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Gage Studies

Six Sigma – iSixSigma Forums Old Forums General Gage Studies

  • This topic has 27 replies, 28 voices, and was last updated 13 years ago by Manu.
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  • #69437

    Georgette Belair
    Participant

    If each rock has its own tolerances,  you will want to perform a gage study for each.
    Because, you need a representative sample of the process to calculate an accurate percent variation.  Otherwise,  using the two processes together will give you an inacurate account of how well your measurement system is doing (Your part-to-part variation will be artificially high,  causing the total variation to be that much larger,  hence the   percent variation caused by the measurement system will look inaccurately low.  – – You’re Gage R&R will look better than it actually is…

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    #69441

    Ken K.
    Participant

    I’m assuming your choices are to run one anlaysis on just the little rocks vs running two analyses: one on little rocks and one on big rocks.
    A lot depends on the nature of the variation in your gage. I would venture to guess that the repeatability would be pretty much the same regardless of the size of the rocks. But, the reproducability could be different if the operators react to the different rock sizes differently.
    Whether or not the GR&R is affected by rock size, certainly you’ll need to be able to compare the GR&R to the tolerances of both rock sizes, and more preferably, you’ll be able to compare the GR&R to the total variation, including the part-to-part variation for both large and small rocks. The problem with getting part-to-part variation outside of a Gage R&R study, such as historical data, is that it will include the measurement system variation.
    My advice – it is usually easier to run both studies (one for large rocks and another for small rocks) than to mess with all the assumptions you’ll have to make otherwise. If running the Gage R&R is very expensive or consumes resources, then if may be worth the “mess”.
    I recommend you have a statistician help you sorting out that mess.
    Ken K.

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    #28028

    Cindy
    Participant

    If you make 2 different products (big rocks and little rocks) but you only have 1 measurement system – should you perform a gage study using a mixture of rocks or should you perform 2 separate gage studies.  I believe the 2nd one is the correct answer – but am having trouble communicating the statistical reasoning behind it……….

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    #69452

    Ravi Khare
    Participant

    I would vote for two separate studies.
    The part-to-part variation between little rocks and big rocks is by design, where p-p variation between either all little rocks or all big rocks is a result of process variation.
    One of the major purposes of a Gage study is to compare the R&R (error) with the part to part variation occuring due to process variation. The purpose of a measurement system is to discriminate between products that are different on account of process variation.
    If little rocks & big rocks are mixed, this would lead to a huge part to part variation; and wrongly give you an excellent R&R/ P-P variation ratio, putting you in a lull of false security.
    Ravi Khare
    [email protected]
     
     

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    #69478

    Durga Kant Jha
    Participant

    Hi Cindy
    Gage R&R is the statistical tool used to validate the measurement system of the project Y in Measure phase. Idea is to find out the variation present in the measurement system & minimize it the best possible. Two R’s maning here is Repeatability & Reproducibility. Repeatability gives you the measure of variation within the operator & the measuring instrument. Reproducibility gives you measure for variation among the operator. So there are two factors involed basically in any measurement system & those are
    1. Operator. 2. Measuring Instrument.
    You have mentioned that the measuring instrument is the same for both the measurment. But people who are involved in the measurement could be different. Even if they are same % gage R&R is calculated with respect to the USL & LSL of the Y. In both the measurement USL & LSL is different. Also repetability capability of the measuring instrument might depend on the rabge of value it is intended to measure. I beleive this is the statistical reason for your answer going for two seperate gage r& r analysis for both the measurements despite the fact that the measurement system is the same.
     
