Failsafing and measurement control charts
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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

FAILSAFINGANDMEASUREMENT CONTROL CHARTS

John R. Grout & Brian T. Downs

Cox School of Business, Southern Methodist University


Outline
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


Shingo on spc
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.”


A recent quality listserv posting
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


Motorola s findings about c p 2
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


3 kinds of inspection
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


Four elements of failsafing
Four Elements of Failsafing

Informative Inspection:

1. Successive Checks

2. Self -Checks 4. Failsafing Devices

3. Source Inspection


Examples
Examples

  • Stem tightener

  • GM-nut welding


Zqc vs spc x bar charts
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.




Comparing zqc and spc for individuals when cp 1
Comparing ZQC and Cp < 1SPC for Individuals When Cp < 1





Using control limits and poka yoke
Using Control Limits and Poka-Yoke Cp > 1

  • 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.


Prevention using spc
Prevention Using SPC Cp > 1

  • 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.


Prevention using source inspection
Prevention Using Source Inspection Cp > 1

  • “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


Source inspection s relationship with spc
Source inspection’s relationship with SPC Cp > 1

  • Source Inspection and SPC can be used together:

    • Use SPC to identify the existence of special causes

    • Use source inspection to prevent their recurrence


Conclusions about failsafing
Conclusions about failsafing Cp > 1

  • 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


Current and prior results spc and mistake proofing
Current and Prior Results Cp > 1SPC and Mistake-proofing


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