One-piece flow: getting Lean with cellular manufacturing

What is cellular manufacturing?

Unlike traditional mass or batch-and-queue production systems, cellular manufacturing subscribes to a “one-piece, pull production” system. Like its name suggests, cellular manufacturing relies on workcells to facilitate flow with production flowing from one cell to the next and no work-in-process in between.

Cellular manufacturing first gained popularity as a key concept within the Toyota Production System before it eventually became a Lean/JIT production fundamental, especially for a method known as “one-piece flow.” The origin of cellular manufacturing may have its roots in the automobile industry, but it can work on just about any type of project in any kind of business.

One-piece flow

One-piece or single-piece flow is systematically different from a traditional batch setup where several units of a product are processed at one stage and moved onto the next step as a group. Here, inventory typically waits in queue for processing.

There is little relationship between the manufacturing steps and each step is controlled by its own independent schedule. This type of batch-and-queue production typically results in:

• High amounts of work-in-process inventory
• Long manufacturing lead times
• Poor on-time delivery
• Higher rate of defects

On the other hand, one-piece flow cultivates a smooth, connected flow between each of the manufacturing steps by targeting the flow within the workcell. As the name implies, a single piece at a time is moved between operations in the cell. It maintains the lowest level of WIP by only ever working on one item at a time.

Workcells

A workcell can be defined as “a work unit larger than an individual machine or workstation but smaller than the usual department.”

Cells are grouped by the products or parts they produce, and each cell represents a specific processing sequence. Everything within the cells – people, equipment and materials – are absolutely necessary to that sequence. The three types of workcells in manufacturing are:

1. Functional cells: These cells are designed with like equipment to complete a specific function, e.g. shipping, testing, welding, etc. Functional cells are not inherently Lean and can easily become a repository for excess inventory, defective products, overproduction and other manufacturing wastes.
2. Mixed model cells: Also called group technology cells, these cells work well in Lean facilities. Like in cellular manufacturing, several operations are laid out in the cell in a series. Products made in these cells are similar and rely on quick changeover times for a smooth workflow.
3. Product-focused cells: These are considered to be the ideal Lean manufacturing cell. Processing steps are arranged in the order of operations and only one type of product is run through the cell at a time.

Cells are set up in the sequence of production, with workstations positioned close together; the most common layouts are a U, a T, an S, and a Z shape.

U-shaped cells were Toyota’s introduction to cellular manufacturing, and the shape became the ideal over time because it allowed operators to communicate better and move around quicker. While a U shape is usually the easiest to set up, a Z layout might make more practical sense for cells housing larger equipment.

Cell layout varies, but each one is designed to address the Eight Wastes of Lean: defects, overproduction, waiting, unused talent, transportation, inventory, motion and extra-processing. For example, stations are placed close to each other in a logical, linear order to cut down on the unnecessary waste of motion.

Setting up a workcell

Successfully implementing cellular manufacturing hinges on the design and the flow of each individual cell. Remember that simplicity is key when designing a cell; it makes material flow much smoother.

There are many different approaches one can take to designing a workcell, but the Lean consultants from Strategos have defined four tasks of cell design:

1. Select the products: Which products belong together in a cell? For mixed-model cells, be sure you’re not attempting too much variety – one-piece flow will not work if machine changeovers take too much time. It can be beneficial to complete a process map for a better understanding of the rate of production.
2. Engineer the process: After choosing what products will be manufactured in workcells, it’s time to determine what exactly goes in that cell. What process steps must be included? What equipment is absolutely necessary? How many workstations should be housed inside the cell?
3. Define the infrastructure: Infrastructural elements directly support the process but don’t actually touch the product. For instance, how is material being handled? Should there be a limit placed on WIP? How is production scheduled? This may be a time to implement other Lean tools like Kanban and Takt time.
4. Lay out the cell: Because cells are completely self-contained, once you have determined the operators, equipment and machines needed, you are now trying to optimize the space.

What are the benefits of cellular flow?

Ultimately, cellular manufacturing optimizes Lean workplaces for a smooth workflow. It lays a strong foundation for JIT production by supporting one-piece flow, a production method favoured by the majority of operational excellence experts.

• Improved quality: Cellular manufacturing facilitates quality control by building quality into the product at each station. Because products are much more visible when they’re not moving in a batch, it’s easier to detect problems, problems are likely to be caught earlier, and you can retrace the product’s journey to identify where the issue occurred.
• Efficiency: Using workcells scales down production making processes more efficient and cohesive. Problems with equipment or within the cell itself can be remedied faster because they’re easier to locate. Additionally, when operators are moved around between stations, their work becomes less monotonous and they’re more likely to produce quality parts efficiently.
• Flexibility: While some Lean professionals argue cellular manufacturing hampers product flexibility, it makes production scheduling more flexible so it’s easier to accurately meet the customer demand. This prevents overproduction, excess inventory and extra processing.
• Safety: With so much focus on Lean, it’s easy to overlook the safety benefits of cellular manufacturing. A cutdown on transportation in the facility means less material is moved, reduced forklift traffic, and a lower chance of forklift accidents.

Although focusing on maximizing efficiency and productivity is central to workcell design, it’s important to not overlook safety. Safety is especially critical when it comes to automated cells where operators will be working alongside robots.

You will need to plan where the robotic machinery will be located, where human operators work, and how to keep them separated. Consider how to protect both from workplace conditions and be sure to install the necessary barricades and safety barriers.

Lean manufacturing can help a business achieve improvement, and what ultimately makes Lean techniques so successful is a one-piece flow system that utilizes cellular manufacturing.

Any business in any industry can use this manufacturing method to enhance efficiency and quality, while also reducing the 8 types of waste. A smooth flow will connect all the aspects of production and ensure that everything works cohesively.