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Your comments are welcome. If you have any additional information about poka-yoke, mistake proofing and related topics, please send me a note to jgrout@campbell.berry.edu or call (214)768-2597.
John R. Grout, Cox School of Business
Southern Methodist University
ASQC: Texas Quality Expo
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What will Quality be in the year 2000?
2 & 3. Less advantage because everyone left in the market will be doing it.
4. "packaged" cookie-cutter approaches will not be sufficient
5. More cost-effective because you start with pareto charts that have big spikes. limited changes have big impacts. As the big impact projects are completed there are "diminishing returns" Solutions that were justified for big pay backs are not justified for smaller projects.
What causes defects?
Cultural factors include awareness, attitudes, incentives, reward systems, commitment, etc.
Variance is the random and inherent differences between process outcomes or outputs
Complexity is loosely defined as opportunities for defects to be caused. It corresponds to the number of separate parts, lack of part commonality, etc.
Mistakes are human errors that result from incorrect intentions or executing correct intentions that result in unintended outcomes.
What tools are used to achieve zero defects?
In my opinion, the tools at the top of the page tend to be more widely known and used than the tools at the bottom of the page.
That's why I am very pleased to have this opportunity to talk to you today. I want to inform you about some very powerful, under-utilized tools.
They are very easy to understand.
They are grounded in common sense.
They tend to be inexpensive to implement.
Good examples probably already exist in your facility
My goal is to get you to notice the existing examples and to create more examples (A change in focus).
Mistakes: to err is human
Have you ever done the following:
It happens to workers too.
The mistakes I would like to focus on are some times called slips. They occur when an action is executed on "autopilot" but has an unitended result.
Mistake-proofing usually(but not always) will involve precluding behaviors that under other circumstances would be correct.
Putting green parts on widgets is a correct behavior when the widgets are green, but not when they're red.
You have two options:
If your paying a good wage and really want your money's worth, make your employees earn their money. Demand vigilance, expect their full attention to repetitive jobs for a full shift. Judge them on outcome.
If you don't feel that you need to make employees jobs harder, If you think their attention could be used in other more humane & profitable ways , Mistake-proof the process.
Ever seen and error cause removal or corrective action that says "instructed worker in correct procedure and gave disciplinary warning"? Is the error cause really removed?
A New Attitude toward Preventing Errors
"Think of an object's user as attempting to do a task, getting there by imperfect approximations. Don't think of the user as making errors; think of the actions as approximations of what is desired."*
*Source: The Design of Everyday Things, by D.A. Norman, 1988, Doubleday
Quality professionals are not the only group interested in the outcomes of processes. Psychologists have been interested in human error. Here's what Donald Norman said in a very interesting book titled The design of every day things
The human brain's default mode of operation is pattern recognition and autopilot execution. If the pattern is familiar, a behavior that has been successful in the past is "launched." It's only when feedback suggests that things are not going as planned that more in-depth though is called up.
A New Attitude toward Preventing Errors
*Source: The Design of Everyday Things, by D.A. Norman, 1988, Doubleday
Donald Norman has some recommendations for responding effectively to how the human mind works.
What would YOU do?
GM welding machine
A) Make a worker responsible for it
B) Track the proportion of missing nuts on a chart
C) Increase preventative maintenance
D) Make the machine stop when the isn't present
E) Change the fasteners to a "clip-in" device
"We have an operation which involves welding nuts into a sheet metal panel. These weld nuts will be used to attach parts to the car later in the process. When the panel is loaded by the operator, the weld nuts are fed automatically underneath the panel, the machine cycles, and the weld nuts are welded to the panel. You must remember these nuts are fed automatically and out of sight of the operator, so if the equipment jams or misfeeds and there is no part loaded, the machine will still cycle. Therefore, we have some probability of failure of the process. An error of this nature is sometimes not detected until we actually have the car welded together and are about to attach a part where there is not a nut for the bolt to fit into. This sometimes results in a major repair or rework activity.
To correct this problem, we simply drilled a hole through the electrode that holds the nut that is attached to the panel in the welding operation. We put a wire through the hole in the electrode, insulating it away from the electrode so as it passes through it will only make contact with the weld nut. Since the weld nut is metal, it conducts electricity and with the nut present, current will flow through, allowing the machine to complete its cycle. If a nut is not present, there will be no current flow. We try to control the process so that the machine will actually remain idle unless there is a nut in place."
Ricard, L.J., "GM's just-in-time operating philosophy", in: Y.K. Shetty and V.M. Buehler, (Eds.)., Quality, Productivity and Innovation. Elsevier Science Publishing, New York, 1987, pp. 315-329.
