Working with flow

Photo: Rexroth.

One of the goals of the lean workcell is to eliminate all non-value added movement; hence the preferred U-shape. With the worker in the interior of the U, minimum movement is required to shift the workpiece or assembly from one workstation to the next. When a process is finished, the worker simply turns around and is back at step one.

The workpiece may be carried from one value-added operation to the next; however, when it or the holding fixture are too heavy, they’re transferred mechanically between workstations.

Very heavy parts are transported on belt conveyors, but manual push or gravity conveyors are ideal for moving other parts between workstations. They’re easy to service, which minimizes downtime and end-to-end connections makes moving workstations within a workcell efficient. The curved “corners” of the U-shape can pose a problem. As potential dead space, they may act as a mini storage area that encourages a return to batch processing. A ball roller transfer should facilitate part movement through the corners.

It’s important each workstation or machine is designed to fit within a minimal envelope. This eliminates excess flat space and avoids the storage of 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. Smaller, minimal size workstations and machines eliminate unnecessary steps taken by the worker between subprocesses.

Significant floor space is saved by properly sizing the workstations and machines. Although tempting, standardizing for all processes should be avoided. Each should optimize assembly subprocesses, which in most cases will vary from workstation to workstation. This customization is achievable with virtually any structural material. To save on costs and minimize environmental impacts related to disposing of inflexible welded steel structures, preference should be given to material that’s reconfigurable and reusable.

The modular characteristics of extruded aluminum bolt-together systems make them perfect for lean manufacturing.
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 eliminate 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. Work should proceed with the minimum number of interruptions. Therefore, all parts should be supplied from outside the workcell. The use of gravity feed conveyors or bins fits the simplified design of the lean workcell. Parts bins load from behind (outside the working area) allows the worker to continue production without interruption as gravity carries the parts to the reach area. And reconfigurable, easily stackable bins provide flexibility.

Although bins are ideal for small parts, many assemblies require larger parts that are delivered in bins or boxes without entering the work space.

Keep the cell clear
Gravity feed conveyors serve this purpose well. An additional gravity feed conveyor mounted in the reverse direction handles scrap or containers that must be removed from the cell. Lift assist devices are recommended for heavy parts or boxes of parts that load onto a case lifter and raised to the proper work height with electric, pneumatic or hydraulic power.

Ready availability of work-critical information also adds to efficiency. Supplying the right information, 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 the downtime spent chasing after a busy supervisor. You should never mirror a cell: most of all your standard equipment, fixtures, and assembly aids become difficult to use because they’re designed for the standard right-to-left cell configuration.

Following these processes and going with the flow makes production more efficient, reduces costs, eliminates waste and accomplishes the lean mission.

Richard Kunst is president and CEO of Cambridge, Ont.-based Kunst Solutions Corp., which publishes the “Lean Thoughts” e-newsletter. Visit www.kunstsolutions.com. E-mail rkunst@kunstartofsolutions.com.

Find this article in the April 2014 issue of PLANT.