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FMEA Occurance Rankings: Frequency of Offset due to Tool Wear Vs OOS

Discussion in 'FMEA - Failure Modes and Effects Analysis' started by _Zeno_, Jun 22, 2016.

  1. _Zeno_

    _Zeno_ Member

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    In a machining environment, the insert cutting a diameter will wear causing the size to change. When determining the Occurance Ranking, do you use the frequency that a tool has to be offset to maintain size, or how frequent the process (with the current detection and prevention methods in place) produces a part out of size?
     
  2. ncwalker

    ncwalker Well-Known Member

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    If I understand - as the tool wears, you are making offset adjustments and at some point, the cutter has worn too much and it is replaced. Based on this I would say that if the offset adjustment is PART of the control plan and not a reaction to an OOS part, then the occurrence is the replacement frequency. But if the offset adjustment is NOT part of the control plan, itself being a reaction to an OOS or at least OOCL situation, then the occurrence needs to be the frequency of making the offset.
     
  3. yodon

    yodon Well-Known Member

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    What you describe, to me, would indicate that you KNOW the sequence of events leading to the [bad thing happening] will definitely occur so the likelihood of occurrence is whatever highest ranking you have. And it sounds like there are 2 failure modes: whatever happens that causes you to make adjustments and whatever happens to require a tool change. The mitigations would be adjustments for the former and tool replacement for the latter.
     
  4. Bev D

    Bev D Moderator Staff Member

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    I assume that you are looking at an out of spec part as the failure and tool wear as the cause, yes? If this is the case the occurrence rate to use is the rate of OOS parts if there were no adjustments. the control is to make adjustments either by number of parts machined or by using a control such as Bob Doerring's Hi-Lo chart or Wheeler's sloping limits chart.

    You should also add tool breakage as a cause and work hardening (if applicable).

    I calculate occurrence rates of the failure separately for separate causes if they are knowable (either theoretical, which can usually be done stable things like tool wear or empirically when it is simple and straightforward like with tool wear.
     
  5. luisdiaz980

    luisdiaz980 New Member

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    The occurrence is the element to consider in the ranking; the preventive preventive actions (as the offset) that are part of the process decrease the ranking
     
  6. _Zeno_

    _Zeno_ Member

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    Understood, but to the original question, what say yea?
     
  7. Bev D

    Bev D Moderator Staff Member

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    I would use how frequently the part would be out of spec if the tool were not adjusted for the original occurrence rate. adjusting the tool is your control. you can use the frequency of occurrence with the adjustment in place as the residual occurrence rate. if this si still too big it will indicate that the adjustment process needs to be improved or another control needs to be developed.
     
    Mark Paul likes this.

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