as the plug gauge wears, it seems to reason that you would have to go closer to the center to reach this point so the diameter would decrease with time.
And I understanding this correctly?

 

[Jim Wynne]

Re: Thread Gauge Calibration Failure

After many years of passing calibration results, we recieved a thread gauge back that failed for pitch diameter being too large. Unless I am drawing a blank on the pitch diameter definition, this seems impossible. How could the plug gauge pitch get larger from wear? The pitch diameter is where the thread thickness is the same as the open space, so as the plug gauge wears, it seems to reason that you would have to go closer to the center to reach this point so the diameter would decrease with time.
And I understanding this correctly?

You should ask the calibration lab about this.

Has the same thread gage been calibrated before by the same lab? If so, do you see a progression in size, or a "sudden" jump?

Can you do your own three-wire check?

Although I've never personally seen it happen, I know that thread gages can "grow" via galling, which is the process of material being compressed into the gage threads from the parts being checked. Aluminum has a tendency to do this.

 

[Hershal]

Re: Thread Gauge Calibration Failure

A dimensional expert should be able to help with a reply shortly.

 

[DRAMMAN]

Re: Thread Gauge Calibration Failure

Also ask them how they do the calibration? I once audited a well known thread ring & plug gage mfg. They also did our calibrations. We used MANY thread gages and were concerned about the quality. For calibration they simply used the 3-wire method using them cadillac mics after letting everything stabalize for a week.