     
     
     
     

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    #69487

    Mooers
    Participant

    Hi Cindy,
    If you are just doing gage R & R’s there is only need for one study.  QS states that the “family” of gages can be studied.
     
    jp

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    #69503

    Stephen Curtis
    Member

    Gage R&R is intended to be a study to measure the measurement error in measurement systems.
    We look at two components, gage variation (repeatability) and operator variation (reproducability).
    The 10% rule tells us that If a measurement instrument does not discriminate to 10% of the part tolerance, the measurement instrument has no feasibility to measure the process variation and if the percent of tolerance consumed by the R&R does not exceed 10%, the measurement system is excellent.
    The three elements of gage accuracy are linearity, accuracy and stability. Linearity is how the size of the part effects the accuracy of the measurement system.
    Gage linearity is used to identify how accurate current measurements are through the expected range of the measurements.
    It also gives answer to the question “Can we measure correctly all of the parts that have different sizes?
    Linearity = slope * process variation
    The closer the slope is to zero, the better the gage linearity.
    In this case it would seem to give the answer that it doesn’t really matter whether you have one or seperate R&R studies.

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    #69521

    Patrick Swift
    Participant

    Hi,
      be very careful with this one. The final R&R value that most stats packages spit out is the ratio of variation in the measurement system v’s the variation in product measurements. i.e. R&R = 15% means that 15% of the variation in product measurements is due to the measurement system.The key phrase here is variation in ‘product measurements’, this figure will be larger if you measure big rocks & little rocks then if you only measure one type, & hence the R&R figure will be smaller. Whatever R&R figure ( or no. of distinct categories figure) you end up with only applies to that range of product measurements that you used in the study. In this case you will need to perform seperate studies for big rocks & little rocks (probably not what you wanted to hear).
    If you have time,what I suggest you do is perform 3 studies using the same operators and gage for each. One study on little rocks, one on big rocks, and the other using big & little rocks. You should end up with a smaller R&R figure for the 3rd study (due to the larger figure for overall variation). By doing this you will end up with a better understanding of what the actual results mean, and how part selection can effect the final data.
     
    Good luck
    Patrick

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    #69528

    Jim Parnella
    Participant

    Do the gage capability studies on BOTH kinds of rocks. Reason: the variability is very likely to be proportional to the mean. Therefore if you combine both kinds of rocks in the same study, your variability estimates will be too high for the small rocks and too small for the big rocks.That said, what kind of ROCKS do you manufacture? 

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    #78679

    S. Kodali
    Member

    If the size distribution of the rocks are normal, then you have to do only one anaysis. If it is bimodel, then group them into two and measure independently. You should also define the objectives of the measurements required for analysis.

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    #85576

    vali ramin
    Member

    Hello
    please send me some information about gage R&R
    thank you

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    #85580

    Pai
    Participant

    Show a little initiative! No one has the time to send you everything you want in an easy to digest form — you’ll have to take a class to get that.
    Here are some resources from this site (try the search, it’s v. good):https://www.isixsigma.com/dictionary/Gage_R&R-147.htm
    https://www.isixsigma.com/st/msa/
    Raj

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    #86102

    tony pattanayak
    Member

     I plan on introducing an ergonomic fixture to be used with a current gage/measurement system. The fixture is a lever which will be used to actuate a pump instead of by hand and the measurement is the weight loss of each actuation.  I wanted to do a Gage R&R study showing that the operator variation created by using the fixture is not significant. Can I use the same operator w/ fixture(s) and w/o fixture as appraiser 1 and 2 or should a different type of study be performed. I wanted the reproducibility to be less then 10% using the standard formula.

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    #115794

    Dan VanSickle
    Participant

    The QS9000 Standard states in 4.11.4.S;
    The supplier should repeat gage R&R studies when warranted by measurement system change (including operator) and have a systematic method to improve gaging.
    We now do yearly gage R&R studies on each gage that measures our product. The statement above says we SHOULD do a study when we change operators. Does this mean MUST. We change operators often and this would interupt production too much. Can we gather data from our actual measurement of the product of each operator instead of all these studies.
                              Dan

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    #119345

    SoqratIS
    Member

    The following books are recommending reading for an  introductionary understanding of SPC and statistical analysis.
                Box, Hunter, Hunter. Statistics For Experimenters. New York: John Wiley & Sons, Inc., 1978.
                Griffith, Gary K. The Quality Technician’s Handbook. Fourth Edition. New Jersey: Prentice Hall, 2000.
     
