THE NINE PRINCIPLES OF LEAN MANUFACTURING
di Kevin Gingerich Industrial Control Design
Tra i migliori articoli pubblicati nel 2007 da Industrial Control Design, c'è anche questo testo di Kevin Gingerich sulla Lean Manufacturing.
In today's manufacturing environment, assembly work is routinely characterized by short production cycles and constantly diminishing batch sizes, while the variety of product types and models continues to increase. Constant pressure to shorten lead times adds to these demands and makes the mix truly challenging, even for the most innovative manufacturers.
The ability to respond quickly to rapidly changing customer demands requires the use of manufacturing systems that can be re-configured and expanded on the fly, and which can accommodate advances in assembly techniques without making any initial manufacturing investments obsolete.
Lean manufacturing, an approach that depends greatly on flexibility and workplace organization, is an excellent starting point for companies wanting to take a fresh look at their current manufacturing methods. Lean techniques are also worthy of investigation because they eliminate large capital outlays for dedicated machinery until automation becomes absolutely necessary.
Indeed, the concept of lean manufacturing represents a significant departure from the automated factory so popular in recent years. The "less is better" approach to manufacturing leads to a vastly simplified, remarkably uncluttered environment that is carefully tuned to the manufacturer's demands. Products are manufactured one at a time in response to the customer's requirements rather than batch manufactured for stock. The goal is to produce only the quantity required and no more.
And since limited numbers of parts are produced, it may be necessary to change processes during the day--to accommodate different parts and to make maximum use of personnel, equipment and floor space. The flexibility inherent in manual assembly cells is therefore preferable to automated assembly. This requirement for maximum flexibility creates unique demands on the lean workcell and the components that make up the lean workcell.
Granted, the lean approach is not the solution for all manufacturing problems. But it does offer a uniquely flexible solution for assembling more complex products. This guide describes 9 basic lean manufacturing principles that should help you evaluate lean manufacturing solutions for your own applications.
The 9 principles discussed are: Continuous Flow, Lean Machines/Simplicity, Workplace Organization, Parts Presentation, Reconfigurability, Product Quality, Maintainability, Ease of Access, and Ergonomics.
1. Continuous Flow
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.
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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 down time. 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.
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 re-configuration is necessary. Casters may be quickly mounted to the T-slotted profiles to allow movement without the use of fork trucks or other lifting equipment.
3. Workplace Organization
A smooth, uninterrupted flow of completed workpieces is the desired result of a properly designed lean workcell. Nothing can slow or stop this flow faster than the loss or misplacement of tools. Thus, all tools used at a workstation should have their own holder. There should be exactly as many holders as there are tools so that the absence of a tool is quickly noticed. Using a modular tool holder system with a specific holder for each tool is ideal. If holders can easily be added to or taken away from a workstation, this simply adds to the flexibility of the workstation and increases its usefulness in a lean manufacturing process.
To minimize downtime, backup tools should also be available at any automated workstations. These tools should be out of the worker's way until a failure occurs at the automated workstation. Of maximum benefit are tool holding structures that allow tools to be swung or slid into the work space and easily returned to the storage position when no longer needed. Information Boards.
Naturally, the ready availability of work-critical 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.
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. The bins in the photo use a key stud to lock them in position. Bins are 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 work space. 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.