Six Sigma Fundamentals: A Complete Introduction to the System, Methods, and Tools

2017-07-07 02:10:07

Poka-yoke—Mistake proofing

Shigeo Shingo is considered the father of poka-yoke, and is often quoted as saying, "The idea behind poka-yoke is to respect the intelligence of workers by taking over repetitive tasks or actions that depend on the vigilance of memory." This process includes adding features to a design or process to assist the operator in the performance of the task.

Poka-yoke, in any of the various spellings, is another name for error- or mistake-proofing a design or process. The focus of poka-yoke is to sensitize us about the differences between prevention and detection and to do something about it. Prevention prevents errors from occurring or prevents those errors from causing defects. Detection identifies a defect and immediately initiates a corrective action to prevent multiple defects from leaving the workstation.

Detection devices are used to deal with an error that is difficult to eliminate, or is in the process of being located. The main idea of a prevention approach is to keep an error from producing multiple defects. Sometimes the error (or root cause) of the defect is hard to find. In this case, it is often profitable to create solutions that detect and react to an error or a defect instead of preventing an error or a defect. Such devices are detection devices.

Using detection devices in error-proofing is different from regular product or process inspection. Error-proofing initiates a corrective action once an error or defect has been detected. Regular product or process inspection should not be referred to as error-proofing unless the inspection is tied to an immediate corrective measure. An example of inspection that is not tied to an immediate corrective action is SPC (statistical process control) or a continual inspection with a process controller.

A number of criteria should be considered when choosing between a prevention or detection approach. The proposed device must be considered for its ability to:

Prevent an error that causes the defect, or initiate a corrective action before multiple defects occur.

Be designed and installed quickly and easily.

Be cost-effective to implement and easy to maintain.

The need for poka-yoke

History shows that no matter how much we train the operator or document the process, human error occurs. Poka-yoke is the methodology of reducing or eliminating human error, which causes defects. The methodology is based on two essential attitudes about human behavior: mistakes are inevitable, and errors can be eliminated. Based on these attitudes the following corollary assumptions may be made about work processes and workers:

Few workers make errors intentionally.

Error is inherent in the nature of humans.

Human errors are invited to occur by processes that do not use error-proofing.

A poka-yoke methodology alters the work environment with a goal of reducing human errors and their defects.

What is the current use of poka-yoke?

Poka-yoke is currently incorporated into the design and manufacturing processes and products used by many companies as a tool to stay globally competitive. It improves product quality for both internal and external customers by preventing defects from getting into the product. (In some cases, the distinction is made between error-proofing and mistake-proofing. The first is associated with design, the second with manufacturing.)

Mistake-proofing reduces costs primarily by reducing waste, because when less material is scrapped, more material is transformed into saleable product. Error-proofing improves the design by allowing a greater quantity of good products to reach the final customer without rework. Error-proofing also improves the quality of the workers' lives by improving worker safety. It is not always necessary to create all designs and processes with error-proofing in mind. Trade-offs among quality, price and delivery must be considered. Sometimes focusing on mistake-proofing is just as acceptable.

Error-proofing benefits

The earlier an error is identified, the less cost is invested in that part. If a bad or defective part is not identified until final inspection, it will go through all areas of the plant pointlessly, as it is not able to be used. All of the labor and material used in the plant operations will have been wasted. Therefore, one of the goals of error-proofing is to identify errors as close as possible to the point at which they are created.

For example, consider a door that has a defect. If the defect is not caught at the time it occurs, the door will travel through all areas of the plant, be installed, painted and have trim applied. All of those processes will waste time and material on a defective part that should have been spent on usable parts.

When analyzing the cost of defective parts versus the cost of implementing a mistake-proofing solution, consider the following factors in the equation:

Cost of time to work on the defective parts at all stations between the point where the error is made and the point where the error is identified.

Cost of all materials used at the stations between the point where the error is made and the point where the error is caught.

Cost of time spent in rework.

Cost of material used for rework.

Cost of time spent inspecting defective parts and/or inspection processes that would no longer be necessary with the proposed error-proofing solution.

Lost production due to time spent on defective parts.

Impact of lower customer satisfaction and lower worker morale resulting from producing defective parts.

Definitions

To understand poka-yoke, we must first distinguish between errors and defects. The two are not the same.

Errors. An error is any deviation from a specified manufacturing process. Errors can be made by machines or people and can be caused by previous errors. Not all errors result in defects, but all defects are created by errors. This means that if errors can be prevented, defects will be avoided. Errors that cause defects are located and eliminated through the use of error-proofing tools and procedures. The following items are examples of errors:

The wrong option package in a vehicle is sequenced into the assembly.

An oil sender unit is defective.

