Avoiding Mistakes

Definition

"Poka Yoke" is a Japanese phrase meaning "mistake avoidance".

It is any method of "mechanically" preventing mistakes, or detecting and reporting a mistake as soon as it is made. 

Here are two examples:

Mistake Prevention:  a USB plug can only be inserted right-side-up.

Mistake Detection and Avoidance: an automobile chimes if the headlights are still on after the key is turned off.

Techniques

There are 3 main techniques misstake avoidance used in manufacturing:

  • Contact: Test the shape, size or other physical parameters of an input, tool or output, or test for proximity.  An example is a press with twin "Cycle" buttons, so the operator cannot leave one hand inside the press when it cycles.
  • Count: Test the number of movements used in a manufacturing process.  Do not permit the product to exit the machine until the correct number of actions has been taken.
      • This is also known as the "fixed-value" method.
  • Sequence: Test that the correct sequence of movements was performed.
      • This is also known as the "motion-step" method.

Principles

A key assumption is that a human operator will eventually become fatigued or distracted. Therefore the poka yoke technique avoids depending on human diligence.

Practitioners attempt to devise inexpensive and simple mistake avoidance techniques. 

Design for Poka Yoke.  For example, design the USB pin so it can only be inserted correctly.

Design and manufacture for Poka Yoke.   For example, design an item to be non-symmetric, then use a jig to enforce the orientation of that item.

Another manufacturing solution pushes the responsibility upstream.  For example, request that the internal supplier provides exactly the parts needed to assemble one widget.  If any parts are left over, then an error was made.  This assumes that the supplier process is perfectly capable of accurate and complete delivery.

Three common methods are: contact; count; and sequence.

Contact methods are recommended where:

  • Incorrect assembly creates a significant defect
  • Parts have small features that are hard to align
  • Parts have only subtle orientation clues that make it difficult to determine up/down, back/front, or left/right
  • Parts have complex designs which confuse operators

Count methods are recommended for repetitive tasks.  For example of a detection method: the operator is to tighten six bolts.  If each bolt originally is wrapped in a colorful plastic film, which is removed automatically as the bolt is tightened, then it is easy to detect that a bolt was not touched.

Sequence methods may be compared to opening a safe.  The correct sequence of actions "unlocks" the jig to allow the completed part to be removed.  Another sequence approach is to detect that the first component has been removed from its dispenser before unlocking the second dispenser.

Self-Check Poka Yoke devices alert the operator of his or her own mistake in time for corrective action.  This differs from a "Successive-Check" system, where the recipient checks the product upon receipt from the upstream process.

Summary

Do not blame the operators for making mistakes.
Instead:
Make the process mistake-proof!

By Oskar Olofsson



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