Cells and Continuous Flow
Updated: Feb 26, 2021
The preferred shape of the lean workcell is U-shaped. Each subprocess is connected to the next in order of process. With the worker in the interior of the U, minimum movement is required to move the workpiece or assembly from one workstation to the next.
Ultimately, one of the goals of the lean workcell is to eliminate all non-value-added movement; hence its U-shape. When the worker has finished the process, he simply turns around and is back at step one.
The workpiece may be carried from one value-added operation to the next. However, there are times when the workpiece or the fixture holding the workpiece is too heavy and must be transferred mechanically between workstations.
Although very heavy parts may be transported on belt conveyors, manual push or gravity conveyors are ideal for moving parts between workstations. Their minimum complexity makes them easy to service and minimizes downtime. In addition, they are easy to connect end-to-end, which makes it easy to move workstations within a workcell. The curved “corners” of the U-shaped workcell can pose a problem. As potential dead space, they may act as a mini-storage area, thereby encouraging a return to batch processing. Instead, the use of a ball roller transfer should facilitate part movement through the corners of the U-shape.
Machines in order of process
Parts arrive as needed
One-piece flow (small lot flow)
Non-cyclical work done outside cell by support people
Since continuous-flow, one-at-a-time manufacturing is another goal of lean manufacturing, it is important that each workstation or machine be designed to fit within a minimal envelope. The minimal envelope ensures the elimination of excess flat space at the workstation or machine. This is done to avoid the possibility of storing parts or subassemblies at the machine. Storing parts increases work in process and results in “batch” processing, which subsequently defeats the purpose of lean manufacturing. In addition, smaller, minimal-size workstations and machines eliminate unnecessary steps taken by the worker between subprocesses.
Finally, significant floor space may be saved by properly sizing workstations and machines. Although tempting for the sake of conformity and standardization, the deployment of standardized machine bases or workstations for all processes should be avoided. Each machine base or workstation should be designed to optimize assembly subprocesses, which in most cases will vary from workstation to workstation. This customization can be achieved with virtually any structural material. To save on cost, however, as well as to minimize the environmental considerations related to disposing of inflexible welded steel structures, preference should be given to material that is reconfigurable and reusable. The modular characteristics of extruded aluminum, bolt-together systems make them perfect for the implementation of lean manufacturing concepts.
Moreover, in a continuous improvement environment, all workstations and workcells must be easy to modify as process improvements are identified. In addition to their superior flexibility in layout and design, lightweight aluminum structures are easier to move when reconfiguration is necessary. Casters may be mounted to the stations to allow movement without the use of fork trucks or other lifting equipment.
Naturally, during the average work shift, additional parts will be required for the workcell. Traditional methods of resupplying workstations are not useful in a lean workcell. Each worker should go about his work with the minimum number of interruptions.
Therefore, all parts should be supplied to each workstation from outside the workcell. The use of gravity feed conveyors or bins fits the simplified design of the lean workcell. Parts bins should load from behind (outside the working area of the workcell) so that the worker may continue production without interruption. Gravity carries the parts to the worker’s reach area. Bins should also be reconfigurable.
Bins should be easily stackable and provide the ultimate in flexibility when reconfiguring the workplace.
Although bins are ideal for small parts, many assemblies require larger parts. These may be delivered in bins or boxes. Again the parts should be delivered to the workcell without entering the workspace. Gravity feed conveyors serve this purpose well. In the event that scrap or containers must be removed from the cell, an additional gravity feed conveyor may be mounted in the reverse direction. In instances where parts are very heavy, lift assist devices are recommended. Heavy parts or boxes of parts can be loaded onto a case lifter and raised to the proper work height with electric, pneumatic, or hydraulic power.
Naturally, the ready availability of workcritical information also adds to efficiency in a workcell. Supplying the right information at the workplace, such as assembly processes, work instructions, repair procedures, or even production targets, allows workers to make the right decisions and act on them on the spot, limiting downtime often spent chasing down a busy supervisor. As with everything in a lean workcell, the information board should be simple, easy to reposition, and reusable.