FAILSAFING AND MEASUREMENT CONTROL CHARTS

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FAILSAFINGAND MEASUREMENT CONTROL CHARTS

• John R. Grout Brian T. Downs
• Cox School of Business, Southern Methodist
  University

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Outline

• Shingos views on SPC
• Introduction to Failsafing
• Using self-checks instead of SPC
• Source inspections relationship with SPC
• Conclusions about the using failsafing

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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.

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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, ....._at_someserver.com

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Motorolas findings about Cp gt2

• ...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

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3 kinds of inspection

• Judgment InspectionSorting out the defects
• Informative inspectionprovides information about
  the process that is used to control the process
  more effectively.
• Source InspectionChecks proper operating
  conditions and prevents a mistake from being made

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Four Elements of Failsafing

• Informative Inspection
• 1. Successive Checks
• 2. Self -Checks 4. Failsafing Devices
• 3. Source Inspection

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Examples

• Stem tightener
• GM-nut welding

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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.

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Comparison of Control limits and Specification
Limits When Cp 1
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Comparison of Control limits and Specification
Limits When Cp lt 1
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Comparing ZQC and SPC for Individuals When Cp lt 1
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Comparison of Control limits and Specification
Limits When Cp gt 1
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Comparing ZQC and SPC for Individuals When Cpk gt 1
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• ZQC Is Inferior to SPC whenever

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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.

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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.

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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

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Source inspections 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

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Conclusions about failsafing

• Shingos 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

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Current and Prior ResultsSPC and Mistake-proofing