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SPC implementation

Discussion in 'ISO 9001:2015 - Quality Management Systems' started by luckycharm, Oct 30, 2018.

  1. luckycharm

    luckycharm Member

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    Hello guys,
    Can I please get clarification / help in SPC implementation.
    Do we still need to do control charting when the specification limit is too narrow (ex 25 - 25.5mm).
    Is there a rule when to apply Control Charting with reference to specification limit.

    Hope to have your opinions / practises.
    Thanks!
     
  2. tony s

    tony s Well-Known Member

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    What about the measured values of the actual production output? Are the actual outputs stay at the target? What are the maximum and minimum measured values? Have you compared the process spread against the specification spread? Control charting is a valuable tool in reducing the variation on the process in order to improve capability.
     
  3. Miner

    Miner Moderator Staff Member

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    SPC is independent of specifications. SPC is used to help you stabilize your process making it predictable and minimizing the resulting process variation. The resulting stable process may or may not be capable of meeting the specifications. However, even if it does not meet specifications, it will have fewer defectives than if it were not stable.
     
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  4. luckycharm

    luckycharm Member

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    Hi Sir Tony S.,
    So far the measured values are within the specification limits which we can say that the process is stable and in control thus control charting is deemed unnecessary anymore. Thanks!
     
  5. luckycharm

    luckycharm Member

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    Hi Miner,
    Yes I recognize the fact that SPC is use to analyze and improve process variations.
    My confusion is, when in my SPC training, the Trainor said " no need to do control charting when the specification tolerance is narrow".
    SPC or Control Charting is best used when the process specification tolerance is at least with 10 range gap because measurement variability is high.

    Any insight on that note?!
     
  6. Andy Nichols

    Andy Nichols Moderator Staff Member

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    I'd suggest maybe the trainer was adding some hypothesis?
     
  7. luckycharm

    luckycharm Member

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    what do you mean by this?
     
  8. tony s

    tony s Well-Known Member

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    You need to compare your Process Spread (6sigma) with the Specification Spread (USL-LSL). If the 6sigma is well within the USL-LSL (see the figure below), then you can consider simplifying your control charting method. Example: instead of using a variable control chart, substitute it with an attribute control chart.
     

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  9. Miner

    Miner Moderator Staff Member

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    Actually, you cannot assume the process is stable and in control simply because the measured values are within spec. Only by plotting the values on a control chart can you tell whether the process is stable and in control.
     
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  10. Andy Nichols

    Andy Nichols Moderator Staff Member

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    This hypothesis...
     
  11. Bev D

    Bev D Moderator Staff Member

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    O the garbage people teach...

    Miner is correct. The thought that a process that within specification is stable and in control is rubbish...

    SPC is appropriate regardless of the data's variation compared to the spec limits. Even a stable process that runs within spec can - and will - shift or drift over time. The whole purpose of SPC is to detect these changes BEFORE product goes out of spec. and to maintain reduced variation when characteristics are subject to stack-ups, wear etc. I have attached a brief excerpt of my SPC training material that begins with just two examples of 'in spec' processes that went out of control and we didn't detect it until bad parts were made because we weren't using SPC. the examples include the multi-vari of the actual variation and the specification and the accompanying SPC chart if were to have had one.
     

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