Calibration and Heirarchy of Standards

[Mark W]

I work at a small automotive manufacturing company (approximately 100 employees at my division) and have been working in the quality department for a couple of years now. Our calibration program is pretty weak and we are attempting to rebuild and strengthen the system.
While working on this project I am strugling with the concept of "calibrating against certified equipment having a known valid relationship to nationally recognized standards". I am not sure if a gage has to be calibrated directly to our highest in-house standard, or if calibrating a gage to this standard and then using this gage to calibrate another measuring instrument is acceptable or not.
A typical example is as follows. I was informed by someone that our gage pin set(pins in increments of .001") has to be sent out to an outside lab for calibration and thereby recieve a certification that is traceable to NIST. The only gaging that we currently do this for is our highest in-house standard, a set of gage blocks. These blocks are used to maintain traceability to NIST with our in-house calibrations. The pin set is a shop floor working set that is used for a variety of tasks, i.e. measuring a depth over a radius broached feature with OD Mics., go/no-go diameters of holes and slots, etc. My question is this. Is it acceptable practice to calibrate a set of .0001" OD mics. against our highest level gage block set, and then calibrate the pin set with the micrometers? I understand that the measuring accuracy of what I am using the pins for will probably impact the justification for this so what if instead I carried out the same procedure but instead of using an OD micrometer, I used a laser micrometer with an accuracy of +/- .00002"?
I am trying to justify what are acceptable calibration methods to industry standards at a level that will ensure our gages will adequately provide quality product at the most economical cost. Please help.

 

[Kevin Mader]

Mark,

I have used a similar approach with our inhouse calibration activities with success. The registrar who reviewed this area was concerned that the general calibration record keeping was current, complete, was done by a trained associate, and demonstrated the "10 factor". The Registrar was also looking to see what percentage of calibration we did inhouse vs. sending out. He mentioned that he was happy to see that only the gaging we did not have the inhouse expertise to handle was sourced out. Remember: compliance and "effectiveness".

Using a micrometer to calibrate a pin gage, under most circumstances, is sufficient. Using a super mic or a laser mic is better, but not necessarily necessary (use the 10 Factor, 10x as sensitive). Use what gives you the most security. Just be sure that you are aware of the class of the Master gage blocks (probably the highest class) and the pin/plug sets and if the pin/plug sets are plus or minus pins.

I have read that many companies tend to send out all their gages for calibration to either avoid having to understand the qualifying/calibration activities or because they don't want to do it. This is neither cost effective or necessary. Your attempt to contain cost is, in my opinion, correct. A good, quick read you may want to consider is "Managing the Metrology System" by C. Robert Pennella. It will give you a pretty good overview of the Metrology process.

Back to the group...

[This message has been edited by Kevin Mader (edited 02-12-99).]

 

[Marc]

All I can do is echo the 10X factor and that even though you send them out, someone still has to understand measurement systems (per MSA). Being automotive you have to do more than for ISO.

 

[Mark W]

Thanks for the input. I'll be sure to pick up a copy of that book you suggested.

 

[Batman]

I too echo Kevin and Mark, and add a couple of items to consider.
If the pin calibration by your own equipment serves, and can help reduce costs, go for it. I have seen this work well, and I have seen it deteriorate. Pins for example will wear at the edges, depending upon the product being measured, and unless you can detect this with your calibration equipment you could pass bad parts. Using a super mic or Laser, care must be taken to be able to detect this wear.

Also, since the Laser is a two-point contact method, any odd-numbered lobing on the pins may not be detected.

If you are using these pins to measure and approve features that are "critical," the calibration procedure and records should stand up to scrutiny.

I recommend good calibration procedures, good records, and a controlled environment for that calibration.

Just a couple of things to consider.