Implementing Poka-yoke: Strategies and 3 Detection Methods

Poka-yoke is a vital part of Jidoka in the House of Lean Production. Its main goal is to prevent errors and defects. The manufacturing industry utilizes Poka-yoke as a standardized work technique.

The main idea behind Poka-yoke is to stop errors and defects before they happen. Using Poka-yoke has proven to be highly effective in improving production efficiency in manufacturing. Now, let’s explore the definitions, benefits, and examples of Poka-yoke:

What is Poka-yoke?

Poka-yoke is a concept from Japan. Shigeo Shingo coined the term, and it’s all about stopping mistakes and ensuring products are perfect without any problems.

The concept helps find potential defects early on by preventing different errors during production. Here are some common mistakes that often happen in manufacturing:

• Steps are missing in the process.
• The process has faults.
• Incorrectly installed workpiece
• A piece is missing.
• Using the wrong parts. 
• The workpiece is processed incorrectly.
• Incorrect machine operation.
• Errors in adjustments.
• Incorrectly installed equipment.
• The tools and jigs are not well prepared.

Poka-yoke uses machines or methods to help spot mistakes quickly. Its main goal is to ensure the production process doesn’t progress unless everything is correct.

Read more: Lean Manufacturing: Definition & 3 Benefits

Two types of Poka-yoke actions

When it detects an error, poka-yoke takes two types of actions: turning off the engine or giving a warning. Here’s the explanation:

Shutdown

Shutting down the engine is the most potent action of Poka yoke. Here are some examples:

• The drilling operation stops when the light sensor fails to detect the required number of holes in the workpiece.
• The machine refuses to start if the workpiece is not positioned correctly. The on switch sends a weak electric current to the reference pin, and the engine only starts when it actively contacts each pin.

The full-work system is another way of shutting down a process. In this system, a supplier stops providing materials to the next stage when it has reached its maximum capacity. The system operates at total capacity when it achieves the desired number of parts.

Warning

The Poka yoke warning system sends out an alert when something goes wrong. It does this by making a buzzer or light go off. A well-known example of this is the andon board used by Toyota. It lets the group leader know when there’s a problem by showing the process number, playing music, or sometimes both.

In Toyota’s case, team members activate the andon by pulling a cable along the production line. The line keeps moving until it reaches a specific spot. Since each step in the process has its own fixed location, team members can finish at least one cycle of work. Stopping the line during an operation reduces the chances of making mistakes.

Three detection methods in Poka-yoke

Poka-yoke detects deviations in the workpiece or work method through the following ways:

Workpiece deviations

This method employs sensing devices to identify product weight, dimensions, or shape abnormalities. Here are some examples:

• Weight: Establish a standard weight and utilize a balance to weigh each product. 
• Dimensions: Standardize thickness, inside and outside diameters, and employ limit switches, stoppers, jigs, photoelectric eyes, and similar tools to detect deviations. 
• Shapes: Establish standard angles, number and position of holes, curvature, and detect deviations using limit switches, tracking pins, or similar detectors.

Work method deviations

This Poka-yoke method uses sensors to find mistakes in everyday actions. For example, a counter actively tracks the number of weld points on the workpiece. The clamps stay locked until reaching the correct number of weld points. This way, the work arrangement ensures that the next step can’t happen until the completion of the previous phase.

Deviations from fixed values

Counters play a crucial role in this scenario. They offer valuable utility, such as:

• You can utilize the limit switch to count the holes drilled in the workpiece.
• When you reach a specific count, you can switch the weld tips. The counter actively halts the welder once achieving the desired count. It remains inactive until you install a new weld tip.

Two types of sensors in Poka-yoke

Sensor technology constantly evolves, particularly in Poka yoke. In this context, we can classify sensors into two categories: contact sensors and non-contact sensors. Now, let’s delve into a more detailed explanation:

Contact sensors

The most common types of contact sensors are:

• Limit switches and micro switches: These sensors detect the presence of workpieces, prints, or tools. They are inexpensive and durable, making them suitable for various applications.
• Differential transformer: This sensor detects changes in the magnetic field depending on the level of contact with the workpiece.
• Touch switch: A gentle touch on the antenna activates it, allowing it to detect the presence, size, or damage of an object.

Non-contact sensors

The device can detect interruptions in the light beam, nearby solid objects, metal tracks, fibers, color, ultraviolet light, infrared light, counting errors, electron beam, size, pressure, temperature, electric current changes, and vibration. Here are some commonly used devices:

• Photoelectric device: This device is often used as a light screen to make sure the machine area is clean before using it. It can also measure actions, detect falling objects, and determine the size of workpieces.
• Metal parts detector: This device counts installed screws, checks for any removal of parts from the press, and ensures the guard is closed.
• Temperature: Thermometers and thermocouples detect changes in temperature for molds, motors, and curing ovens.
• Pressure: The pressure gauge can find blockages in pipes and detect excessive force in the machine.
• Current fluctuation: This is commonly used in spot welding to check for secondary currents that could weaken the strength of the weld.

Poka-yoke implementation strategies

There are two key indicators to ensure effective Poka-yoke implementation:

• Poka yoke must check 100% of items.
• Poka yoke must provide immediate feedback for countermeasures.

Implementing Poka yoke is straightforward and logical. Here are some steps for you to follow:

1. Identify company operations or work processes.
2. Analyze the causes of failures using the 5-whys method.
3. Determine the most suitable Poka-yoke method.
4. Test the chosen plan and evaluate the results.
5. Train operators, assess performance, and measure the success of the implemented Poka-yoke.

Poka-yoke implementation examples

In the automotive and consumer product industries, the pictures below demonstrate the application of Poka yoke. The floor shop team members are the ones who came up with most of the Poka-yoke ideas.

The car manufacturing industry faces challenges when loading cars into freight cars. Loading the car into the carrier proves difficult due to inappropriate height. Therefore, we need a method to prevent damage and ensure safety during car transportation.

To fix these problems, we implement Poka yoke in the following ways:

• We cut the wood to the right length and labeled and color code it.
• We attach the chain to the wood and position the house.
• We updated the standardized work to include a check for level.

Previously, detecting unfilled boxes in the consumer product industry posed a challenge for companies. Occasionally, boxes would remain unfilled due to machine errors, yet they were still closed and sent for shipment.

To address this issue, companies employ Poka yoke through the following measures:

• They install pedestal fans near the conveyors.
• They blow off unfilled boxes from the conveyor.

Read more: Jidoka: Definition and its 2 Implementation Strategies

Summary

Poka-yoke is a manufacturing method that prevents defects and errors in production. It involves two actions: stopping the machine and providing a warning in case of a mistake. Poka-yoke is commonly found in manufacturing companies that heavily rely on devices.

Having understood the concept of Poka yoke, the next chapter will discuss involvement, the core of the House of Lean Production.

References

Dennis, P. (2017). Lean production simplified: a plain-language guide to the world’s most powerful production system. Crc press.