Mistake Proofing (Poka-Yoke)
Description: 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 synonomous term. The concept was first put to widespread use by Shigeo Shingo within the Toyota Production System (TPS). Objective: 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.
Connection to the FMEAThe FMEA has three categories of:1) Severity 2) Detection 3) Occurrence 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. Examples of Poka-YokaAll of these are there to 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 feasible and economical. Others 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
Example:
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.
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