| Written by Mark Buzinkay
The idea of one-piece flow
One-piece flow—also known as continuous flow—is a lean manufacturing principle that focuses on moving one workpiece at a time through each step of a production process, rather than processing large batches. The goal is to streamline workflow, reduce waste, and improve quality and efficiency.
One-piece flow is built on several key concepts that aim to optimise production efficiency. At its core, it involves moving a single product or component through each process step without delay, promoting a smooth and continuous workflow. This approach significantly shortens lead times by eliminating the wait times associated with batch processing. Because items are processed one at a time, work-in-progress inventory is kept to a minimum, reducing clutter and streamlining operations. Additionally, defects are detected immediately since each unit is inspected as it moves through the system, allowing for quick corrective action. The method also enhances flexibility, making it easier to adapt to changes in product design or customer demand. Lastly, it helps improve space utilisation on the shop floor, as there is less need to store large quantities of partially completed goods.
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The effect of a batch size
Some machines process a single piece at a time, others, a load of identical pieces; others yet, sets of matching pieces.... In the pipe model, cumulative volume, through one operation for one product over time, is a straight line, but it only truly is if the product is a liquid or a powder pouring into a container. With one-piece flow, it is actually a staircase with such small steps that it looks like a straight line.
If the operation is performed in batches, it looks different depending on whether you are considering it with the range on the long time axis with respect to the processing time of a batch or with the range used for planning the work of production operators. Then the steps are high, and the cumulative volume function cannot be confused with a straight line within a shift or a day.
The corporate planning staff is far enough for the staircase to look like a straight line again. Up close, to production operators, it doesn't. And the lumpiness of the process affects their work, the lead time through the operation, and the output quality.
Each piece must wait for a full batch to accumulate before it is processed, and it waits again to be picked up from a processed batch. Between the buffers in front and back of the operation and the batch in process, there are, in steady-state, an average of two batches of work-in-process that need to be managed, and a lead time that is proportional to the batch size.
In addition, the wait delays the discovery of quality problems and this is a strong motivation to pursue one-piece-flow. Finding, or developing, a machine able to process a single piece in 1/50th of the time the current machine takes for a 50-piece batch is sometimes a technical challenge that the transition to one-piece flow has to wait for new technology. It is often feasible, particularly when the current machine is underutilised, and the alternative does not have to match its capacity. None of these issues are perceptible from headquarters.
Comparison: One-Piece Flow vs. Batch Production
| Feature | One-Piece Flow | Batch Production |
| Lead Time | Short | Long |
| Inventory | Low | High |
| Flexibility | High | Lower |
| Quality Control | Immediate feedback | Delayed detection |
| Space Requirement | Less | More |
| Changeover Time Impact | More significant | Less significant |
Cell design for one-piece flow
Cells should be designed with a one-piece flow. Initial cell implementations normally do not involve the automation of materials handling between machines within the cell. The most common pattern is for parts to be hand-carried between machines, and the major difference with what happens in a job-shop is that instead of having one operator pick a part in an input buffer and placing it in the output buffer, the parts move directly from one machine to the next with no buffer in between. The parts may even be loaded and unloaded in the same machine by two different operators.
While one-piece flow clearly minimises the amount of WIP in the cell, the corresponding inventory reduction is not the main point. The main benefits of one-piece flow are the quality improvements that accrue due to the following:
1) One-piece flow minimises the transit time of each workpiece through the sequence of operations in the cell. As a consequence, if any of the machines in the cell start making defective parts, the operators detect it before the machine has had the time to do more damage.
2) By preserving process sequence, one-piece flow facilitates the tracing of problems. Not only does it make it easy to isolate the defective part, but it also allows us to observe the parts made before the first defective part to see, for example, whether the deterioration of the process was gradual or sudden.
One-piece flow is not applicable with machines that process multiple identical parts at the same time when their entire capacity is required. While such machines are undesirable, they are sometimes inevitable, at least until engineers find an alternative that works one piece at a time. Often, these machines are monuments and cannot be circumvented nor replaced, but sometimes they are small enough to be integrated into cells. On a smaller scale, these machines are to cells as monuments are to the plant as a whole: they are rogue machines, and are not allowed to impose their mode of operation on the rest of the machines in the cell. While all the other machines work in one-piece flow mode, a full load accumulates in front of the rogue machine, and the downstream process works off a completed load.
Image: Chaku-chaku line (bottom) compared to a conveyor-based assembly line (see also: The assembly process)
Chaku-chaku lines
Chaku-chaku lines are an advanced form of cells. To appreciate the value of the concept and to be able to implement it, you need to experience of first implementing cells. Chaku-chaku lines are not fully automatic. They are semi-automatic operator-machine systems embodying a philosophy of what is best done by people and by machines. The term "chaku-chaku" literally means "load-load". As a mode of operation, it is opposed to "datsu/chaku-datsu/chaku", which means "unload/load-unload/load".
A chaku-chaku line is still a cell, but compared to a machining cell it shows a higher density, two buffers removed through the conversion of batch machines to one-piece flow, automatic unloading retrofitted to the machines, as-you-go switches that allow the operator to start a machine while walking over to the next one, chutes to transfer parts between machines and one-touch setups between products. When it comes to an assembly line, the transition to a chaku-chaku assembly line involves the conversion from a straight line to a U-shape, the removal of a conveyor, bringing the assembly stations closer, introducing simple automation steps (such as driving screws), automatic unloading at each station, chutes between stations, as-you-go switches, one-piece flow and one-touch setups.
FAQ
What is the main advantage of using one-piece flow in manufacturing?
One-piece flow significantly reduces lead time by eliminating delays between production steps. It improves efficiency, exposes problems early, and enables faster customer delivery. This streamlined approach also reduces waste and excess inventory, supporting lean manufacturing principles.
Is one-piece flow suitable for all types of manufacturing?
Not always. One-piece flow works best in environments with stable, predictable demand and standardised processes. It's ideal for low to moderate product variety. Hybrid approaches combining batch and flow may be more effective in highly customised or irregular production settings.
How does one-piece flow affect quality control?
One-piece flow enhances quality control by enabling immediate detection of defects. Since only one unit is processed at a time, any issues are caught quickly, preventing defective products from moving further down the line or accumulating unnoticed in batches.
Takeaway
One-piece flow is a powerful lean manufacturing principle that enhances efficiency, reduces waste, and improves product quality by eliminating the delays and excess inventory associated with batch processing. Its successful implementation requires thoughtful cell design, operator involvement, and often technical adaptations to equipment. While unsuitable for every scenario, one-piece flow provides significant operational advantages when applied appropriately. It encourages faster feedback, better problem tracing, and greater responsiveness to customer demand. Understanding its impact from a production and planning perspective is key to unlocking its full potential on the shop floor.
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Glossary
Work-in-Progress (WIP) refers to inventory that is partially completed but not yet ready for sale. It includes materials, components, and labour invested in products that are between the start and end of a production process. High WIP levels can indicate inefficiencies, such as bottlenecks or overproduction, and tie up capital and space. Lean manufacturing aims to reduce WIP to improve flow, responsiveness, and quality.(2)
References:
(1) Baudin, M. & Netland, T. (2023). Introduction to Manufacturing. Routledge.
(2) Womack, J. P., & Jones, D. T. (2003). Lean Thinking. Free Press.
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