Sampling Inspection 100 % Inspection

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    Ibrahim Gouda

    We are using 100% inspection for testing the items related to safety, and we have to use sampling inspection if we applying destructive tests, But what if we have to apply destructive testes for items related to safety ?!! I have such case that I need to test Electrical Fuses for its failure at certain over-voltage  or over-current  values, so we use destructive test meanwhile these fuses are safety items..!, how can we avoid the risks of sampling inspection? What is the recommended technique to ensure the zero defect of these fuses.
    Please advise, Thank you
    Ibrahim Gouda 


    Rene A van Leeuwen

    Hi Ibrahim,
    My cold thursdaymorning brain tells me that when destructive test need a 100% score you should identify causes of malfunction of the fuses. Use this in stead of the destuctive test.
    Hope this helps you on the right track. René



    Performnce limits are seldom inspected 100%, regardless of whether they are sfety items or not. Imagine the following situations:
    The aircraft’s airframe must withstand load factors up to 6G. Pemanent deformation is accepted as long as it does not result catastrophical.
    With a 150 pounds person seated and with the safety belt secured, the car seat must withstand a crash of 70MPH and keep fixed to the car structure.
    The weel bearing must pass the 1,000,000 turns test.
    The fuse must burn with a current of 12A during 0.01s.
    In all these cases, one must find the critical Xs that define the critical Y, find the range where the Xs can vary enssuring the fullfilments of the requirements for Y, and set controls for the Xs (wich may include 100% inspections), not for the Y. A very small sample inspection can still be performed to the Y also as to check that the Y=f(X1,X2,…,Xn) relationship still works, but never 100%.
    In the case of the airframe and the car seat, may be it is enough to check the chemical composition and structure of the materials (on every batch of raw material) and the geometrical dimmensions of the parts (some dimensions may need to be checked 100%).
    In the case of the bearing, you must add other Xs such as quantity of grease, surface finishing of the rolling elements, hardness, etc.
    In the case of the fuse, you will probably know. I imagine for example that in the wire type fuses the comosition and diameter of the wire should be critical to the current needed to make it fail. Once the permisible limits for them are found, you can check the composition in every batch of raw material and the diameter in every coil of wire (or even in every segmet of every fuse).
    Sometimes, it is just impossible to test the performance limit and then it is impossible to validate the tolerances defined fot the Xs, so one must relay on theory, mathematical models, scaled tests, etc. Examples: The bridge must withstand a 100MPH wind. The building must withstand an earthquake of 6 degrees without unrepareable failures, and of 8 degrees without catastrophical failure. After a 30MPH crash, the passanger must escape without permanent injuties. You will never know for sure until it happens for real.

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