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FAILSAFING AND MEASUREMENT CONTROL CHARTS

FAILSAFING AND MEASUREMENT CONTROL CHARTS . John R. Grout & Brian T. Downs Cox School of Business, Southern Methodist University. Outline. Shingo’s views on SPC Introduction to Failsafing Using self-checks instead of SPC Source inspection’s relationship with SPC

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FAILSAFING AND MEASUREMENT CONTROL CHARTS

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  1. FAILSAFINGANDMEASUREMENT CONTROL CHARTS John R. Grout & Brian T. Downs Cox School of Business, Southern Methodist University

  2. Outline • Shingo’s views on SPC • Introduction to Failsafing • Using self-checks instead of SPC • Source inspection’s relationship with SPC • Conclusions about the using failsafing

  3. Shingo On SPC: • “statistics is no more than qualified guess work.” • “...control charts only help maintain the accepted defect rate -- they cannot reduce defects to zero. • “It took 26 years for me to free myself completely from the spell of inductive statistics.” • a look at SQC methods as they are actually applied shows that feedback and corrective action ... are too slow to be fully effective.”

  4. A recent quality listserv posting Subject: Shingo's poke yoke system: Shingo presents and expands on some rather interesting concepts, but he is also somewhat "blasphemous " when it comes to SQC. SQC may be statistically sound, but it is not always the best way to get to zero defects. Comments pro and con would be appreciated. Thank you, .....@someserver.com

  5. Motorola’s findings about Cp >2 ...it became evident early in the project that achieving a Cp greater than 2 would go only part of the way. Mistake-proofing the design would also be required ... Mistake-proofing the design is an essential factor in achieving the [total number of defects per unit] goal. Smith, B. IEEE Spectrum 30(9) 43-47

  6. 3 kinds of inspection Judgment Inspection—Sorting out the defects Informative inspection—provides information about the process that is used to control the process more effectively. Source Inspection—Checks proper operating conditions and prevents a mistake from being made

  7. Four Elements of Failsafing Informative Inspection: 1. Successive Checks 2. Self -Checks 4. Failsafing Devices 3. Source Inspection

  8. Examples • Stem tightener • GM-nut welding

  9. ZQC vs. SPC: X-Bar Charts • ZQC uses specification limits as the signal for remedial action. • SPC uses control limits as the signal to adjust the process.

  10. Comparison of Control limits and Specification Limits When Cp = 1

  11. Comparison of Control limits and Specification Limits When Cp < 1

  12. Comparing ZQC and SPC for Individuals When Cp < 1

  13. Comparison of Control limits and Specification Limits When Cp > 1

  14. Comparing ZQC and SPC for Individuals When Cpk > 1

  15. ZQC Is Inferior to SPC whenever

  16. Using Control Limits and Poka-Yoke • Use go/no-go gauge to perform 100% inspection using control limits. • Go/no-go gauges convert measurement data into binomial data. • Information lost: -- find patterns in data -- ability to track the process variance. • ZQC without measurement sampling would not lead to on-going variance reduction and quality improvement.

  17. Prevention Using SPC • The purpose of SPC is to indicate the existence of a special cause. Once its existence is known, efforts are made to determine the special cause. The special cause should be eliminated and step are taken to prevent the cause from recurring. • SPC is silent about how to prevent the cause from recurring.

  18. Prevention Using Source Inspection • “It had dawned on me that the occurrence of a defect was the result of some condition or action, and that it would be possible to eliminate defects entirely by pursuing the cause.” (Shigeo Shingo) • Source inspection is 100% inspection of the presence or absence of causes. • Source Inspecting causes using failsafing devices is appropriate for binary decisions

  19. Source inspection’s relationship with SPC • Source Inspection and SPC can be used together: • Use SPC to identify the existence of special causes • Use source inspection to prevent their recurrence

  20. Conclusions about failsafing • Shingo’s self-checks “only help maintain the accepted defect rate [as measured by process capability] -- they cannot reduce defects to zero.” • SPC is the preferred method of informative inspection for measurement data. • Source inspection can be used effectively in combination with SPC

  21. Current and Prior ResultsSPC and Mistake-proofing

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