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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:

Contact method - identifies defects by testing product characteristics.

Fixed-value - a specific number of movements every time.

Sequence method - determines if procedure were followed.

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.

Examples of Poka-Yoka

All 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 and contradicts the direction of Lean manufacturing.

Spell check on work editor

UPC codes and scanners

Snooze

Circuit breaker

Operator training

Low fuel alarm and visual indicator on vehicle

Instructions on assembling furniture or light fixture

Automatic save features on software

Pull down menus

Inspection

The hole in the upper portion of a sink to prevent overflow

Pressure relief valve on hot water heater

Coffee maker shut off feature when pot is removed

Tinting of contact lenses to assist in locating them

L or R on the contact lens case to indicate left eye and right eye

Kill switch of jet-ski attached to life jacket

Product labels, keyboard labels, cash register labels

And/On lights for materials, maintenance, or assistance

"Are you sure you want to DELETE" reminder on software programs

Sorting of suspect material

Force detection monitors on machines

Infrared imaging surveys

TPM boards for quick visual recognition

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.

Example:

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.

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