[geetarman]

Bart,

I worked for McDonnell-Douglas and Boeing for over 40 years. I measured things for size, flatness, roundness, torque, force, air and liquid flow. I have been around the track a few times. Invariably, when one takes shortcuts in maintaining the accuracy ratio between masters and standards, things get hosed up and parts that should interchange don't fit.

That is probably the biggest reason why parts from different lots and manufacturers sometimes have assembly problems. The first place you want to look is at their calibration lab and procedures governing handling of standards.

I worked for a part time lab that did gage block calibration for Boeing in Mesa.

I also worked for Boeing in Mesa. At Boeing, we would send a set of gage blocks for recalibration because we did not ( at the time ) have equipment to calibrate gage blocks. We would send a set of blocks to this house and a lot of blocks that were unpopular sizes ( those not needed for stack buildups ) would be rejected for being oversize or undersize. These were blocks tha had not been used.

It just happened the guy who owned the business with a partner was an elder at the church I attend and we were elders together. He needed some short term help as the business built up and I hired on part time.

What I saw was a business partner who, in order to meet schedule, would take a MASTER block from the calibration house reference standard set and send that to the customer to complete their order. What that did was destroy the traceability of the standard that was used to calibrate the set sent in by Boeing. That is what accounted for unused blocks being thrown out. The whole traceability from end use item to the working standard was destroyed.

What I did was ensure that gage block calibration equipment was purchased for Boeing-Mesa and we stopped sending blocks to this calibration house.

Long story short, the cal house was bought out and then went out of business. And it went out of business because they did not pay attention to the history and uncertainties that derive from Grand Masters, Masters, Transfer and working standards and started sending out bogus data.

Most companies keep logs of their gage blocks and I know for a fact the transfer standards at Boeing in Saint Louis have history going back to the 1940s.

Some gage blocks will, depending on how they are constructed, will grow a micro inch over a year or lose a micro inch a year, but if a log is kept long enough, you will see those blocks settle out. If the standard that is being used to derive the value of YOUR standard is continually being changed out, all those uncertainties can be positive or negative to your results and the history on your blocks is compromised. More than one company has gone belly up because they did not pay attention to their calibration system.

It still irritates me to think how we were hurt because we paid attention to a calibration certificate that meant nothing. Sad to say, a lot of calibration houses are that way and the only one that is going to tell you the truth is NIST. You are going to pay a lot for their service but they know what they are doing.

If all you want is a cert to satisfy an auditor, set up your own cal lab in your business. Problem is you might just get an auditor like me and I do know what I am doing and would shut you down in a heartbeat if you were giving me a line of baloney and I know where to look to see if you are on the up and up.

Calibration is serious business and in the aerospace industry a "valid" cert for a catastrophic failure is no comfort to grieving families.

Too many people view calibration as a necessary evil and try to cheat any way they can and hope they don't snag a savvy auditor who can shut them down.

I guess you can say I am still passionate about the discipline. I really got into it and enjoyed it.