All the latest news from the industry weekly compiled by the editorial team for you free of charge.
This eMail is already registered.
An unexpected error occured.
Please accept our Terms of Use.
Registration successful.

ADDITIVE MANUFACTURING Zero Defect Manufacturing - Poka Yoke Overview

Apr 12, 2019

The cover story describes the concepts, methods & applications of Zero Defect manufacturing, also referred to as Zero Defect Quality (ZDQ) & Zero Quality Control (ZQC). Starting from causes of defects to differences between defects & errors to types of defects & mistakes, this article gives an indepth insight along with the implementation steps described to help the readers. - Shekhar Deshpande, Director, Shilpa Consultants

Many things can go wrong in the complex environment of the workplace. Every day, there are opportunities to make mistakes that will result in defective products. Behind Poka Yoke (mistake proofing) is the conviction that it is not acceptable to produce even a single defective product. To become a world-class competitor, an organisation must adopt the philosophy and practices of producing zero defects. In this context, Deming appropriately expressed, “The problem is your process & not people; 85% of the reasons for failure are deficiencies in the systems & process rather than the employee. The role of management is to change the process rather than badger individuals to do better.” Mistake-proofing methods are one of the proven means for achieving this goal. Shigeo Shingo, the founder of Poka Yoke, realised the limitations of statistical techniques & devised a three-step process of ZQC (Zero Quality Control). Based on profound wisdom from proverbs, like, ‘To err is human,’ and ‘Prevention is better than cure’, Poka Yoke is a technique-driven approach to achieve ZDQ (Zero Defect Quality). The common sense approach, relying primarily on machine vigilance rather than human vigilance, allowed Toyota to become so profitable.

What causes defects?

Defects are caused by process variation from:

  • Poor procedures or standards

  • Machines

  • Non-conforming material

  • Worn tooling

  • Human mistakes

Except for human mistakes, these conditions can be predicted by use of control charts and corrective action can be implemented to eliminate the cause of defects. Simple errors, the most common cause of defects, occur unpredictably. The goal of Zero Defect Quality is zero! Make certain that the required conditions are in place & controlled to make acceptable product 100% of the time.

It will be important to understand the definition of mistake made by Shigeo Shingo:

Mistake: The execution of a prohibited action, the failure to correctly perform a required action or the misinterpretation of information essential to the correct execution of an action

Mistake proofing: The use of process or design features to prevent manufacturing of non-conforming product. Shingo differentiates between mistake & defect as cause & effect relationship.

Types of human errors

Shingo further defines, ten types of human errors as follows:

Forgetfulness: We forget things when we are not concentrating, e.g. latching the door with key inside or stationmaster forgetting to lower crossing gate

Misunderstanding: The mistake that occurs when we jump to wrong conclusions e.g. a person not used to a car with automatic transmission steps on the brake thinking it is clutch

Misidentification: We view it very quickly or see from far distance, e.g. ₹ 500 for ₹ 50 in earlier times

Amateur inability: New worker does not know operation or is just barely familiar with it

Willful neglect of procedure: When we decide that we can ignore rules under certain conditions, e.g. crossing a street against a red light because there are no cars in the sight at the moment

Inadvertent neglect of procedure: We are absent-minded & make mistakes without knowing how they happened, e.g. someone lost in thoughts tries to cross street without even noticing red light

Slowness: Mistakes occur when our actions are slowed down by delays in judgment, e.g. a person learning to drive is slow to step on the brake

Lack of standards: When there are no suitable instructions or work standards, e.g. a measurement may be left to an individual worker’s discretion

Surprise error: When machine works differently than expected, e.g. a machine might malfunction without warning

Intentional error: Deliberate mistakes resulting in sabotage

Types of inspection

For avoiding the defective products, traditionally, two types of inspections are used:

Judgement inspection also called as final inspection or standard inspection. It assesses quality of production outputs based on given sampling plans or sorts out defects from good products. This does not really improve the process & should be used only in short term as it only discovers defects. The two risks associated with sampling inspection is producer’s risk (rejecting a good lot of production) & consumer’s risk (accepting a defective product which gets dispatched to consumer).

Informative inspection also called in-process inspection. It assesses process by inspecting outputs and using information gained to control the process (a feedback loop). This has relatively less risks & reduces the probability of defects to the extent of time interval between sampling.

Source inspection Shingo, however, has recommended source inspection, i.e. assessing beforehand whether the conditions necessary for high quality production exist. This completely eliminates the possibility of mistakes & subsequent defect. Preference of source inspection is also based on an important rule called 1-10-100 Rule, which states that as a product or a service moves through the production system, the cost of correcting an error multiplies by 10.

Activity

Cost

Error in order corrected ay entry

$ 1

Error detected in billing

$ 10

Error detected by customer

$ 100

If dissatisfied customer shares the experience with others, the cost is $1000.

