Error proofing (EP) and Mistake proofing (MP) are techniques used to minimize errors in production processes. Their main goal is to detect and eliminate errors in the initial stages of the process before they affect the quality of the final product.
Mistake proofing – meaning and role from defect detection the point of view
Mistake proofing includes all tools that allow us to capture a problem generated in an earlier phase of the production process. Most often these are End of Line testers (EoL) or Dock Audits (DA). They are used to verify the functionality and correct operation of a given final product.
Such equipment allows us to check whether our product is compliant. If the result of such verification is negative, it means that we have already generated a scrap. So MP is linked with a detection action. Mistake proofing allows us to prevent non-conforming final product from reaching the customer.
Of course, under one important condition – if we have it properly calibrated. Why? Because it may also turn out that in the case of an example End of Line tester or Dock Audit or another station located at the end of the production line, our station was decalibrated before packing and even non-compliant products were sent to the customer as NOK parts.
Mistake proofing examples
Examples of the use of mistake proofing are below:
- Fuel tank leakage test. The tank is submerged under water and the amount of air bubbles appearing on the surface is checked.
- Fuel lines leakage test. Performed by verifying the pressure drop over time
- Forward and backward sliding tests for front car seat guides
- Headrest up and down force
- For the steering column, it is a measurement of friction by simulating turning the steering wheel to the right and left
- Verification of correct operation of car lamps
- Car window movment verification
Error proofing – prevention is better than cure
However, what we want to implement are all types of error proofing solutions. In Japanese, they are named as Poka-Yoke. This technique consists in introducing such changes in the production process or product design that will make it impossible or much more difficult to make a mistake.
Fig. 1. Error Proofing and Mistake Proofing stages
Why is it so important to us? Because in this case we will not produce a defective product. As you can see, we have two strategies for quality control mangement. We either focus on Detection or Occurrence. Occurrence is more recommended, also from the IATF 16949 standard requirements point of view.
Error proofing types
What are the types of error proofing? One of the most frequently used solutions in the automotive industry are mechanical poka-yokes. They allow us to have one particular orientation base for the component at the production station.
Thanks to this, they force the appropriate position of a given product to perform a specific sequence of operations on it (e.g. screwing or welding).
Another common type of error proofing relates to proximity sensors. Their purpose is to verify the presence and check whether all components required in a given production cycle are present at a given station. Often their use is associated with visualization.
What is it about? If a component is missing then:
- it is not possible to start the production cycle
- a red light is on, informing about the absence of such a subcomponent and information about its placement on the station. Only then can the cycle be started.
Error proofing strategy implementation
As part of the error proofing strategy implementation, you can also implement various types of solutions that eliminate the possibility of making a mistake from the point of view of similar elements. Mostly produced close to each others.
Here two categories can be distinguished:
- components similar in terms of the applied plastic isolation color (e.g. electrical wires)
- components with a similar structure (e.g. on steering columns, such very similar elements are motors or I-shafts)
Another type of error proofing and very often used, in particular when it comes to issues related to welding, is counting elements and informing about errors. How it’s working? When we take the part from the left side, we place it on the welding machine base plase. After the welding process, we put by hand through the frame on the right and place the welded component in the container.
What does it give us? We receive information that this particular part has passed the entire production process and has not, for example, been put down somewhere by the operator at the workstation.
Error proofing implementation
As you can see, when it comes to the issue of error proofing, there are many possibilities for their implementation. However, the starting point for their implementation is what PFMEA contains, i.e. in the production process design . This is a risk assessment, repeatedly referred to IATF requirements. The main criterion concerns how big impact the generated defect will have on the client when it reaches him. What questions do we need to answer?
- What problem will we be dealing with? Most often we focus on the problem related to safety, homologation or legal requirements
- Will it be a problem regarding the lack or reduction of functionality (window lift will work slower and louder)
- Is the defect related to aesthetics? For example, small scratches that are only visible when I get closer to the bumper
What else is worth remembering?
Implementing error proofing or mistake proofing means that we are halfway there. Why? Because we need to remember about two important points:
- Each implementation of the above equipment should result in the creation of appropriate inspections for Maintenance. With one of the complaints, we “boasted” to the client that we had implemented EP. After a few months we received a complaint about the same type of defect. It turned out that we did not take into account the appropriate frequency of preventive inspections during the implementation. Finally, the pin included in the EP after a few weeks was worn out enough to allow us to change the subcomponent’s orientation angle when during screwing.
- Calibrators. Before each start-up of the production line, appropriate calibration parts are used to check the correct operation of the EP and MP. Here, we must remember that (just like error proofing itself) it should be verified with the appropriate frequency. In addition, they should have the right color (as an element of identification management) that would prevent it from being sent to the customer together with the final products.
Summary
Of course, it is known that it is not possible to implement error proofing tools everywhere, because sometimes these End of Line testers, equipment and workstations defined as mistake proofing are required in the project as additional certification. Thanks to this, we know that our product meets all the requirements from the point of view of design evaluation set before us by the client.
Dariusz Kowalczyk