    Chapter 13 in Griffith’s book has a great intro to gage R&R and the chart he uses is helpful in writting something in Excel to do the number crunching.

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    #123269

    naveen adlakha
    Participant

    gage study is basically a study of gage by which we are making checks this process will tell us about the perfection of our gage.gage r&r tells us about the repeatibility and reproducibility of our gage and our operator

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    #132319

    Leon Edward
    Participant

    Cgk is being used by an automotive customer in Australia for gage capability and it is calculated differently. we have not seen this .

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    #134773

    johansson
    Participant

    You can use only one rock (big or little) to measure your measurement fixture (system).  The result will apply for other.
     
    Max

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    #138055

    howe
    Participant

    There’s some great information her. I have a question please.
    I believe the information is saying, if the gage doesn’t come within 10% it should be replaced. So it would be true that for attribute data a 80% agreement is not satisfactory and another measurement system is recommended?
    Please

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    #147096

    sreedharan
    Member

    We have to measure 2 separate rocks with 2 separate gages.

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    #147148

    Monk
    Participant

    Shree
    The purpose of the gauge R&R is to understand whether the measurment system is good enough to measure the required variations in the product or parts or rocks.
    So whether you have to use the same measurement system or a different onw will depend upon the following criteria – Is the measurement system good enough to measure the variation to the required scale? If the answer is ‘yes’…we can go ahead with the same measurement system.
    Remember that ‘big’ rocks and ‘small’ rocks have not been defined. If in both the case the measure is the diameter in inches, definately the same measurement system can be used. But if in one the dimension is in microns and the other the dimension is in metres, then definately different systems will be needed.
    Monk

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    #150007

    RR Researcher
    Member

    How does one account for ‘gauge block’ changes while performing a gage study? For example, if one has a test device (the gage) that measures valve crack pressure, but the valve changes each time it is tested due to it’s design, wear, etc. (assuming that the most repeatable gage imaginable were used for the study), the gage will likely indicate a poor R&R result, even though it is actually very good. I assume that one has to have non-changing gauge blocks before commencing the study, but should’t one be using the actual product(s) that will be tested on the device? Is it realistic to create custom, stable gage blocks before beginning such a study to get a true understanding of the gage’s precision?

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    #150019

    Paul White
    Participant

    I think this is a valid approach to validate the measurement system as long as the non-changing blocks represent the variation in the process.
    I have seen a similar approach used within the auto industry to validate vehicle wheel alignment characteristics (toe, camber etc). I.e. When the vehicle is driven off the rig the suspension characteristics change. We used master gauges to represent the vehicle in a non-changing environment.
    The other approach would be to select ‘gauge blocks’ from the same batches and conduct a destructive (nested in Minitab) R&R study. Used this approach to measure flex strength of headliners which is a destructive test.

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    #157778

    CHristophe Niel
    Participant

    Gage evolution with repeated measurement is a quite common situation in electronic industry.
    One practice seen in publications is to modelize measurement evolution due to repeatability (linear or polynomial regression), and to correct measured values accordingly. Computation of R&R indicators is tehn done considering “corrected values”.
    Do you think this paractice is acceptable ?

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    #161725

    Jonathan Barker
    Participant

    In a recent project during the MSA the process I was looking at had no historical standard deviation, during the gauge R&R the %P/T calculation for the measurement system I designed was 25% suggesting a secondary %R&R calculation was required. However the lack of process standard deviation made me look at the % contribution and number of distinct categories instead – this showed some 10% variation was due to the measurement system, mostly repeatability and the rest was part to part, distinct categories was 4. I concluded the measurement system was capable – is this a valid approach in this circumstance ?

