By Gerald Najarian
The terminology “cellular layout,” “overlapping operations” or “machine cells” now has a familiar ring in manufacturing parlance. What these terms mean for manufacturing in the twilight of the 20th century and in the dawn of the new century is more important than their place in the terminology. The potential for waste avoidance and improved quality and productivity of Lean manufacturing and particularly the cellular manufacturing component of Lean business is dramatic.
Cellular operations must be seen in contrast to the traditional approach to production. Traditional manufacturing operations are organized by functional specialty into plant departments. This means, for example, that all presses are in one department, all milling machines in another, welding machines in another and so forth. Workers are spread out along linear-shaped production lines and inevitably produce goods in large batches or lots. Batch-oriented linear production means that an operation completes all of the shop order and then moves it to the next operation for further processing.
In a Lean production cellular organization, just the opposite is the norm. Production is organized by product rather than by function with equipment dedicated or partially dedicated to a family of products. Production in such cells is conducted sequentially in the order in which operations must be performed to produce the end item. Workers in this type of environment are in close proximity to one another and are performing operations on lots of one.
Work Cell Organization
Henry Ford’s early assembly line is the original prototype of the product focused work cell. In the early Ford factories, the work cell produced automobiles in lots of one. The automobiles being produced had a synchronized flow of fabrication, sub-assembly and final assembly in the sequence of operations necessary to produce the end item—A Model T. The legendary River Rouge plant had a four-day production cycle from processing of ore into steel through final assembly. Steady demand and a monolithic product line facilitated this high-volume linear operation.
The work cells of our era are much different. We no longer have the luxury of steady and predicable demand for an uncomplicated product line. The Ford assembly line model was inflexible and oriented to high-volume production. There are four primary characteristics of the modern Lean production work cell organization:
- Product/product–family focus. Just as in Ford’s early factories, product/product family organization and layout are what constitute a work cell oriented plant. Equipment is dedicated to the product line.
- One at a time production. A lot size of one is what causes overlapping operations. In a one-at-a-time production environment, the succeeding operation is started immediately after the part emerges from the preceding operation thereby creating a batch of one piece. Earlier approaches to machine cells, while having the equipment dedicated to product lines, still produced parts in large batches.
- Flexible output levels. The traditional approach to production lines is to have the line speed set at one level—the highest. To realize the benefits of Lean processes, today’s work cell flexes the output level to the ultimate level of demand by the customer whether the customer is the end user or a succeeding work cell. All work cells must have their output levels synchronized to avoid batch production. Flexing the rate of production flies in the face of traditional thinking about efficiency which seeks to keep equipment fully utilized.
- Operator multi-tasking. In a Lean production work cell, operators typically operate more than one machine concurrently. This may not sound unusual for in traditional operations operators are often responsible for more than one machine. However, in these cases, the operator is usually running similar machines making different parts. In a Lean production work cell, an operator is concurrently operating machines performing sequential operations on one part.
Lean production work cells are a practical application of a once popular phrase: managing for results. Managing for results in a work cell environment asks for only the amount of production needed during the period with only the amount of people required to produce that amount. In this way, we optimize the result rather than utilization as was the case in the past.
People and Layout
An essential precept of Lean work cells is the idea of moving people to the work instead of bringing work to the people. In a work cell this concept begs the question of line layout for, if operators are to be responsible for numerous machines making the same part, they will need the appropriate proximity to those machines. This concept of “operator in motion” contrasts with the usual picture of the worker sitting at the machine for the full shift. In a Lean production work cell, the operator is usually standing and moving a few steps in various directions to manage the operation of the machines.
The layout implication of moving people to the work is abandonment of the linear approach to production line operations. Replacing the linear operation are two similar production line configurations: the U-shaped line and the parallel line. These are variations on the theme of having workers in close proximity so that they can do more than one thing if the line must be slowed down while providing the flexibility to speed up production by adding more people and having them perform singular tasks in the work cell. In a U-shaped production line, operators stand in the middle of the U and—as work progresses from one part of the U to the next—can operate different machines by merely turning around. The operator has 360-degree mobility with the consequent ability to do work on both sides of the U or at the adjacent work stations.
Cellular operation is not just another way to layout a plant and organize production. There are tangible benefits to be realized from Lean production work cells.
- Lower total labor cost. Although flexing work center output may seem to work against lower labor costs, the fact is that a U or parallel shaped work centers and worker mobility results in more effective use of the total workforce across all centers.
- Reduced material handling. As lot sizes approach a size of one within sequential operations, the need for batches of goods to be moved from operation to operation diminishes.
- Improved quality. When goods are produced in lots of one in sequence, quality problems surface as they occur and therefore must be solved immediately. Immediate quality problem detection eliminates the hidden plant capacity required for rework of defective parts.
Less work in process. When parts are produced in large lots and in unsynchronized operations, work in process accumulates in the queue with the consequent congestion and carrying costs of such inventory.
A fresh look at cellular organization, based on its roots in the assembly line model, can lead to production layout and management which optimizes workforce efficiency in ways previously unheard of.
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