Konnyaku Stone

A product usually goes through a series of processes before it comes out as the ideal and final piece. Its raw materials may have certain characteristics or flaws that need to be removed as it goes through the production process. A perfect diamond cut isn’t normally achieved outright. It undergoes treatments and various techniques until the flaws are removed and the overall quality is enhanced. Removing imperfections to ensure product quality is the underlying concept of the konnyaku stone.

What is Konnyaku Stone?

Konnyaku stone is a tool that is used by Toyota to remove imperfections from manufactured car parts. It’s also referred to as the devil’s tongue by Toyota workers. It’s not a stone per se but a mixture of gritty resins that were shaped into a stone.

A konnyaku stone is used to smoothen metal surfaces which are what car panels are made of. These panels would have blemishes and imperfections in them once they are made. As part of the quality control process, a worker will inspect a car part for any flaws and will use the konnyaku stone to smoothen and perfect it.

Toyota knows what their customers are expecting from their cars. Blemish-free and smooth auto body panels are what makes their car shine from the outside. And this is the ultimate objective of applying the konnyaku stone. To further strengthen the process, there are checkpoints in the process to inspect and sign off parts that have been subjected to the konnyaku stone. This way, only those parts that meet the requirements will be passed onto the next process.

Removing Imperfections as Part of Quality Control

Although a konnyaku stone is a physical tool, the concept of removing imperfections as part of quality control can be applied to any business operation. Checkpoints, inspection, checklists, and quality standards serve as konnyaku stones as they are used to examine any work-in-progress item for imperfections and flaws. The team should then be pushed to take action if issues, flaws, or physical imperfections are found. These enable teams to improve their processes to ensure quality is built in every step of the way.


Konnyaku Stone – Toyota Production System – Image Source: Toyota.co.uk

Whether using an actual tool to remove imperfections or implementing processes and procedures to spot and resolve any issues, what’s important is there are measures in place to establish quality. Employees should have the means to inspect product quality and resolve any issues. This allows them to take ownership of the products they produce and the process they use. Having ownership of the process allows them to think of ways to continuously improve their work and consistently provide high-quality products.

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