MSA samples Preparation.

abdelouahed

Starting to get Involved
Dear Experts;

Please I'm come back to asking you about the best method to prepare or select the MSA samples for a GRR study. because we get a remark from an IATF auditor about the way on how we prepare the sample (next year it will be considered as Major NC ).

Actual method (Wrong method according to the auditor) :
we are using a digital Micrometer to measure the height of piece with a specific value +/- tolerance ( i.e. : 1.05 +/- 0.05 mm), so we prepare manually 10 samples representing real measurable parts. The sample parts must be selected from the process and represent the entire production operating range.
For example we prepare one sample on the min. tolerance value, then we adjust on tools to get a sample on the min. tole. +0.01 mm ... till the max. tole..

According to the auditor when we adjust manually the the height of the sample, we effect the %PV and then we modify on our measurement system which is not correct.
and he said that the correct method is by taking one sample per day for several days. but like this we will get 10 samples with the same height value because our system is stable.

Could you please support and advise me with the correct method on how can i prepare or select the samples in a similar cases.

Best Regards.
 

Miner

Forum Moderator
Leader
Admin
It all depends on how you intend to use the measurement device:
  • If you intend to use it to measure the part to its specification, part selection has no impact on the % Tolerance metric and your method would be acceptable.
  • However, if you intend to use the device for process control (e.g., SPC) then your auditor is correct. Part selection does impact the process variation (PV) and therefore the %PV/%SV metric, so the parts must represent the actual process variation.
An option that your auditor MAY find acceptable is to use the attached document and to manually enter the PV from a capability study rather than use an approximate PV from 10 parts. This will depend greatly on how well your auditor truly understands MSA, so it is risky.
 

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abdelouahed

Starting to get Involved
Thanks a lot Mr. Minner for your usual cooperation and support.

Normally we are using our equipment to measure the part to its specification, the operators adjust to tools and when the measure of the sample be under the specification tolerance then he start the production ( measuring only the first piece).

As you said "This will depend greatly on how well your auditor truly understands MSA" , because I'm using this method for more than 10 years and i have past more than 20 audits (IATF & VDA) and never i get a NC or a remark about the MSA method.

regards.
 

Miner

Forum Moderator
Leader
Admin
I would first explain to the auditor that the device will only be used for inspection, and therefore % Tolerance is the only applicable metric. Once they understand that, show them that part variation is not part of the calculation of % Tolerance, and therefore has not impact.

I would also refer to the MSA manual, 4th edition, page 77, where it states "The criteria as to whether a measurement system's variability is satisfactory are dependent upon the percentage of the manufacturing production process variability OR the part tolerance that is consumed by measurement system variation. The final acceptance criteria for specific measurement systems depend on the measurement system's environment and purpose and should be agreed to by the customer."
 
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