A hose clamp is not positioned correctly during the assembly.

A worn installation tool causes molding clips to be installed incorrectly.

A door is left open on an assembly finishing line.

Defects. A defect is the result of any deviation from product specifications that may lead to customer dissatisfaction. (In some industries the word defect is unacceptable for legal reasons. If that is an issue, then the term non-conformance is used). Product specifications are set by customer requirements, which are translated into design and manufacturing requirements. A "deviation from product specifications" means the product is not produced according to the manufacturing plan or requirements. A product must fall into one of two categories to be classified as a defect:

The product has deviated from manufacturing or design specifications.

The product does not meet internal and/or external customer expectations.

Note that the definition of a defect applies whether the receiver of the defective part is the final customer to receive the product (external customer), or the first operator outside of a group of processes in the manufacturing system (internal customer). Examples of defects (the result of errors) include:

A vehicle equipped with the wrong option package (e.g., options with premium sound, air, ABS or heavy-duty suspension).

An engine oil/coolant leak.

Squeaks, rattles or loose parts.

Parts that become damaged during assembly.

Now that we have examined the difference between errors and defects, let us look into the distinction between error-proofing and mistake-proofing.

Error-proofing. Error-proofing is based on two essential attitudes about human error: mistakes are inevitable, and errors can be eliminated. Error-proofing is a process improvement that is designed to prevent a specific defect from occurring. It is generally associated with design, since it is a system that:

Prevents personal injury.

Prevents faulty products.

Promotes job safety.

Prevents machine damage.

Error-proofing can have different definitions, but they all have the same core elements that focus on elimination as follows:

The application of tools and devices to a process to eliminate the possibility of errors occurring.

The application of tools and devices to a process to eliminate the possibility of defects that have occurred continuing on to the customer.

The use of functional design features to eliminate the possibility of parts being assembled incorrectly.

Mistake-proofing. Mistake-proofing, although often used interchangeably with error-proofing, focuses on reduction of the defect and is associated with manufacturing:

The application of tools and devices to a process to reduce the possibility of errors occurring.

The application of tools and devices to a process to reduce the possibility of defects that have occurred continuing on to the customer.

The use of functional design features to reduce the possibility of parts being assembled incorrectly.

Another way of differentiating between the two variations of poka-yoke is to say that mistake-proofing focuses on reducing risk, while error-proofing focuses on eliminating the risk of errors occurring.

Generic five-step process for poka-yoke

The generic 5-step process shown in Table 4.8 is a proven method that can be applied to most error-proofing situations. It shares a similar framework with many other methodologies, including phases for problem definition and solution review and implementation.

Table 4.8: Generic five steps for poka-yoke
Step 1: Define the problem What is the problem? Whom does it affect? (Internal/external customer) Where does the problem occur? When does the problem occur? How did the problem occur? How many defects are produced? Problem statement What will determine that the problem has been solved?	Tools Pareto diagrams Run charts Scatter diagrams Floor plans Brainstorming
Step 2: Implement Interim solution How can we keep the effects of this problem from affecting the internal/external customer? How do we contain the problem?)	Tools Brainstorming, etc.
Step 3: Define root cause People Method Machine Material Measurement Environment	Tools Fishbone diagrams Brainstorming Five whys Performance checklist
Step 4: Define and select solution Recommended action Rationale Pilot plan Problem solved according to criteria stipulated in step 1?	Tools Brainstorming Pilot programs Run charts Scatter diagrams
Step 5: Implement solution Solution Necessary communication

Generic mistake-proofing implementation process

Table 4.9 shows the generic mistake-proofing implementation process.

