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Failure Modes in FMEAS

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  • #44647

    BillC
    Participant

    I need some clarification from the FMEA experts!
    You are manufacturing a “widget”, and you have to evaluate the cosmetic appearance after a finishing step.  A scratch-free finish is desired by the customer.
    Is a scratch a failure mode, or the effect of a failure mode? I feel that this would be the EFFECT of a failure mode, but some argue that “scratch” would be a valid failure mode.
    Any comments?

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    #143503

    GB
    Participant

    The scratch is a symptom (effect) of some underlying root cause.

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    #143505

    Brit
    Participant

    The scratch is the result (effect) of something failing (failure mode).

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    #143509

    Rodrigo
    Member

    my humble opinion is that “scratch” is a failure mode.
    Having said that it is important to determine where it ocurred and what was the cause of it.
    either way you will always have to investigate the process so not to have scratches (since they are part of product specification).
    hope it helps

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    #143514

    Szentannai
    Member

    Hi,
    imagine the manufacturing process: you can end up with a scratch on the surface due to a failure in almost any of the process steps. E.g. one step is polishing the surface and the failure mode could be using sandpaper instead of a soft cloth (obviously I’m making this up on the fly :)) ) the effect will be a large number of scratches on the surface. In the process of packaging the pieces the failure mode could be dropping them from the table – and the effect will be some scratches again.
    Regards
    Sandor

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    #143515

    GB
    Participant

    Rodrigo,
    why target the symptom?   how about the root cause?

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    #143517

    Brit
    Participant

    I guess we can agree to disagree.  What failed is the failure mode.  The scratch or the actual finish of the surface didn’t fail – something caused it – that is what failed – if the process worked perfectly as designed, the scratch wouldn’t exist.   Having the effect as the failure mode isn’t really in the spirit of the tool, I think. Your analysis won’t get near enough to the root cause of the problem, and I would venture to say you would have a hard time actually calculating the rpn improvement for a scratch rather than something causing it.

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    #143518

    Rodrigo
    Member

    hbgb
    it was not my intention to induce you in error with my post. The scratch situation can be evaluated in terms of severity, ocurrence and probability of detection, hence being a failure mode (am I right?). However, the most important thing is to look for the cause (root cause) of the scratch. It is my belief that the root cause would come out of the FMEA (or the need to realy drill down this problem).
    my FMEA table has a root cause section
    does it make sense?

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    #143523

    GomezAdams
    Participant

    Function : Provide smooth finish.
    Anti-function or failure mode : Does not provide smooth finish (eg surface scratched , pitted ,….)
    Effect : Degraded visual aesthetics,customer dissatisfaction,rejected,…
    Cause / Mechanism of failure : Place in cause(s) or mechanism(s) here. Each must be evaluated separately.

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    #143534

    Nandz
    Participant

    I agree with you that a “Scratch” is an EFFECT. This is a result of the failure of your process, in this case, your Finishing step, to perform it’s intended function. By design, you set up your process to be scratch free but along the way there is/are some deviation/s from it. That deviation can be called Failure Mode.
    Hope this helps!

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    #143540

    BillC
    Participant

    Thanks for all the opinions on this. I was thinking in terms of failure modes associated with input varaibles, which lead to “adverse” effects.
    Input: Polishing wheel RPM
    Failure mode: RPM too low
    Effect: Scratched surface
    Cause: Motor wear
    Something along these lines.
    The corrective action would be to improve the motor design or something like that.
    Feel free to keep the ideas coming. It is great to see all your ideas.

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    #143542

    Anonymous
    Guest

    BillC,
    The question you need to ask yourself is … are cosmetic defects the same as a functional defects?
    If they aren’t then cosmetic defects are subjective, and subject to misinterpretation. No surface is ever scratch free .. the use of a microscope will soon confirm this. Scratches and Digs are common in optical processing – no lens is ever perfect – but we can still form functional images!
    Therefore, you would be advised to devise a system for quantitative inspections – since human inspectors are unreliable. The purpose of the measurement system would be, in this case, to measure the degree of scratches and digs. Video systems or a stylus surface probe are ideal, but you should not try to identify or recognise defects; instead the system should measure the ‘signal-to-noise’ ratio and record the maximum as the system scans the surface. There are some commercial systems avaiable to do this .. such as Dr. Shenk
    I can also comment on some of the variables mentioned:
    Polishing a surface on a wheel usually takes a number of process steps. You start off fairly rough and progressively move towards a finer slurry. This means using separate wheels and keeping each process step clean. The final stage often uses oil, which should be filtered during lapping. Obviously, each substrate has to be devoid of damage and clean, and the weight holding the sample on to the lapping table down has to be optimised. As the Japanese would say – you can’t make chicken soup out of chicken sh*t.
    Andy

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    #143544

    Churchill
    Participant

    Let me imagine your process first: in a polishing process, your inputs including operator, widget, sandpaper, etc. And in your outputs, including some voice of your customer, eg, scratch free. Normally, process inputs should be noun.
     
