Thin Flow

When we talk about flow, we refer to “one-piece flow” or “continuous flow.”

Reading articles by Kaneyoshi Kusunoki, the second head of TPS, he often mentions the term “thin flow.”

Eiji Toyoda told us, “We don’t have money, so we can’t buy new equipment. To become profitable with the available equipment, we have to make the material flow as thin as possible and operate with Just In Time.”

Kusunoki, K. (1997, December 5). 生産技術開発とトヨタ生産方式の確立. https://www.jsae.or.jp/files_publish/page/180/interview42.pdf

This was Kusunoki’s reference to Eiji Toyoda’s teachings, but he also mentions “Thin flow” in his book.

So, what is “Thin flow”? And if there is a “Thin flow,” there must also be a “thick” or “fat” flow. Let’s think about the difference.

The main characteristic of “thin” flow is that it has minimal Muda, as Eiji Toyoda stated. Excess waste makes the flow noncompetitive. It should avoid overproduction and other types of Muda. Just because it’s flowing doesn't mean it’s free of Muda.

Another key point is that this isn’t about a conceptual or aggregated flow but the precise, real-world flow of how things actually happen.

An engineer drew a flowchart to illustrate the factory's operations. It was a simple horizontal diagram with blocks and arrows that traced a neat sequence. But the actual flow was much more complex. When I asked why not draw it exactly as it is, the engineer replied, “It’s too complicated to draw.” And what benefit do we get from drawing something oversimplified? An inaccurate map can be just as harmful as having no map at all.

When the engineer draws a large arrow, many movements aren’t captured accurately. There are many processes inside each block—some sequential, others parallel. Between these processes, multiple paths exist. Movements happen all over within the arrows. There are numerous temporary storage locations, or sometimes they are undefined. Material handling isn’t standardized; many assume someone else will handle it. The information flow was even worse—multiple handwritten papers, created randomly, requesting extra parts outside the regular system. In the end, within that big fat arrow, it was chaos.

Within large rivers or oceans, many currents flow. Some areas flow faster, others reverse, and every current has both positive and negative effects. Currents bring essential moisture, heat, and nutrients, but can also be destructive. Understanding currents has helped humans sail, fish, farm, harness energy, and more. Underestimating or ignoring them can lead to drowning. Currents aren’t always visible, so sailors and fishermen learn about them through experience.

This is similar to what I think the “thin” flow is.

A large “fat” flow is like an ocean or river. Inside are many “thin” flows—currents. For those immersed in the flow, the current is life or death. From a big-picture view, it seems like one large flow, but beneath the surface, it’s different—treating a “fat” flow as one can mean sacrificing many things along the way.

Small causes can impact the “thin” flow—rocks change currents, small amounts of water cause shifts, temperature and wind affect it. The same applies to factory “thin” flows. Unclear locations lead to violations of the 5S principles. A missing return route for empty boxes causes disruptions. Undefined paths create traffic jams. Without proper standardized work and Kanban, overproduction is likely to occur. Since small factors influence the “thin” flow, we can improve it through Kaizen.

In Japan, there's a saying: “Thin flows collected will become a large river.” 【細き流れも大河となる】. Consistent small efforts over time yield significant results. If you want Kaizen to be truly effective, you need to focus on the “thin” flow.