The Ultimate Guide to Poka-Yoke

Poka-yoke is a lean manufacturing tool that refers to “mistake-proofing” or “error-proofing” a process. In Japanese, poka-yoke translates to “avoid” (“yokeru”) “mistakes” (“poka”).

When a mistake reaches the customer, it becomes a defect. Poka-yoke aims to prevent defects by catching, correcting, and eliminating mistakes at the source.

History of Poka-Yoke

The term “poka-yoke” was coined by Shigeo Shingo, a key contributor to the Toyota Production System (TPS), in the 1960s. Shingo noticed that in one process at the Toyota plant the workers often forgot to place a required spring under a switch button. He redesigned the process to be completed in two steps: first placing the required springs in a placeholder, then inserting the springs into the switch. As a result, if a spring remained in the placeholder, it would be clear to the worker that they had forgotten to place it in the switch. Then, they could correct their mistake immediately.

Shingo originally called the technique “baka-yoke,” or “idiot-proofing,” but changed it to the milder “poka-yoke,” or “error-proofing,” after an operator became offended.

Poka-Yoke Examples

The principles of poka-yoke have been applied across industries. For example, preventing mistakes before they occur is critical in the medical industry; failing to prevent even a tiny mistake can have tragic consequences. Having measures in place to mark the correct body part for surgery, and even enforcing the washing of one’s hands, can prevent serious consequences.

Poka-yoke is also present in our everyday lives. For example, microwaves are designed to stop running if the door is open to prevent waves from escaping, and many new cars have error-proofing features such as alerts that let the driver know if the car is leaving a lane. Some examples of poka-yoke are so commonplace that we often wouldn’t think of them. For example, we often take for granted the spell-checking functions on our phones, internet browsers, and software that prevent us from making spelling mistakes in our communications.

Poka-yoke provides many benefits to manufacturing processes, the foremost being an improvement in overall quality control. By integrating poka-yoke inline, mistakes are either prevented or caught shortly after they happen. This prevents defective products from making it to the end of the process. As a result, a higher quality of output naturally follows.

Further, these solutions get to the root cause of problems and prevent them from becoming bigger problems. Solving problems immediately takes significantly less time than trying to rework products when they’ve made it to the end of the line.

Applying poka-yoke in your factory can lead to a variety of other benefits, such as:

Reducing time spent on training
Since workers won’t need to look out for as many problems, you won’t need to spend as much time training them on these issues.

Promoting a culture of continuous improvement
Poka-yoke fosters an improvement-oriented approach by encouraging employees to solve problems immediately and address the root cause of problems to prevent them from becoming bigger problems. It’s a great tool to implement within a larger lean manufacturing framework.

Improving safety
You can improve the safety of your factory by putting measures in place to prevent workers from entering unsafe situations.

Reducing waste
It may go without saying, but producing fewer defective products means you’re also wasting fewer materials. You also save time by spending less time reworking defective products: in addition to cutting down on the number of defects that need fixing, it’s faster to fix a defect earlier rather than later.

Higher productivity
Your processes will run more smoothly when errors are prevented and defects are detected and solved immediately. As a result, faster, more efficient processes and less waste result in higher overall productivity.

Poka-yoke can be implemented at any point in a manufacturing process in which a mistake can be made. Here are some situations where it would be particularly important to mistake-proof a process:

• At a hand-off step in a process, when output is transferred to another worker
• When a minor error early in the process will cause major problems later in the process
• When the consequences of an error are expensive or dangerous

Here are some common types of errors that can occur in manufacturing processes:

Processing errors
Missing a process operation or not performing it according to the standard operating procedure

Setup errors
Setting machine adjustments incorrectly or using the wrong tooling

Missing parts
Not all parts included in the assembly, welding, or other processes

Improper parts/items
Using the wrong parts in a process

Operations errors
Carrying out an operation incorrectly or having the incorrect version of the standard operating procedure

Measurement errors
Making an error in measurement in a process, machine adjustment, or the dimensions of a part coming from a supplier

The American Society for Quality (ASQ) suggests the following process for mistake-proofing a procedure:

Detecting and Identifying Defects

The TPS defines three methods (known as setting functions) for identifying defects:

• Contact method: identifies defects by testing the product’s physical attributes such as shape, color, or size
• Fixed-value (or constant number) method: requires that a certain number of movements are made and alerts the operator if this number has not been met
• Motion-step (or sequence) method: determines whether the steps in a process have been followed

These methods might be performed during inspections at different points throughout a process. Defects might be identified during a successive inspection, which is done at the next step of the process by the next worker; a self-inspection, in which workers check their own work immediately after doing it; and a source inspection, in which conditions are checked before the step takes place. Source inspections are often automatic, and the process cannot proceed until conditions are met.

You can design a process to signal when an error has occurred using two types of signals, known as regulatory functions:

• Warning functions are bells, buzzers, lights and other sensory signals that alert the worker to the error.
• Control functions prevent the process from proceeding until the error is corrected (if the error has already taken place) or conditions are correct (if the inspection was a source inspection and the error has not yet occurred).

While a control function makes it impossible for the process to continue if an error exists, a warning function sounds an alarm to alert the operator to the defect.

As IIoT sensors, device integrations, and manufacturing software become more accessible, it is becoming easier than ever to error-proof manufacturing processes. Here are a few examples of ways to implement Poka-Yoke:

• Use pick-to-light systems to light up the correct bin or part needed during a process step
• Use break beams to detect whether the operator has reached into the correct bin, and prevent the process from advancing to the next step until the correct part is obtained
• Use a digital scale to detect whether a product weighs as it should and halt the process if the weight does not meet the requirements
• Integrate tools such as torque drivers and calipers to perform operations to exact specifications
• Require products to pass machine vision inspections before allowing them to proceed down the line

Recent developments in manufacturing technology have opened up a variety of possibilities for error-proofing processes in your factory. Implementing poka-yoke in your factory can have immediate benefits to the quality, efficiency, productivity, and safety of your operations, and it can contribute to your factory’s continuous improvement for years to come.