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Session 1547

TEACHING ZERO QUALITY CONTROL CONCEPTS IN MECHANICAL

ENGINEERING TECHNOLOGY

Ratan Kumar & Bill Watt

Department of Engineering Technology

University of North TexasDenton, TX 76203.

ABSTRACT

Zero Quality Control (ZQC) is a quality control approach for achieving zero defects. ZQC is

based on the principle that defects are prevented by controlling the performance of a process so

that it cannot produce defects, even when a mistake is made by the machine or a human operator.This is done by combining four basic elements1: i)source inspection ii) 100 percent inspection

iii) immediate feedback and iv) use of poka-yoke (mistake-proofing) devices. ZQC is widely

gaining popularity in the industry. It is well established in Japan and its practice is catching on inUSA. At the department of mechanical and manufacturing engineering technology at the

University of North Texas, we strongly feel that this important tool needs to be addressed in our

Quality Control class. A plan has been made to cater to this desire, and strategies have been

made to incorporate it in other classes as well.

INTRODUCTION

Shigeo Shingo2is credited with starting the Zero Quality Control (ZQC) quality system. He was

a leading proponent of statistical process control in Japanese manufacturing in the 1950s, but

became frustrated with the statistical approach as he realized that it would never reduce productdefects to zero. Statistical sampling implies that some products to go untested, with the result

that some rate of defects would always reach the customer. ZQC tries to achieve zero defects in

all products. This helps in maintaining customer satisfaction and loyalty, in reducing the costassociated with scrap, rework and downtime, and in attaining a companys ability to adopt lean

production methods with smaller inventories.

The majority of the defects that arise inside a factory3originate in four Ms (Materials,

Machinery, Manpower, Methods) and one I (Information). However most of the defects can be

traced to the people involved in the above sources. Simple mistakes are the most common causeof defects, and they are the hardest to prevent. Since the goal of ZQC is to prevent all defects,

these simple mistakes must be caught first. ZQC does not point fingers after the mistake has beenmade or hassles people to perform better next time. Instead it uses devices to keep errors fromever turning into defects in the first place. It talks about mistake-proofing the process and not

fool-proofing it. ZQC uses control function that ensures that the necessary conditions are

present to make good products.

The traditional quality improvement cycle1is Plan, DO, Check (figure 1a).This cycle catches

and corrects defects after they occur, but it cant make sure that work is done according to plan


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in the first place. The ZQC approach integrates the Check and Do stages. This gives instantfeedback so problems can be corrected before defects happen.

Figure 1a): Traditional Quality Improvement Cycle1 Figure 1b) ZQC1

ELEMENTS OF ZQC SYSTEM

The ZQC system clearly recognizes that to err is human. It is letting the errors turning into

defects that causes the problem. ZQC prevents defects by integrating Check and Act stages of the

quality improvement cycle as shown in Figure 1b. It does it by combining four basic elements:

1.

It usessource inspectionto catch errors before they become defects. There are three basicapproaches to the inspection of products: judgement inspection, informative inspection and

source inspection. In judgement inspection, a person or machine simply compares the

product with a standard, discovers items that dont conform and rejects them as defects.However it does not prevent the defect from occurring as this type of inspection generally

takes place at the end of the process. On the other hand informative inspection focuses on the

defect-producing process of a problem so that the problem can be corrected. StatisticalQuality Control (SQC), successive checks and self-checks are examples of informative

inspection. SQC relies on sample rather than checking every unit. As a result it cannot ensure

that every product is good. In successive check (Figure 2), people in the next process inspectthe units that are passed on to them and there is a delay in the detection of mistake. In self-

checks (Figure 2) the operator or assembler checks his/her own work afterwards for defects.It gives a quicker feedback but cannot catch all defects. Source inspection (Figure 3) catcheserrors and gives feedback about them, before further processing can take place, so that the

errors do not turn into defects. This is the integration of Check and Act stages that was

discussed earlier. This inspection is done by humans as well as by poka-yoke devices, which

will be discussed later. An example of source inspection might be using a switch that haltsthe equipment if a part is fed in upside down, or a pin that physically prevents the insertion of

a workpiece the wrong way.

