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Cpk for dimension which require min and max (to check position inside USL and LSL)

Discussion in 'SPC - Statistical Process Control' started by danut horincas, Mar 24, 2023.

  1. danut horincas

    danut horincas New Member

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    Hello ,
    What are normal aproach in these situation?Cpk for min +Cpk for max or Cpk for both min and max in the same time (in the same lets say tabel /chart)

    Thanks
     
    danut horincas, Mar 24, 2023
    #1
  2. Bev D

    Bev D Moderator Staff Member

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    Welcome danut :)
    The ‘normal’ approach is to list the worst (lowest) one. Some people advocate for reporting both values especially if both are less than 1.33.

    However, I always ask why you are trying to calculate Cpk? Is it required by a Customer? The process capability indices are really not very helpful or statistically very valid. They are more misleading than helpful.
     
    Bev D, Mar 24, 2023
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  3. Miner

    Miner Moderator Staff Member

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    These are normally designated as Cpku (for USL) and Cpkl (for LSL). Cpk = Minimum (Cpku, Cpkl).
     
    Miner, Mar 24, 2023
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    Bev D likes this.
  4. danut horincas

    danut horincas New Member

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    I measured pieces with ODmin =29.94 mm and ODmax =29.98 mm because of ovality .USL are 30.2 mm and LSL are 20.87 mm.
     
    danut horincas, Mar 25, 2023
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  5. tony s

    tony s Well-Known Member

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    This would mean that your process has a tendency to produce parts that will go outside the USL than the LSL. Your process capability is lower at the USL than at the LSL.
     
    tony s, Apr 8, 2023
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