    For widget, one failure mode is scratch, one potential effect of failure is ¡°customer rejects¡±, when you try to find potential cause, maybe you need ask times ¡°why¡± to find root cause.
     
    Hope it is of a help.

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    #143564

    billybob
    Participant

     
    Hello folks,
    I look at  FEMA failure modes this way…
    Take for example a brake drum:
    Potential Failure – premature wear of the drum
    Potential Effect of the Failure – Fracture of the brake drum
    Cause of the failure – scratch on the brake drum
     
    Process control – 100% automatic finish testing.
    Its not that the scratch is the failure; its that the scratch will lead to some other failure that the customer will occur , the failure mode
     
    Later,
    Billybob

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    #143607

    Deanb
    Participant

    Bill C:The question regarding if a surface scratch is a failure mode or a cause begs a basic question inherent in FMEA that says failure modes tend to represent BOTH the effects and the causes of other failure modes. This is an important concept in the tool because exploring the relationships between failure modes forward and backward usually is necessary to generate sufficient insights around the situation at hand. The RPN scores will typically suggest a multiple or comprehensive solution on several levels is needed. Applying FMEA thinking on a single failure mode level limits the logic, power and effectiveness of the tool.Therefore, the scratches probably are failure modes for customer rejections, yet are the “effects” of other backward modes (causes). Your job is to characterize both the forward and backward relationships around this issue.

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    #143615

    Mayes
    Participant

    Using the definition on this site “An effect is that which is produced by a cause”, the scratch would be an effect.

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    #143616

    ww
    Member

    I cut it twice and it’s still too small!  Failure mode, doesn’t meet specification requirement.  Root cause(s) why did it happen?  The specification sez no scratches so the failure mode is not meeting the specification requirement.  Root cause= why, how…  Have fun!

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    #143621

    Deanb
    Participant

    It is at a minimun an effect for sure, but not exclusively so. Wouldn’t the scratch also be a potential cause of other problems, such as rejected product, backcharges, or other technical nonconformances? The scratch is merely a failure mode within a continuum of failure modes and effects. To find the best economic solution requires a full picture. Perhaps buffing the surface to prevent customer rejection is a better economic solution than fixing a backward root cause. Finding the best place to fix the problem is part fo what FMEA seeks to accomplish.

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    #143624

    Zimmerman
    Participant

    In an FMEA, the scratch is a failure mode.
    The potential effect is a dissatisfied customer.
    The possible causes of the scratch would go in the “potential causes” column of the FMEA form.
    Some possible functions of the blemish-free surface could be “improve appearance” or “enhance appearance” or “attract user”

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    #143626

    Deanb
    Participant

    Unless the continuum concept is applied to FMEA then the “chicken or egg” debate such as we have seen here is easily precipitated. Such debates are unnecessary and wasteful. FMEA contributes much more if it is embraced to characterize the relevant continuum of failure modes and effects, and not merely to render judgements from observing a single layer of a problem.

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    #143632

    Adnan
    Participant

    I believe that the scratch is an effect. Cauese and effect analysis is required at the finishing step of the process. Are there other machines not producing this effect? maybe you need to compare your machine with other machines process capabilities.
    Regards

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    #143656

    Rhineg
    Member

    Note that “scratch” can be both a noun and a verb.  As such, all that attention to polishing speed & pressure, etc. may be competing with those dirty gloves the operator is wearing while unloading the piece.  So look at scratch at both ends of the spectum – noun and verb.

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    #143704

    BillC
    Participant

    Kent,
    I suppose that you are correct in one respect. The effect of a scratch is a dissatisfied customer. You could keep going with this however, where the dissatisfied customer is a failure mode and the effect is lower sales volume.
    I chose “Scratch” as the effect in this case. On the C&E matrix, which you do before the FMEA, scratch is a Y variable that gets a high importance rating because the customers have specific criteria for these cosmetic defects. Each input (X) is evaluated for it’s correlation to the (Y’s). Once the critical X factors are defined with the use of the C&E matrix, we look at the failure modes associated with all the influential X variables (i.e RPM).

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    #143775

    SEIDEL MURIEL
    Member

    Gentlemen:
    A scratch is the effect of a failure mode because it was originated by a human error.  For example if during the finishing of a journal or shaft  an scratch is left over  its surface this could lead to a premature shaft fracture.
    Seidel Muriel

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    #144397

    Orang_Utan
    Participant

    Yes, your KPIV identified in C&E matrix is your failure mode.

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    #144398

    Orang_Utan
    Participant

    With this sentencing technique and do no mixed up effect and cause.A failure mode is due to a cause(s) and resulting an effect.

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