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18_posting_-_Statistical_Process_Control

18_posting_-_Statistical_Process_Control - Statistical...

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11/19/2010 1 Statistical Process Control (SPC) "By a small sample, we may judge of the whole piece." Miguel de Cervantes
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11/19/2010 2 In order to measure quality we need… The average (mean) of the observations: = = N i i x N X 1 1 The standard deviation of the observations: N X x N i i = - = 1 2 ) ( σ
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11/19/2010 3 Average & Variation example Number of pepperoni’s per pizza: 25, 25, 26, 25, 23, 24, 25, 27 Average: Standard Deviation: Number of pepperoni’s per pizza: 25, 22, 28, 30, 27, 20, 25, 23 Average: Standard Deviation: Which pizza would you rather have?
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11/19/2010 4 When is a product good enough? Incremental Cost of Variability High Zero Lower Tolerance Target Spec Upper Tolerance Traditional View The “Goalpost” Mentality a.k.a Upper/Lower Design Limits (UDL, LDL) Upper/Lower Spec Limits (USL, LSL) Upper/Lower Tolerance Limits (UTL, LTL)
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11/19/2010 5 But are all ‘good’ products equal? Incremental Cost of Variability High Zero Lower Spec Target Spec Upper Spec Taguchi’s View “Quality Loss Function” (QLF) LESS VARIABILITY implies BETTER PERFORMANCE !
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11/19/2010 6 Capability Index (C pk ) It shows how well the process can produce the performance measure to fit the design specification - - = σ σ 3 or 3 min X UTL LTL X C pk
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11/19/2010 7 Example 1: Capability Index (C pk ) X = 10 and σ = 0.5 LTL = 9 UTL = 11 UTL LTL X
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11/19/2010 8 Example 2: Capability Index (C pk ) X = 9.5 and σ = 0.5 LTL = 9 UTL = 11 UTL LTL X
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11/19/2010 9 Example 3: Capability Index (C pk ) X = 10 and σ = 2 LTL = 9 UTL = 11 UTL LTL X
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11/19/2010 10 Capability Index (C pk ) It shows the % of + 3 σ the process can produce to fit the design specification based on a certain tolerance level.
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