Over the last 30 years that poka-yoke devices have gained awareness in the manufacturing world, many different kinds of poka-yoke devices and applications have emerged. Before grouping all of these, it is mandatory to understand all methods to reduce defects. Next we will explain the scope of poka-yoke systems and explain how they can be classified.
How to reduce defects?
In the graph below, we have tried to visualize all methods how to reduce the defect ratio in relation to their effectiveness. The portion which is covered by poka-yoke is what will be described in detail in the next section.
The proactive approach for defect reduction will not allow defects to occur and is therefore most effective. Ideally you design your product or process in such a way that defects cannot arise. As this is very difficult, the second most preferred proactive method is preventing defects to occur.
The reactive approach checks immediately after the operation if an error has occurred. If yes, it is caught by the system. The defect is not prevented, but detected and will not pass to the next process step. If it cannot be detected in the process, it should be detected later on by standard inspection.
The supportive approach facilitates production, but is does not prevent nor detects defects. In order to help your process from being defect free, visual control and aids can be installed to help the operator.
Poka-Yoke in Prevention and Detection systems
Poka-yoke devices can be very simple and very complex. For example a pin to ensure correct positioning in a jig that makes use of the asymmetric design of the part versus a large wire harness or sensors that automatically inform the operator in case of a abnormality.
Regardless of the degree of simplicity and the cost of the device, all poka-yoke devices tend to fall into 3 setting methods categories and 3 regulative action categories. These categories were established by Shigeo Shingo in 1985 and are still applicable today. Below summary is based on his writing in his book: Zero Quality Control: Source Inspection and the Poka-yoke system (1986).
The setting method classifies poka-yoke based on the technique used for defect prevention.
1. Contact method:
The contact poka-yoke works by means of a sensing device. A defect is “sensed” (detected or prevented) by testing the dimensions, shape, size, color or other physical attributes of the product. It is compared to the requirements of the product.
Contact methods are generally mostly applicable in fast production processes, in unusual production processes and in unpleasant work circumstances (hot, cold, dark, loud, bright, humid,…).
They overall support four areas of problems according to Paul Dvorak:
- when incorrect or incomplete assembly of a component results in a defect
- when unmet critical features result in a defect
- when differences in front/top or top/bottom result in defects
- when designs are confusing and complicated
- a sensor that detects the presence of a screw
- a locating pin on a die to ensure correct assembly
- an asymmetric feature that prevents incorrect installation of a part in a fixture
Generally contact methods are often very simple in design and easy to use for operators.
2. Fixed-number method:
The fixed-number method is applicable in processes where the same step or operation is executed multiple times. The method fixes the number of movements, the number of parts and the conditions that are mandatory in the operation not to result in a defect. It notifies in case one is forgotten or prevents from forgetting a step. The counting behavior is essential in the fixed-number method.
- applying 4 felts on a frame. If there are still felts left in the box, a mistake has occurred.
- tightening 5 bolts on a assembly. A counter registers how many bolts were tightened at the required force and prevents passing through the assembly before all bolts were tightened.
- drilling 6 holes in a work-piece. A counter counts the number of movements up and down.
- counting the number of components necessary to complete the operation (and put in kitting box). In this case the operator will find the components that are left and will realize that something went wrong.
- the temperature of the injection liquid is too high. The operation will not take place as long as temperature is not reduced.
3. Sequence or motion-step method:
The sequence method is used in production processes where 1 operator needs to perform a set of operations in a specific sequence. It defines the number of steps in 1 operation and informs when 1 is forgotten. The difference with the fixed-number method is that the operator does not perform the same operation multiple times but performs several activities sequentially. A sensor could be used to confirm if a step occurred out of sequence. This sensor could be connected to a timer and in case the defined time interval has passed, the operator can be warned.
- an indicator board that visualizes each step. First a red light must be inserted, then an orange light and finally a green light. If this sequence is not respected, it will result in a defective streetlight.
- the use of a checklist to complete all process steps. (the checklist needs to be filled and approved in order to proceed with the process)
- pick-to-light systems where all components must be taken by the operator
The contact, sequence and fixed-number method can definitely be combined in one production process. For example when 2 springs and 3 felts need to be applied and the presence of the springs is being confirmed by means of a proximity sensor.
The regulative action according to Shingo (1986) classifies poka-yoke based on the purpose of the device, by the action that is executed by the poka-yoke.
1. Control action (Preventive):
The control action installs an automatic adjustment in the process to eliminate the possibility of a defect. It prevents the defect from occurring and is therefore most effective in achieving a zero-defect process.
2. Shutdown action (Reactive):
The shutdown action reacts by stopping the machine in case an irregularity has occurred. The next part will not be processed as the machine is stopped until the operator allows it to continue. The defect is prevented in this way to reach the next process step. This method results in machine down time and is therefore only used in processes where defects are very expensive, when rework is impossible and when this machine is not the bottle-neck in your production process. In case there is a capacity issue on the particular machine and the defect is unique, not recurring, other methods are preferred. For example, ensuring the defective part is taken out automatically and allow the machine to continue its operations. The defects can then be reviewed by the operator and countermeasures can be put in place. When the defect is reoccurring, off-course shut-down is preferred.
- If the light bulbs are not inserted in the correct sequence, the barrier will not open and the produce cannot proceed to the next station. Only when all 3 color sensors are triggered in correct order, the barrier opens.
3. Warning action (Reactive):
The most soft action that can be taken by a poka-yoke is a warning action. Here the operator is warned or notified when a mistake has been made. This method is used mostly in processes with a small amount of defects and if rework is allowed. The defects will still rise if the operator does not respond to the warning signal.
Examples are visual flashing lights, audio sounds, alarms, etc.
As a conclusion, all poka-yoke systems can be placed in the matrix below. Try to look at your manufacturing site and identify some poka-yoke from each combination between setting method and regulative function.
Note: Multiple sources were used for summarizing the content on this page. You can find these sources on the following page: References.