Table 4.9: Generic mistake-proofing implementation process
Task	Major steps	Specific individual characteristics
Preparation of application area	Step 1. Deliver mistake-proofing overview	Mistake-proofing definition Approaches to mistake-proofing Other definitions Mistake-proofing preparation and deployment steps Function of mistake-proofing devices Examples of mistake-proofing devices Benefits to mistake-proofing
	Step 2. Creating a mistake-proofing log	Establish format for collection, storage, maintenance of mistake-proofing information Designate persons) to update mistake-proofing log
	Step 3. Prioritizing defects collection points in the application area	Identify all data Collect existing quality data from plant sources Collect current and historical data from all collection points Categorize collected data according to defect type Summarize data for each defect type using a Pareto chart Select top 3 to 5 defects
	Step 4. Choosing defects	Complete part description Identify the defect Identify the financial impact of the defect Identify where the defect was found Identify where the defect was originally made Identify time and distance of defect Select defect for which mistake-proofing is to be developed
	Step 5. Document rationale for defect selection	Define the reasons for the selecting the defect Document the reasons for selecting defect in mistake-proofing log
Deploy mistake-proofing to application area	Step 1. Establish objectives for implementation of mistake-proofing	Review the current and historical data Estimate and document improvement Calculate the expected impact
	Step 2. Deliver mistake- and error-proofing implementation worksheet	Define and discuss the purpose of the mistake-proofing implementation worksheet (MPIW) Discuss the key components of the MPIW
	Step 3. Define the source error	Complete the heading area on the error-proofing implementation worksheet (EPIW) Describe the defect on the EPIW Document the defect rate on the EPIW Identify where the defect was found on the EPIW Identify where the defect was made on the EPIW Describe the current process where the defect was made on the EPIW Identify any errors or deviations from process standards on the EPIW Investigate/analyze the causes for each error/deviation
	Step 4. Create and install selected mistake-proofing devices	Develop ideas for improving, eliminating and detecting errors on the EPIW Create an error-proofing device and test for effectiveness
	Step 5. Measure, document and standardize results and benefits	Measure and document the improvement on the MPIW Measure and document the impact on the MPIW Compare the mistake-proofing results and objectives Update standardized work sheets and other appropriate documents
	Step 6. Notify other areas	Consider who may benefit from the improved process Fill in the appropriate suggestion form Consider ways to visually post improvement Congratulate the team on their achievement

Examples of poka-yoke elements

In a typical mistake-proofing situation there are three elements for consideration: inspection systems (i.e., successive check, self check or source), setting functions (e.g., contact, fixed value and motion step) and regulatory functions (e.g., control and warning). Using three distinct examples, let us examine these elements:

Example 1. Some manual transmissions allow drivers to start the car while in gear, causing the car to lunge forward or backward unexpectedly. In some newer models with standard transmissions, the car cannot be started unless the clutch is depressed.

Inspection system: self check.

Setting function: contact.

Regulatory function: control.

Example 2. Vehicle name plates are rotated 180 degrees, causing them to be installed upside down. A larger hole was created on the right-hand side of the install area and a smaller hole on the left-hand side. A large pin is placed on the right-hand side of the back of the name plate, with a smaller pin on the left.

Inspection system: source.

Setting function: contact.

Regulatory function: control.

Example 3. During the assembly of a simple pushbutton switch that consists of an ON button and an OFF button, workers sometimes forget to install the small springs that go under the buttons. When this error is discovered during a subsequent inspection or by a customer, company inspectors are sent to examine every switch assembled over that same time period. Faulty switches are disassembled and correctly reassembled. At the beginning of the operation, workers take two springs out of a box and place them on a small dish. If any springs remain on the dish after switch assembly, the worker recognizes that a mistake has occurred and reassembles the switch correctly.

Inspection system: self check.

Setting function: fixed value.

Regulatory function: warning.

Lean manufacturing process

The lean manufacturing error-proofing implementation process is another example of an error-proofing methodology at many corporations. This process is made up of two groups of tasks: preparation and deployment. Both tasks contain sequential steps to follow when implementing error-proofing; each is discussed in the following sections.

Preparation. This includes the start-up steps workers take when preparing to implement error-proofing. The goal is to create an understanding of the situation before rushing to implement a change. The steps for preparation are:

Deliver an error-proofing overview.

Create an error-proofing log.

Prioritize defects.

Choose defects.

Document the rationale for defect selection.

Deployment. This includes the steps to implement a standardized method of error-proofing. This is a proven process that, when followed correctly, will error-proof any given operation or piece of equipment. This phase should be started only after the preparation phase. The steps in deployment are:

Establish objectives for the implementation of error-proofing.

Complete error-proofing implementation worksheet.

Define the source error.

Create and install selected error-proofing devices.

Measure, document and standardize results and benefits.

Notify other areas.

Specific relationship of six sigma and lean

It appears that the lean methodology is running parallel to the six sigma philosophy. In fact, in some cases the six sigma methodology incorporates the concepts of lean manufacturing. That is because with lean, you have to know what your customers require and understand concepts such as mean time between failures with equipment in your process. You have to calculate takt times to know how much product you produce per second and match production to your customers' demands. In other words, in any lean environment, part of what you are trying to create is the atmosphere for people to look creatively at what they do on a daily basis and enable them to do it better and smarter. That is precisely what six sigma is trying to do, but in a slightly different way. The difference is that six sigma is focused on a systematic approach (via the DMAIC model) to eliminate variation, and the lean methodology is focused on eliminating waste through correction, overproduction, processing, conveyance, inventory motion and waiting by using the 5Ss (sorting, storage, shining, standardizing and sustaining). A sixth one, safety, has recently been added.) The goal is the same.