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    #161728

    ncwalker
    Participant

    Jonathan,And you have stumbled upon the inherent weakness of the MSA manual. You get your result and … what next? The results are in terms like NDC and %GRR, but your gage is measuring things like mm or ft-lbs.If you do not have long term process variation to look at, you are correct in using the study variation. Because it is all you have. But your question is “now what..”Answer: Because your Gage R&R is “close” to being unacceptable, it behooves you to do it again in a few months when you have some good, long-term process data.A measurement system is part of a process – no more, no less. We do FMEAs, process studies, etc. to determine which parts of a process we have to watch closely (inspection every hour) from those that we can watch occasionally (inspection every tooling change). I am sure you understand that concept – driven by tolerance you are trying to hit and repeatability of the process. The measurement system is no different.A borderline Gage R&R means you have to keep an eye on it with more calibrations, more remastering, etc. It is simply risk management.Your NDC of 4 is troublesome. I have seen in older literature where NDC of 4 is acceptable. But as time has passed, I have seen that number climb from 4 to 5 to 6 and without a lot of reasons as to why.The NDC is basically the ratio of the noise of your parts to the noise of your gage. (Scaled a bit because it is a vector). You are saying roughly that your gage noise is one-fourth your part noise. Is that good? Sadly, it is really a financial decision.Let’s say you have a spec (for easy math) of 1 mm +/- 0.2 millimeters. Your categories you can discriminate are (roughly)
    0.98 to 0.99
    0.99 to 1.00
    1.00 to 1.01
    1.01 to 1.02This is better than a go/no-go gage and certainly accurate enough to show drift using pre-control or another acceptable method. BUT – the outer bands have enough inaccuracy that you could pass a “slightly” bad part. So if the characteristic was important, you are probably ok. But if the characteristic was CRITICAL, you may want to ONLY accept the .99 to 1.00 and the 1.00 to 1.01 bands which would guarantee that you never passed a bad part. Assuming your process was centered, you might be throwing away 1/3rd of your good parts “to be safe”.Your production department would hate you :-).(And I wish I had a better answer. I am trying to work out what to do with the Gage R&R numbers myself. Life would be easier if the process spit out the +/- of the *GAGE*. Then you could just set your inspection limits directly and the “good enough” question would go away.)

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    #162641

    Jeremy
    Participant

    I have an R&R study question, but let me set up the scene first.
    I did a study on an M8x1.25-6H plug gage in a hole with a max of 18 and a min of 12. Nominal is 15 and it is measured by counting the number of engaging threads. I was the first of three appraisers. First, we took measurements in sequential order from 1 through 10. Next we took measurements in the following order: 2, 4, 6, 8, 10, 1, 3, 5, 7, 9. The third trial was a random order of choice determined by the appraiser. Once I have all the data collected, I then plug it into a spreadsheet provided by my Dept. Manager (Who is a Blackbelt). The spreadsheet will give me percentages of EV, AV, GRR, and PV. I can choose one tab to view %Tol or I can choose another to view % Total Variation. On the %TV tab, it shows %EV=54.63, %AV=22.06, %GRR=58.92, and %PV=80.80. On the %Tol tab it shows, %EV=2.95, %AV=1.19, %GRR=3.19, and %PV=4.37. My question is when determining if my measurement process is okay, should I go by % Tol or % Total Variation? I believe I should go by % Tol since I am trying to determine where my tolerance is being consumed. This then would lead me to believe that the tab labeled % TV is breaking the %GRR up even further, telling me where I need to improve. Is this thinking correct?

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    #163020

    Manu
    Participant

    Hi Cindy.
    You should go ahead and perform two different gage studies. The reason behind is, that there are two different products having different characteristics and the Quality of each product is judged diffrently.
    Whenever you have such a doubt just keep this in your mind as a golden rule : Who are the customers of that service or that product . If the customers have different expectations from the different products that you have then those products have to be dealt separately and their characteristics have to be measured separately.The same or different measurement system does not make any difference in your approach. If you have two measurement systems for each product use them respectively for each.
    This has helped me a lot and I am sure that will help you as well.
    Cheers
    Manu
     
     
     

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