Since the mid-1990's, almost every factory I've been to has had or has claimed to have had manufacturing cells in one form or another. I have seen everything from true continuous-flow cells (very rare) to a bank of like machines brought together to form a "cell." In most cases, however, management would often admit that they haven't seen the kind of improvements they expected from implementing such cells. Let's look at common types of cells and how (and whether) they fit into a lean manufacturing environment.
1. Functional cells - Functional cells are cells consisting of like equipment. For example, a factory that does primarily machining operations might have a bank of lathes together in a "turning cell." Another example would be a cell consisting of several sets of like test equipment. These cells are called "functional cells" because they perform a specific function (as opposed to manufacturing a complete product, assembly, etc.).
Though there are exceptions, in most cases functional cells do not fit into a lean manufacturing environment. In a factory consisting of functional cells, product often travels from cell-to-cell, to have various operations done. Functional cells often create a haven for several types of muda:
· Inventory – WIP Inventory often accumulates in front of equipment and/or processes in a functional cell.
· Transportation – Product often moves from cell to cell throughout the factory.
· Waiting – Product often waits long periods of time without being operated on.
· Over-production – Functional cells usually have large, expensive equipment. Emphasis is usually on keeping the machine running. However, there is usually little emphasis on quick changeover. This causes a tendency to overproduce for economies-of-scale’s sake.
· Over-processing – Large, expensive equipment is often used where smaller equipment should be used.
· Defects – Because operators are generally process focused and are usually not aware of the bigger picture (i.e., the completed product), quality suffers in a functional cell. Defects are often created and not detected until much later in the process.
2. Group Technology (or Mixed Model) Cells - These are the least understood of the types of cells. Many people mistake Group Technology cells for Functional cells when in fact they are most closely related to Product-focused cells; that is perhaps the reason many now call Group Technology cells "Mixed Model" cells, showing emphasis on the product. These are cells in which a series of operations for several products takes place. The products are often very similar, and the operations are very similar for each product (though not usually identical). This type of cell can work very well within a lean manufacturing environment, particularly if the organization is characterized by high-mix, low volume products. In such organizations, it is rarely possible to have product-focused cells.
3. Product Focused Cells – Cells that are product-focused typically run one type of product through a series of operations. These are often thought of as the ideal lean manufacturing cell. They are perfect for low mix, high volume environments.
Hopefully, this article can help clear up many of the misconceptions surrounding manufacturing cells. Functional cells, which contain equipment that performs an identical process, are generally incompatible with lean manufacturing. On the other hand, Group Technology (or mixed model) and Product-Focused cells work well for the lean producer. For a high-mix environment, group technology cells, which generally run product families through a series of similar operations, are more appropriate. For a low-mix environment, product-focused cells are ideal. Whether you use Product-focused or Group Technology cells, the most important factor is whether or not you create continuous flow. To learn how best to do that, read our article "Implementing Continuous Flow Manufacturing Cells."