Womack's (1990, 1996) principles for lean manufacturing are quite similar to those associated with six sigma. They are:

Specify the value desired by the customer.

Identify the value stream for each product, providing the relevant values, and challenge all of the wasted steps (generally nine out of ten) currently necessary to provide those values, since these steps are value non-added. However, these non-value-added steps are part of the current value stream.

Make the product flow continuously through the remaining value-creating steps.

Introduce pull between all the steps where continuous flow is impossible.

Aim for perfection, so that the number of steps and the amount of time and information needed to serve the customer is continually reduced.

Caution on lean manufacturing

Even at Toyota, home of lean, or at Omark Industries (now Blount International) and Harley-Davidson, which were both in the news in the 1980s as roaring JIT success stories (Schonberger, 2002), lean manufacturing hasn't always remained as successful over time. The numbers from their annual reports, and those of over 500 other companies in many countries, tell the tale. The analysis, tracking inventory turnover trends for up to 50 years, reveals that in its glory years—the late 1970s and into the 1980s—Toyota's inventories were turning an awesome 60, 70, and 80 times per year. A decade later its inventory turnover had fallen to the 20s, and has dropped steadily since then—all the way to 12.1 in 2001. Blount/Omark roared upward until 1987, but has been in a fluttering stall since then. Harley's top year was 1995. (Inventory turnover is cost of goods sold, from the income statement, divided by value of inventory, from the balance sheet.)

Inventory is the simplest marker of leanness, and it is not only of interest to those with lean aspirations (i.e., the makers, shippers, distributors and sellers). Inventories are tied to cash flow, which savvy analysts on Wall Street monitor even more carefully than they do earnings. However, focusing on just three companies, regardless of their reputations, does not make a story. Nor do the inventory turns of any one- or two-year period. What is interesting is the totality of long-term trend data from the 500-plus companies, which include almost every industry. For what they reveal about lean supply chains, some retailers and distributors are included. About 37 percent, nearly 200 of the database companies, are the success stories: their inventory improvement trend stretches over at least 10 years, and in some cases 25 or 50 years. That group includes such stalwarts in the machining and metalworking trades as Dana, Ingersoll Rand and Milacron in the United States, Lucas in the United Kingdom (a recent acquisition of Dana), SKF in Sweden, and Tata Engineering & Locomotive in India. The other 63 percent make up the bad reviews. Fully 28 percent have stalled or have been fattening up on inventories for at least 10 and up to 50 years. One of them is the world's most esteemed manufacturer: General Electric. That maker of jet engines, electric power equipment, locomotives and major appliances had its "leanest" year in 1973 and has lost ground, in a valley-peak-valley pattern, since then—its six sigma prowess notwithstanding. Twin Disc, a key player in making MRP (material requirements planning) famous in the 1970s, has seen its inventory situation worsen since 1985. A.O. Smith, Toro and Snap-on Tools have been on a downslope since 1985, 1986, and 1989 respectively (Schonberger, 2002).

Why, in the midst of lean, six sigma, 5S, total production management (TPM), and supply-chain management fervor, are so many companies backsliding or why are so many plateauing? In Japan the decade of 1990s was an economic downer. The fortunes of its manufacturers were the same. One reason for worsening inventory patterns in so many companies may relate to Japan's cherished, though fading, reluctance to reduce labor. In the face of declining sales, an excess labor force just keeps producing and producing and producing. Why, then, are some companies not getting lean? The following may give us a clue:

Complacency. This is a fallout of the prosperity of the 1990s.

Stock-hyping deal making. Executives are looking past the basics of good process management.

Growth and retention of unprofitable customers and product variations. This is often the company's fault for not bringing sales and marketing into multifunctional teaming with finance and operations.

Legacies. Mega-machines that produce fat inventories, outsized factories that require marathon flow distances, systems that bog down rather than link up manufacturing and supply chains, and job designs that instill mindless boredom rather than inspiring waste-chopping ideas contribute to this problem.

Retention of command-and-control management. This stifles broad involvement. As control increases, involvement decreases. This is because individuals think or rather perceive their input as irrelevant and or useful when management is commanding and controlling both inputs and outputs of the decision process.

Job-hopping managers and engineers. These people launch initiatives, but do not follow through; they favor what is hot while losing touch with what is still successful.

Corrective responses revolve around reversing these points. Complacency and job-hopping are already on a course of self-correction, and the basics are back in style, especially in the aftermath of the Enron financial manipulation debacle. Clearing out the unprofitable, attacking the legacies, tapping companywide human potential, and maintaining continuity, however, require upgraded awareness and commitment. Those of us in the engineering community bear much of the blame for past failures and must take much of the initiative in achieving truly lean results and making them stick.