DO-

CHECK

PLAN

CHECK DO

PLAN


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Figure 2. Successive Checks and Self-Checks inspection process1

Figure 3. Source inspection keeps errors from turning into defects.1

2. The second unique element of the ZQC system is that it does a source inspection on every

single product i.e. a 100 percent inspection. This is different from SQC, which presupposes

that a certain level of defects is unavoidable. This is inconsistent with the goal of zero

defects. Poka-yoke devices can be used to perform 100 percent inspection on all products atdifferent phases of production.

3. The third element of ZQC is quick feedbackso that errors can be corrected right away.

Traditional inspection methods come short in doing this. The reason is traditional techniqueshappen after the process when errors have already turned into defects. In some situations

they may not even tell the process that a bad product was made. When they do inform the

process, time has already passed and either the process has churned out more defects, or the

conditions that caused the initial defect no longer exist and cant be learned from. In ZQC theinspection is carried out by a system that signals the operator or assembly person about

mistakes and machine errors before they become defects.

4. The fourth element that is unique to and the most important part of ZQC are the use ofmistake-proofing systems calledpoka-yoke. Rather than relying on operators to catch their

own errors or of those of the previous process, ZQC uses poka-yoke devices installed in the

machine to do source inspection and give quick feedback 100 percent of the time. Since thisis the heart of ZQC process, it is being discussed in more detail.


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POKA-YOKE SYSTEMS

Poka-yoke is Japanese for mistake-proofing. These devices are used either to prevent the specialcauses that result in defects, or to inexpensively inspect each item that is produced to determine

whether it is acceptable or defective. A poka-yoke device is any mechanism that either prevents a

mistake from being made or makes the mistake obvious at aglance. It is used to carry out 100 percent inspection and provides immediate feedback and action

when errors or defects occur. However as Shingo2himself mentions, these system are a meansand not an end. Simply putting in such devices does not eliminate defects. It has to be supported

by successive checks and self-checks.

Various examples of poka-yoke system can be had from reference 1,2 and 3. Jon Grout4

maintains an elaborate page on this subject matter. Ricard, L.J.5,

described an example of a poka-yoke device at General Motors (GM). "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 operator loads the panel, the weld nuts are fed automatically underneath the panel, themachine 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 ofthe 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 weldingoperation. We put a wire through the hole in the electrode, insulating it away from the electrode

so as it passes throughit 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 tocomplete 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."

Figure 4 shows some examples of poka-yoke devices employed for defect prevention anddetection.

Figure 4. Poka-Yoke devices.


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ZQC IN MECHANICAL ENGINEERING TECHNOLOGY

Although the concepts of ZQC can be taught in the Quality Assurance course, that is required ofevery mechanical student, its practice can be initiated in several courses. The idea of detecting or

preventing mistakes early on lends itself not only to the laboratory classes but to some classroom

lectures as well. Currently the students of mechanical engineering technology at the Universityof North Texas take about eleven technical courses that have a laboratory attached to them. The

course were it can be first initiated is the first manufacturing course encountered viz.Manufacturing Processes and Materials. Here the students are introduced to conventional

manufacturing tools, equipment and processes. Many of the equipment used are equipped withpoka-yoke devices but there are many areas were mistake proofing can be further extended

specially for teaching purposes. Similarly during the manufacturing process there are several

steps eg. heat treatment, hot and cold forming, chip removal technique etc., that can benefit byintroducing poka-yoke techniques.

