Mistake Proofing is about adding controls to prevent defects, reduce their severity, and detect them if they can occur. Poka-Yoke is often used as a synonymous term. The concept was first put to widespread use by Shigeo Shingo within the Toyota Production System (TPS).
PREVENT a defect from occurring and when this is not possible, DETECT the defect every time one occurs. There are various levels, effort, and costs of error proofing. The team needs to identify the optimal states and examine feasibility of implementation in the new product or process KPIV's.
He recognized three types of poka-yokes:
Defects found in later operations or steps of a process have more costs associated with them. There are more materials, labor, overhead, previously reworked product that are at risk. Ultimately if the product gets to the customer as a defect, the intangibles, such as reputation, can exceed the tangible cost.
For example, a carbon monoxide detector will not be as effective if the inputs of dangerous and tolerable PPM levels are not known, or if the proper location for installation is not clear.
GOOD: Detect defect before proceeding to next step.
BETTER: Detects defects while in process at an operation.
BEST: Prevent defects from occurring at all.
Each process should have error-proofing to ensure defects are not passed on to the next step and that each operation has mistake proofing in place to ensure only defect free parts are accepted in.
The FMEA has three categories of:
Mistake Proofing is applied to reduce the scores in Detection and Occurrence.
Once a mistake proofing device or method is proven it is important that the GB/BB follow up with the Quality Department to update the company FMEA if applicable. Risk reduction is another soft savings benefit from a Six Sigma team (reducing the RPN number of a failure mode).
The Six Sigma team also updates their own project FMEA and this becomes the Revised FMEA in the CONTROL phase.
Their is an initial cost to implementing poka-yokes just like their is a significant cost to implementing SPC. However, this cost will be more than offset in the long term stability and performance of the output. In other words, if the correct poka-yokes are invested these will help reduce variation and shift the mean (or median) in the desired direction.
The chart below show the basic costs at various levels of DETECTION.
The further downstream a defects goes undetected the more risk is associated. If a customer finds a defect there are various Cost of Poor Quality from direct cost to a variety of indirect cost that may not immediately be felt (such as future recalls or lost quoting opportunities).
The Six Sigma team should focus on PREVENTION first. In most cases not 100% defects can be PREVENTED 100% of the time so investments in DETECTION are also necessary.
The examples listed below are devices that help prevent or detect mistakes. However, there are varying degrees of effectiveness and the team needs to balance getting the most effective poka-yoke while being practical and economical.
Some mistake proofing ideas can add time to the value stream and add costs such as an inspection process or sorting process.
More lead time equals more inventory, longer cash conversion cycle, and contradicts the direction of Lean Manufacturing.
More Mistake Proofing examples and photos at www.mistakeproofing.com
It is also important to evaluate if any other problems or risks have been created form the new poka yoke device. The device should not present a new safety hazard, increase risk of existing failure mode or create a new failure mode.
Suppose a newly installed shut off valve on the coffee maker functions to prevent spilling below onto the base or countertop when the coffee pot is removed.
This is a very pleasant mistake proofing device, but if the hot water pump doesn't get a feedback signal and continues filling the filter compartment then this could overflow. Essentially, the spill failure mode has been moved from one location to another.
The laundry tub is deeper so it could hold more water but the sink in the bottom picture has an overflow drain. This offers more risk aversion if the water is accidentally left on with the plug in it.
Think about stretch ideas that further improve mistake proofing of both? Staying somewhat practical with cost in mind, let your imagination take over.
What seemed impractical a few years ago may become practical now or in the future. What value does it add to the customer, maybe the Kano Model could draw out any interest in "delighter" features like this and understand what the customer will pay for them.
Six Sigma Modules
The following presentations are available to download.
Green Belt Program (1,000+ Slides)
Cause & Effect Matrix
Central Limit Theorem
1-Way Anova Test
Correlation and Regression