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.
Missed operation: Leaving out one or more process steps
Processing errors: Process operations are not performed according to the standard work procedure
Errors in part set-up: Using the wrong tooling or machine settings for current product
Missing parts: Not all parts can be included in the assembly, welding or other process
Wrong parts: Improper or wrong part installed in assembly
Processing wrong work piece: Wrong part machined
Mis-operation: Wrong operation performed on a work piece
Adjustment error: Error in machine adjustment and dimension of part
Improper equipment set-up
Improper tools & jig: Wrong tools & poorly designed jigs
Components of Zero Defect Quality (ZDQ)
The philosophy of ZDQ functions by combining four elementary components:
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.
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.
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.
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:
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.
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.
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.