Implementing ZQC in the theory class is another challenge, but automating the process can go a

long way in meeting this task. For example let us look at the machine design class. The studentsfrequently encounter situations where they have to look up values from various tables or graphs,

convert from one unit to another, select an appropriate material etc. These are some perennial

sources of errors and even for students who perfectly understand the theory it leads to incorrectanswers. Even during the process of number crunching in the calculator, errors occur if the

students are not careful. This is so common that we often see the student enter the numbers

several times to check the answer. These are some classical cases of human error in the design

area and leads to erroneous results. Automating the process on a computer can eliminate some ofthese inconvenient activities. A program can be built so that: a) the user can correctly get values

from specific graphs or tables, b) unit conversion can be done, c) selection of materials can be

had by proper query and d) some hints are provided alongway. As compared to a calculator,computers have a much better display and it vastly reduces the error during number entry. Also

since the user enters the values in a tested code, his answers are more reliable than by entering

mathematical variables in a calculator. A properly coded computer program can instantly discard

some wrong data as well. Although several design software are currently there to automate theprocess, yet they tend to overlook certain small things that can make the design process flawless.

In the Mechanical Engineering Technology program at the University of North Texas we have

embarked on the ides of creating some these tools so that the design value entries can be madecorrect. The tool for unit conversion is complete and it recognizes any combination of the

approximately 500 built-in SI, metric, English, and historical measures. Some of its other

features are:

Uses a natural language interface for expressing measurements. For example, converting from "10 cups" to "liters" returns 2.37.

Measurements can be preceded by any of 50 known prefixes.

Measurements are known by their common names, abbreviations, acronyms, and synonyms. For example, "kilometers per hour" can be entered as

"KPH", "km/hr", "kmeters/hour", "kilometers per hour", etc.

Understands any combination of the known units and/or prefixes. For example, converting "45 ft*lbs/second^2" (a measure of force) to "newtons" returns 6.22.


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Returns the remaining units when the units being converted do not represent the same type of physical measurement. For example,

converting "12 joules" (a measure of energy) to "newtons" (a measure

of force) returns "12 meters" (a measure of length).

Other tools are being currently designed keeping in mind the students of mechanical design.

CONCLUSION

ZQC, an important quality tool, is used to achieve zero defects. It combines steps that helps oneto either detect or prevent an error before it becomes a mistake. There are four basic features that

can be attributed to this method. Although it has been used in the manufacturing areas, its use ina class room setting has not been tested. The mechanical engineering technology program at the

University of North Texas wants the students to be exposed to this powerful tool. They plan to

introduce it in several manufacturing courses and its use in the design courses has just begun. Asoftware tool to convert units has been completed and has some attractive features. Several such

small tools are being envisioned that would make number retrieval and processing error free. In

turn this would make the design calculation, either by hand or by some other software, less prone

to human error, and which would capture the spirit of ZQC.

REFERENCES

1. Productivity Press; Mistake Proofing for operators; Portland, Oregon.2.

Shingo, Shigeo; Zero Quality Control: Source Inspection and the Poka-Yoke system;

Productivity Press, Portland, Oregon.

3. Hirano, Hiroyuki; Poka-Yoke: Mistake Proofing for Zero Defects, PHP Institute Inc., NewYork.

4. Grout, J.R. , Edwin L. Cox School of Business, Southern Methodist University, Dallas, TX .

5. 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.

BIOGRAPHICAL INFORMATION

RATAN KUMAR obtained a BME in Mechanical Engineering from Jadavpur University (India), ME in Nuclear

Engineering from the University of Florida and doctorate in Nuclear/Mechanical Engineering from University of

Florida. He has worked as a Mechanical Engineer for three years and since 1992 has taught at American Technical

Institute and currently at the University of North Texas.

GEORGE W. WATT obtained a BS in Mechanical Engineering from the University of Wyoming, MS From the

Air Force Institute of Technology in Aerospace Engineering, and doctorate in Metallurgical Engineering from Ohio

State University. He spent 23 years in the US Air Force and has taught at Utah State University and currently at the

University of North Texas since leaving the USAF in 1985.

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