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MSA with no specification

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  • #28160

    Chaz
    Participant

    I am performing MSA with no specification limits; I am trying to determine how good is my measurment system.
    I read in my software manual that you will look at % of process variation column under combined Gage R&R . If the number is <10% than is good, >10 and <30% questionable. They also reference 1991 AIAG manual.
    I have always thought (i do not have an access to AIAG) that the <10, <.10 and >30% referred to % of tolerance. Am I correct?
    What is a ROT to determine how good is your meas.system, when you have no specification limits ?

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    #69777

    EPS
    Participant

    Hi Chaz,
    Without the specs you won’t be able to calculate “precision to tolerance ratio”, but you can calculate the “precision to total ratio” (sigma measurement divided by sigma total). I assume you know how to make the calculations; othewise, feel free to e-mail me:
    [email protected]
     
     
     

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    #69783

    Ken K.
    Participant

    The same criteria applies when using process variation in the denominator. The AIAG MSA reference manual says:
    “Acceptability Criteria – The criteria as to whether a measurement system is satisfactory are depnedent upon the percentage of part tolerance or the manufacturing production process variability that is consumed by measurement system variation. For measurement systems whose purpose is to analyze a process, a general rule of thumb for measurement system acceptability is as follows:

    Under 10 percent error – accpetable measurement system.
    10 percent to 30 percent error – may be acceptable based upon importance of application, cost of measurement device, cost of repair, etc.
    Over 30 percent – considered not acceptable – every effort should be made to improve the measurement system.
    The final acceptance criteria of a measurement system should not come down to a single set of indices. The performance of the measurement systems should also be reviewed using graphical analyses.”

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    #69786

    Ken K.
    Participant

    I should have mentioned that the AIAG MSA reference manual clearly prefers the metrics based upon the production variation over those based upon the tolerance.
    I quote from the chapter titled “MEASUREMENT SYSTEM DISCRIMINATION”:
    “Thus a recommendation for adequate descrimination would be for the apparent resolution to be at most one-tenth of the total process six sigma standard deviation instead of the traditional rule which is the apparent resolution be at most one-tenth of the tolerance spread.”

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    #69848

    Chaz
    Participant

     

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    #69857

    Ravi Khare
    Participant

    The MSA Manual also talks of Study Variation as the denominator for comparison of R&R.
    The study variation is the one that’s captured within the current Measurement study. The relation between Process Variation and the Tolrance will be based on how good your Cpk is.
    In this case since there are no tolerances, Process Variation or Study Variation would be an appropriate measure for comparison.
    Ravi
    http://www.symphonytech.com

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    #69888

    Kumar
    Participant

    I have seen so much debate about GRR in last several years when I got involved in Six Sigma.  The statistical tools in market seems to create more confusion.  Here are some of my observations and hope they help.
    1) GRR number shall always be looked in reference to the tolerance.  Ultimately what matters is the ability of the gage to resolve a tolerance.
    2) The acceptability numbers are should be also in reference to tolerance.  A common industry standard is 5% for a good gage
    3) The acceptable GRR number is a very much BUSINESS decision.  It is all about RISK management.  If you can handle lot of risk, you can have a sloppy GRR.  In general, I may say that a critical part may require better GRR than a non critical part. 
    4) GRR for a destructive testing cases is in theory not possible (That is what an expert with Ph.D. told me).  However, there are always engineering approached and the results should be interpreted with other information such as part to part variation etc.
    Hope that helps.  -ravi
     

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    #69891

    Ken K.
    Participant

    In response to Ravi’s comments (shown in bold):
    1) GRR number shall always be looked in reference to the tolerance.  Ultimately what matters is the ability of the gage to resolve a tolerance.
    I have to cordially disagree. There are many occasions when the gage is used to take measurements for process improvement activities, such as DOE’s. In these cases, a gage’s ability to detect improvements MUCH smaller than the tolerance can result in a substantial quality imrprovement. In these cases it is more important to have the measurement variation small with respect to the process variation (which I’m assuming to be equivalent to the experimental variation.)
    2) The acceptability numbers are should be also in reference to tolerance.  A common industry standard is 5% for a good gage
    Again, I have to disagree. The accetance criteria for gage R&R is clearly stated in the AIAG MSA Reference Manual. As a matter of fact, suppliers to the “Big 3” and other QS-9000 subscribers are contractually obligated to follow these acceptance criteria unless they have written customer approval for alternative acceptance criteria.

    Under 10 percent error – accpetable measurement system.
    10 percent to 30 percent error – may be acceptable based upon importance of application, cost of measurement device, cost of repair, etc.
    Over 30 percent – considered not acceptable – every effort should be made to improve the measurement system.
    3) The acceptable GRR number is a very much BUSINESS decision.  It is all about RISK management.  If you can handle lot of risk, you can have a sloppy GRR.  In general, I may say that a critical part may require better GRR than a non critical part. 
    I agree.
    4) GRR for a destructive testing cases is in theory not possible (That is what an expert with Ph.D. told me).  However, there are always engineering approached and the results should be interpreted with other information such as part to part variation etc.
    Again, I agree, at least in theory. Close approximations to the correct GR&R results may be possible if nearly homogenious samples can be obtained from a single batch or similar circumstance. These homogenious samples are then treated as psuedo-repeated measurements. Another method is to find non-destructive ways of replicating the taking of measurements, such hanging weights on a strain gage, commonly used for pull breakage tests. Even this may not accurately simulate the actual testing conditions though.

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    #69996

    Ravi Khare
    Participant

    I do share Ravi’s first name but not his views on comparing R&R with tolerance.
    A Measurement System is ment to measure variation in the process, and discriminate between different parts. Part to Part variation is detected and is measurable with some degree of confidence only if the R&R is low compared to the Process Variation.
    The Proess Variation can be grossly different than the Tolerance, as is demonstrated by the varied values of Cpk that you encounter in Process Capability Studies.

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