(Making wrong actions more difficult)
What would YOU do?
L'Hotel Louis XIV
A) Hire a floor manager who can unlock doors when needed
B) Rent only half the rooms in the hotel
C) Tear down and build new building with a bathroom for every room
D) Use a leather strap to keep both doors closed
E) Use electric door locks like a car
Making incorrect actions correct
Unlocking only one door is an incorrect action. But if by unlocking one door both are unlocked, the action becomes correct.
What would YOU do?
Nissan Stanza Van
A) Put a warning label on the fuel door and the sliding door
B) Reinforce the fuel pipe to withstand impact
C) Create a locking system that must be activated in order to take the gas cap off which keeps the sliding door from opening
D) Move the fuel door etc. out of the way of the sliding door (e.g. to the other side)
A)Variation of "be more careful"
B)
C) Nissan chose "C"
D) the best of all options is to make error impossible
Approaches to Mistake-Proofing
Source: Yoshikazu Tsuda, in Quality through Engineering Design (ed. Kuo) 1993.
On those occasions were mistake-proofing is needed there are a variety of approaches that can be used:
1. Reduce ambiguity and confusion in the work place
2. When an error occurs, bells and whistles go off immediately. Process can be reset & rework can be
performed at minimum cost.
3. once a cause is known, you need to prevent that cause from re occurring. Source inspection means inspecting the source of defects or errors. It insures that prerequisites for high quality production are met.
4. If mistakes are inevitable, can we make the impact of the error insignificant?
Examples of Mistake-Proofing
3.5" diskette you cannot insert it incorrectly
Binney and Smith Crayola crayons, light sensor checks each box & stops machine if box is not complete.
Trinity Industries, Layout jig eliminates measuring and makes omitted parts obvious at a glance.
Automobiles key in to turn steering wheel & shift out of park. put in park before key can be removed. (also buzzers, bells, warning lights, etc.)
Computer Software, common DOS error: bad command or file name In Windows you can't point and click something that is not there
Mail-order computer, if you open the wrong end of the box a warning message is displayed on the inner flaps of the both. if the correct end is opened, the process is controlled to minimize errors & ambiguity.
Where Mistake-Proofing Works Well
1. manual operations where worker vigilance is needed
2. where mispositioning can occur.
3. where adjustment is required
4. where teams need common-sense tools and not another buzz-word.
5. where SPC is difficult to apply or apparently ineffective.
Where Mistake-Proofing Works Well
6. where attributes not measurements are important.
7. where training cost and employee turnover are high.
8. where mixed model production occurs.
9. where customers make mistakes and blame the service provider.
10. where special causes can reoccur.
11. where external failure costs dramatically exceed internal failure costs.
6. better for replacing p-charts than for xbar/R charts
7. McDonald's designed system to be mistake-proofing and easy to learn.
8. pokayoke helps keep track of what is being done (Toyota example: correct part doors open automatically after product bar code is read.)
11. In the short run, the cost of quality may be reduced by sorting out defects. (Internal failures cost less than external failures.)
Where Mistake-Proofing does NOT work well
1. Destructive tests.
2. Production rate is very fast.
3. Shifts occur more rapidly than they can be responded to.
4. Self-checks when control charts are used effectively.
no tool is applicable everywhere
1. 100% destructive testing not recommended.
2. as production rate speeds up, cost of momentary delays due to inspection become very costly.
3. If true, you got bigger problems to worry about
4. Despite what you'll probably read if you look into poka-yoke further, SPC is better for managing variance. Poka-yoke is better for mistakes. If someone says pokayoke should replace SPC don't believe it.
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
Sandia Labs (Rook, SCTM93-62(14)) did experiments in 1962 that show undetected omissions occur about once in 33,000 operations. A simple eight step operation will result in 240 PPM due to mistakes. This is much higher than the 3.4 PPM goal of Motorola's six sigma program.
Shingo Shigeo : Defects = Ø is absolutely possible!
John Grout: If Defects = Ø is possible, Mistake-proofing will be in the toolbox of those who achieve it.
Some final evidence
Source: Productivity Inc. and Shingo prize profiles
Let's not wait 30 years...
John P. Lafferty wrote:
Shingo brought his Poka-Yoke devices to America in the mid-'80s...Unfortunately, the reception to Shingo's methods in this country is similar to our response to Dr. W. Edwards Deming in the 50's. It took us 30 years to become convinced Deming was right about statistical control. Must we wait 30 more to believe Shingo?
Please let us combine Deming's control charts and Shingo's control devices, so we all can produce world-class products and guarantee our customers defects equal zero with every shipment.