Causes of defects

There are ten causes of defects in which error-proofing is designed to correct or eliminate.

  1. Missed operation: Leaving out one or more process steps

  2. Processing errors: Process operations are not performed according to the standard work procedure

  3. Errors in part set-up: Using the wrong tooling or machine settings for current product

  4. Missing parts: Not all parts can be included in the assembly, welding or other process

  5. Wrong parts: Improper or wrong part installed in assembly

  6. Processing wrong work piece: Wrong part machined

  7. Mis-operation: Wrong operation performed on a work piece

  8. Adjustment error: Error in machine adjustment and dimension of part

  9. Improper equipment set-up

  10. Improper tools & jig: Wrong tools & poorly designed jigs

Components of Zero Defect Quality (ZDQ)

The philosophy of ZDQ functions by combining four elementary components:

  1. Point of Origin Inspection: As we have seen earlier, there are three types of inspection - Only Point of Origin Inspection actually eliminates defects. It checks for optimum process conditions before processing is done and errors can be eliminated with instant feedback. Corrections are made before defects occur. Here, the focus is on prevention, not detection. It differs from traditional inspections as it catches errors, gives feedback before processing & hence, there is no risk of making more defective products. By combining Check and Do in the ZDQ approach, the Doing is controlled so it cannot be wrong 100% of the time. This may include switches that detect miss-fed parts, pins that prevent miss-feeding, warning lights and sound signals.

  2. 100% Audit Checks: It is Point of Origin Inspection on every piece. It does not rely on sampling. It does not assume that defects will statistically occur & hence prevents defects.

  3. Immediate Feedback: ZDQ sends the operator a signal and alarms the person that an error has happened! ZDQ Inspections is equal to immediate feedback. It minimises the time it takes to carry out corrective action when abnormalities appear.

  4. Poka-Yoke: Uses the devices to ensure that error is prevented

Zero Defect Manufacturing is a quality concept to manufacture with zero defects & eliminate waste associated with defects! “Zero” is the goal, based on the discipline that defects are prevented by controlling the process. No finger pointing takes place. It recognises that it is natural for people to make mistakes. Operators and machines, both will sometimes make mistakes. Find ways to keep errors from becoming defects! It is a method for Mistake-Proofing (Poka-Yoke)–a process that assures that defects are not produced.

Methods of Poka Yoke

Poka-Yoke systems consist of three primary methods:

  1. Contact Method: Do not have to be high-tech! Passive devices are sometimes the best method. These can be as simple as guide pins or blocks that do not allow parts to be seated in the wrong position prior to processing. Take advantage of parts designed with an uneven shape! A work piece with a hole, a bump or an uneven end is a perfect candidate for a passive jig. This method signals to the operator right away that the part is not in proper position.

  2. Counting Method: Used when a fixed number of operations are required within a process, or when a product has a fixed number of parts that are attached to it. A sensor counts the number of times a part is used or a process is completed and releases the part only when the right count is reached. Another approach is to count the number of parts or components required to complete an operation in advance. If operators find parts leftover using this method, they will know that something has been omitted from the process.

  3. Motion-Sequence: The third poka-yoke method uses sensors to determine if a motion or a step in a process has occurred. If the step has not occurred or has occurred out of sequence, then the sensor signals a timer or other device to stop the machine and signal the operator.

Three levels of Poka Yoke devices

Level 1: Eliminate error at the source, before it occurs

Level 2: Detects an error in the process of it occurring, before it results in a defect

Level 3: Detects a defect after it has been made before it reaches the next operation

Characteristics of Poka Yoke devices:

  • Inexpensive solutions to problems

  • Simple and easy to implement

  • Specifically focused applications

  • Developed by every employee

Implementing Poka Yoke

Poka yoke implementation is a 7-step process as follows:

Step 1: Describe the defect. Show the defect rate. Form a cross-functional defect prevention team.

Step 2: Identify the location at which the defect is discovered & made

Step 3: Details the current standard procedures/elements of the operation where the defect is made

Step 4: Identify any errors or deviations from standards where the defect is made

Step 5: Identify the Red Flag condition where the defect occurs. Investigate (analyse) the causes for each error/ deviation. Ask why the error happens until the root cause or source error is identified.

Step 6: Identify the Poka Yoke device type required to prevent the error or defect. List alternative ways/ideas to eliminate or detect the error.

Step 7: Create a Poka Yoke device. Test for results.

Image Gallery

  • An example of a contact device using a limit switch. In this case, the switch makes contact with a metal barb sensing it’s presence. If no contact is made, the process will shut down.

  • Three levels of Poka Yoke devices

  • Shekhar Deshpande

    Director

    Shilpa Consultants

    sbdeshpande12@yahoo.com

Companies related to this article
Related articles