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Are all Bias, Linearity, GRR and Stability to be performed for equipment defined in Control Plan?

Discussion in 'Gage R&R and MSA - Measurement Systems Analysis' started by Pongsakorn, Jan 22, 2019.

  1. Pongsakorn

    Pongsakorn Member

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    Please enlighten me as follows:
    1. Is it true that the equipment defined in Control Plan must be done Bias, Linearity, GRR and Stability completely without exemption?

    2. If there are several equipment with the same brand/model, can we do Bias, Linearity, GRR and Stability for only 1 equipment as representative?

    I could not find this in AIAG MSA Manual, please advise.
     
  2. Golfman25

    Golfman25 Well-Known Member

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    A few years ago we had a finding for no Bias, etc. We only did a GR&R.

    My understanding is you can use only one of the same instruments as a representative sample.
     
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  3. Miner

    Miner Moderator Staff Member

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    Bias and Linearity should be covered by a good calibration program. Stability should be risk based depending on tolerances, sensitivity to environmental factors, etc., leaving R&R. Most customers will accept the creating of a gage family provided the criteria make sense. Similarity of gages, tolerances, features measured, etc. are considerations in making the family.
     
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  4. Andy Nichols

    Andy Nichols Moderator Staff Member

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    Not all equipment is susceptible to all three. I had this argument with a UL auditor who clearly didn't understand...
     
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  5. Pongsakorn

    Pongsakorn Member

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    Andy, is it nonconformance if the stability is not done?
    It is very difficult for me to debate with any customer/auditor, which manual should be used for reference to select proper MSA method to be done?

    In my opinion, Bias, Linearity, GR&R should be done but Stability may or may not be done depending on sensitivity of equipment.
    Correct me if my opinion is not valid.
     
  6. judegu

    judegu Well-Known Member

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    In my work place, we only do Gage R&R for all the measuring equipement in CP. I also wonder why there is no bias, linear, stability studies. However for your reference, no NC was raised during 2nd and 3rd QMS audits. Maybe it is due to the position where my company is in the automotive supplier chain. We are far from the OEMs.
     
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  7. Pongsakorn

    Pongsakorn Member

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    Judegu, this is the reason why I need to know what is right what is wrong and the reason. Auditor and customer might not be clear and ask for unnecessary thing but we should have clear explanation to them.
    I am waiting for the clear answer from Guru. :)
     
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  8. judegu

    judegu Well-Known Member

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    I am not sure about the Auditors` competence on determinging which is right or wrong regarding the MSA. What I do know is that some of the customers don`t know it quite well. They only got very vague understanding on MSA as I do. In my workplace It looks like that we only do the Gage R&R as it is a requirement from the Automotive Standard. Does it(MSA) do any good to the whole process in our company? Highly doubtful.
     
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  9. Bev D

    Bev D Moderator Staff Member

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    Pongsakorn:

    linearity is a natural outcome of a bias study - is the bias consistent or varying over the expected range of part variation.

    Bias occurs in several forms
    - bias of the chosen measurement system from the true value (calibration needs to part of the study design)
    - bias from the first to second (or second to third, etc.) measurement of the same part. This usually occurs when the measurement system effects the part in some way (breakaway torque)
    - systemic bias between operators (reproducibility) or gauges of the same type (calibration/reproducibility between gauges of the same type).

    Stability is indeed something that should be done for 'sensitive' instruments or measuring systems. Good engineering sense can often inform us of what is likely to suffer from stability degradation due to environmental and use conditions. Real understanding of yoru measurement system is needed to make a logical scientific argument for not performing a stability study. short of that only the study can tell you whether your system has a problem with stability or not. I can say that in my experience calipers have no stability problem and visual inspectors do have stability problems. of course there are other systems that also suffer from lack of stability and those who are robust to it. knowledge (not belief or opinion) is necessary.

    as for auditors who have a shallow understanding and need to rely on blind application of the standard - you must either have the scientific knowledge and understanding to provide a reasonable rationale or do the studies.
     
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  10. Miner

    Miner Moderator Staff Member

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    For additional information on bias and linearity, see my resources and scroll toward the bottom.
     
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  11. Pongsakorn

    Pongsakorn Member

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    Hi Bev D, your answer is reasonable. I wonder why AIAG MSA Manual written is such way that the reader has difficulty to understand and interprete. Different person has different interpretation causing an argument especially customer and supplier. You should be AIAG member.
     
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  12. Pongsakorn

    Pongsakorn Member

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    Hello Miner, many thanks, will read through your articles.
     
  13. ncwalker

    ncwalker Well-Known Member

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    The reason the manuals are written the way they are is that the OEMs would never agree to wording that prevents them from asking for more information. They can be summed up in this sentence:

    "We all agree you should do things this way, however, we reserve the right to ask for more ..."

    It is worded this way because there may come a day when a customer legitimately DOES need more and you don't want an unreasonable supplier using the (arbitrary) blue books as a defense to not have to do the right thing.

    This creates the inherent weakness in the system - when someone is in a decision making position that does not understand the requirements, they generally default to a position of asking for "stuff" until it feels right. They don't pay for the extra work, after all, and they don't want their organization to know they may need more training. Which creates this busy work we all as suppliers sometimes face.

    We then have to make the decision - is it more cost effective to fight it? Or just comply (which while cheaper, in the long run, reinforces the bad decisions).

    I have found the best way to defend against these unreasonable requests is with good internal standards. If a supplier quality engineer is asking you for extra, nonsensical things and you can produce a logical, active, QMS controlled standard that appears to be followed, your defense is then "Well, you're asking me to change our standard way of working - we don't do that lightly ..." That can often calm them down ...
     
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