Defect-free production with PokaYoke
In this article we present how to produce without defects with a source inspection system and using PokaYoke, which allow a process-embedded 100% inspection, at a very low cost.
There is a long-standing concern in companies operating in competitive markets not to send defective products to customers. It is also part of the corporate culture the notion of increased costs due to non-quality.
In ascending order, these costs are classified into:
Costs of Prevention: costs of activities and means that aim to avoid the generation of defects. They include, among others, costs related to new product design, supplier development, process capability studies, quality improvement teams, training, quality systems...
Evaluation Costs: costs related to activities and means aimed at the early detection of defects that occur. They include, among others, costs related to receiving inspection of materials, process quality control, final quality inspection, audit, calibration of measuring instruments, ...
Internal Failure Costs: costs inherent to the correction of defects, when detected within the company. They include, but are not limited to, costs related to rejects, scrap, product rework, re-inspection, ...
External Failure Costs: costs inherent to problems caused by defective products detected at the customer. They include, among others, costs related to complaint processing, recalls, returns, expediting, penalties, guarantees, ...
The cost of a quality problem increases exponentially when we progress in the value chain towards the customer, so it is very important to avoid the occurrence of defects or, if this is not possible, to detect them as early as possible.
In its evolution over time, Quality Control (QC) began by focusing on the inspection of finished products, at the end of the process and before shipment, to avoid sending defects to customers. The inspection was done by quality inspectors (controllers), external to the production process. This method had several limitations, despite the mobilization of considerable means: on the one hand it was not possible to guarantee that defective products would not be sent to customers, on the other hand, the defects were detected at the end of the value chain, with all the inherent costs.
QC then moved to a stage in which the inspection of the products was done at the end of each stage of the process, still by inspectors external to manufacture. This method proved to be more effective than the previous one, allowing to reduce (but not eliminate) the number of defective products sent to the customer, and to reduce their cost, since detection was done along the value chain, not just at its end.
Later, consecutive inspection and self-inspection were done by production operators themselves. In consecutive inspection, the operator of each production step inspects the product resulting from the previous step, that is, the internal customer checks the quality of the product that is sent to him by his internal supplier. In self-inspection, each operator checks the product immediately after completing the value-added operation for which she is responsible. The objectives were to involve operators in quality control, giving them autonomy and more responsibility, and to detect defects as early as possible and to immediately take corrective actions. It was thus possible to further reduce the number of defects as well as their cost.
At the same time, process control evolved, with the application of statistical data processing techniques (Statistical Process Control), the subject of a forthcoming article.
The evolution of Quality Control described very briefly in the previous paragraphs allowed to considerably reduce defects and consequently improve the quality sent to the customer as well as the overall costs. Nevertheless, defects continued to be produced and, in certain cases, to escape inspection systems and be sent to customers.
A new paradigm of quality control was then developed: instead of inspecting products to detect defects that might have been produced, the emphasis was now on equipping processes with systems to prevent the production of defects, making product inspection unnecessary in the long term.
DEFECT PREVENTION AND ERROR-PROOF DEVICES
The paradigm of defect prevention thus intends to prevent defects from occurring, minimizing the cost of non-quality, specifically evaluation costs and internal and external failures costs.
An effective defect prevention system is based on the following components:
Source Inspection. It is intended to detect anomalies as far upstream as possible, ideally at the point of occurrence.
100% Inspection of units produced, using mistake-proof systems (Poka Yoke), embedded in the process.
Immediate action to stop production in case of anomaly.
The clever combination of these components makes it possible to inspect the whole production, early detect any abnormality and prevent the production of defects, without incurring the costs usually associated with 100% inspection.
Inadvertent human mistakes are common and, in processes that are not robust enough, can cause defects. Mistake-proof systems are aimed at preventing inadvertent mistakes from happening or prevent them from causing defects.
Mistake-proof systems have long been present in our daily lives, particularly in devices relating to the safety of people at home, in health care and on the road. Here are some examples of these systems:
The Japanese designation Poka Yoke can be translated as "mistake-proof".
Poka Yoke systems, by definition:
Are built in the process.
Allow for control at source.
Allow 100% inspection at no extra cost.
There are two types of Poka Yoke:
Those that prevent mistakes. They are built in the process or in the design of the product itself and ensure that mistakes don’t occur. A poka yoke of this type is, for example, a design of parts that makes it impossible to assemble them in the wrong position.
Those that prevent mistakes from causing defects. This type of Poka Yoke does not prevent mistakes to occur but detects them immediately and stops the process so that a defect is not produced. An example of this type of Poka Yoke is a system that detects the correct positioning of a part inside a press tool: if the part is not well positioned, the system prevents the start of the machine cycle.
Two examples of poka Yoke like the ones just described:
Figure 1 - Part Positioning for Machining
This system, built in a machining equipment, checks the geometry of the part at the entrance. If the part is not the correct one or is wrongly positioned, it will not enter the machining process.
Figure 2 - Tightening Torque Control
This system, built into a multi-axis bolt driving machine, checks the tightening torque, and only releases the part for the next operation if it is within tolerance. Otherwise, the clamps remain close, and an alarm is triggered.
EXAMPLES OF POKA YOKE
Here are some examples of Poka Yoke from Factory Magazine / Nikkan Kogyo Shinbun Ltd "Poka-Yoke: Improving Product Quality by Preventing Defects" (see References at the end):
There are other systems that, in some way, help operators to carry out operations correctly or that even detect and alert if a mistake has been made, but that do not prevent them from causing a defect. These systems, strictly speaking, cannot truly be considered Poka Yoke systems.
THE IMPLEMENTATION OF POKA YOKE
The evolution of automation and detection systems, as well as the most recent digitization of processes, has made it possible to extend the concept of mistake-proofing. Equipment manufacturers as well as process integrators are increasingly aware of the subject, originating an increasing number of equipment designed from the outset with built-in Poka Yoke and Jidoka systems. However, there is still much to be done in legacy processes and equipment.
Many Poka Yoke systems can be developed based on simple principles and using materials and techniques easily available in most manufacturing plants. Production operators, team leaders and maintenance technicians are perfectly capable of developing and implementing such systems, if they are given the motivation, the autonomy, and the right training.
The success of any business necessarily involves delivering the right product to the customer, with the best quality and at the lowest cost.
It is known that the cost of non-quality increases exponentially as we progress down the value chain and get closer to the customer.
It is therefore necessary to prevent the occurrence of defects or, failing that, to detect them as early as possible, without increasing cost, using source inspection and PokaYoke devices, which allow a built-in 100% quality control.
Many of these devices can be developed by people on Gemba, in a short period of time and at a very low cost.
"Zero Quality Control: Source Inspection and the Poka-yoke System"
Productivity Press, 1986
Factory Magazine / Nikkan Kogyo Shinbun Ltd
"Poka-Yoke: Improving Product Quality by Preventing Defects"
Productivity Press, 1989