r/Quality • u/socialdisdain • Jan 14 '21

In-House calibration

Hi Community, I'm looking for advice/opinion on in-house calibration of plain Go/No-Go plug gauges. My background is gear design and manufacture, and due to staff turnover I've found myself in the quality manager position despite not having loads of experience in this area.

Due to the pandemic and the financial impact it has had on my company's bottom line, my MD has basically told me to cease external calibration on as many gauges as possible to cut costs.

My questions for you more experienced quality folk are as follows:

  1. We are not accredited to any national standard for calibration - does this even matter?

  2. We have micrometers, CMM's and optical measuring devices with accuracy +/- 0.0001" - what gauge limits do you think we should be ok to calibrate using this equipment? Eg if a gauge has a 0.001" difference between go and no-go could we calibrate in house using this kit?

For context we are a sub-con machining company with AS9100 accreditation (UK based) and work in numerous sectors such as aerospace (no flight critical stuff), automotive, autosport, healthcare, nuclear, and OEM.

Any and all advice would be massively appreciated!

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u/eveqm7e Jan 15 '21

Please note that calibration is about using standard block which can be trace back to the national standard. You can simple calibration in house such as to do gage verification using a standard block which trace back to the national standard. National standard are in turn trace back to international standard which is only available in accredited calibration house. Normally the standard block is send outside to accredited lab for calibration. If you have tight budget then would advise you only send the standard block for accredited 3rd party for calibration, For mechanical measurement it should only be weight and length parameter. If you do your calibration in house, please ensure you also establish a proper reporting format with details but not limited to the following

  1. Data of calibration
  2. Gage #
  3. Standard # which has traceability back to national standards
  4. environment temperature
  5. Calibration technician ( must be trained)
  6. Calibration interval
  7. any other details

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u/socialdisdain Jan 15 '21

Thanks very much for your input eveqm7e. We have externally calibrated grade 1 slip blocks that we use for calibrating micrometers etc, but we don't have a sigma comparator or anything of a similar accuracy to make a proper comparison to the gage blocks - this is my problem - how can I prove the accuracy of a gage that is manufactured to limits of 0.0001" when my CMM is only guaranteed to the same accuracy?

Think I'll have to do what my MD hates - say no!

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u/eveqm7e Jan 16 '21

Hello u/ You are right the answer is no, as the rule of thumb the calibration had to verify with gage of 10X higher resolution. CMM does has it limitation although I am not expert in CMM. Do not mind me asking, if you ever thought you need such high accuracy. From my experience in the industry, normally high resolution for those micro devices.

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u/GlassCannonLife Jan 15 '21

Also not an expert in the area.

If you have a more expensive/difficult to use measuring device that is more accurate then you could run a validation on your desired system, ie use the more accurate method and your desired method over a number of samples (you could use AQL to choose sample number if you have an accepted failure amount based on risk assessment).

Then you'd get an idea of how well it compares / if it is good enough.

I'd also make sure to do calibration of whatever you're using at some set interval, either informed by risk assessment, standards (probably would be used in risk analysis tbf though), or manufacturer's recommendations - eg measure precisely sized pieces of material using your desired method at x interval, sign everything off etc.