This project involved an engine component which needed capability improvement.

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Background: 

Capability is measured at final assembly.  Measurement determines selecting mating components based on tolerance matching.

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Part bodies are machined on automatic screw machines.  There are 23 characteristics on each part body.

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SPC is used to control each part body characteristic at machining.

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Cpk at final assembly is below 0.9.

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SPC charts are posted in a war room.

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Characteristics have individual action plans for improvement.  No improvement action has improved Cpk.

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Solution:

SPC charts showed trends from characteristic to characteristic appeared to be correlated.  

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The individual characteristic trends resembled the individual trends of stocks in the stock market.  

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Using a Box-Jenkins Time Series Analysis (cross correlation study), it was demonstrated that over-control was being done.  By limiting to three (3) characteristics under control, the over-control was corrected.

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Results:

Capability improved to 1.3 Cpk.