Hierarchy of Calibration in Metrology

[Pumkin]

Hi all,

I'm looking for some information/advice about calibrating metrology equipment. I want to know if there is a standard for how calibration should flow downwards from certified standards.

For example: you have a glass ruler, gauge blocks, microscope, calipers, micrometers, pin gauges, profile projector, etc. Which of these should you calibrate in-house with the metrology equipment you have and which would be the most cost-effective to send to be calibrated and certified.

In this example you could have the profile projector externally calibrated, then use that to calibrate the gauge blocks, then use those to calibrate the micrometers and calipers, then use those to calibrate the pin gauges. You could also use the profile projector to calibrate the glass scale, then use that to calibrate the microscopes, etc. I guess my question is, it seems impossible to in-house calibrate everything so what's the most useful piece of equipment to get done externally?

Also, I've learned that you cannot calibrate item one via item two and then later use item one to calibrate item two but could there be a cyclical effect? Item 1 is used as a standard against item 2, item 2 for item 3, item 3 for item 1?

I've received no formal training or education in calibration and maintaining a calibration lab, and no one has held this position at my company in 10 years. We are looking to be ISO certified. If anyone has any free resources for what I'm sure are basic questions I'd greatly appreciate them.

 

[mhannon]

Welcome!

Deciding what to calibrate in house and what to outsource is all up to you as long as you can back up your decisions, have proper test methods and documentation.
However, you are correct in not being able to calibrate item 1 with item 2, then 2 for 1. That is a no-go. You could technically have one standard and use that to do item 1, then item 1 to cal item 2, and so on.... but I do not recommend that. You will have tolerance, error, and uncertainty stacking and compounding as you get further down the line.

Calibration is an unbroken chain of comparisons all the way back to the national standard. Always maintain NIST traceability.



In my plant, we have a contract with the OEM to calibrate our optical comparators and toolmakers microscopes. They are the experts and can do a better job adjusting back to zero than I can. They also do a full preventative maintenance including cleaning the mirrors and lubrication. All for a very reasonable price in my opinion.

Glass microrules and gauge blocks get sent out to a lab for calibration. This is because (in our plant) they are used as standards to calibrate other devices like micrometers and calipers. We want to maintain at least a 4:1 test uncertainty ratio between the standard and the item being calibrated. In order for me to calibrate a gauge block, I would need something 4 times more accurate than the block being calibrated. Purchasing and maintaining calibration on such precision standards does not become economical. It is cheaper for me to send these items out to a professional Z540/17025 accredited lab.

Basically, I have everything set up so all standards used to calibrate other devices are outsourced. Those standards are used to do in house calibrations of our shop floor tools (calipers, micrometers, rulers...)

We have a MASSIVE pin gauge library. It would be extremely expensive/time consuming to calibrate every pin. We have our internal processes set where any pin must be verified with a calibrated micrometer before use. This allows us to not calibrate the pin gauges, yet still have NIST traceability when making a measurement with a pin gauge.

Again, you can set up your internal processes however you like, as long as you perform "appropriate due diligence" and have documented your process. As someone who is leading a calibration program, you should be working closely with your quality department and engineering to help guide you in some of these decisions.
Also, the particular ISO standard you adhere to can have some impact on how you set things up. Are we talking ISO 9001? Any others?

As far as someone completely new to metrology, I would recommend watching ALL of the Mitutoyo Metrology Training Lab videos by Jim Salsbury. I have started several people here and Jim's videos really help build a solid foundation to build off of later. Watch all of them!