Principle 1: Continuous Flow. This concept requires minimum movement of work piece or assembly from one workstation to the next. For this reason the preferred shape of the Lean work cell is U-shape (as shown in appendix A, figure 7), connecting each subprocess to the next. In this way, all non-value-added movements are eliminated, by carrying work piece from one value-added operation to the next, either manually or mechanically, if the work piece is too heavy. Manual push or gravity conveyors are ideal for transportation of very heavy parts, although belt conveyors may also be used.
Principle 2: Lean Machines (Simplicity). It is important to design each workstation or machine to fit within a minimal envelope, in order to reduce unnecessary steps taken by the worker between subprocesses, and provide continuous flow, thereby resulting in “batch”ť processing. If this principle is not considered, it can increase the amount of work done in the process. The reason is that excess flat space increases the possibility of storing parts, or sub-assemblies, at the workstation or machine. Moreover, to enable continuous improvement of the work environment, the layout and design of the process should be flexible so that all workstations and work-cells can be easily modified, as areas for process improvements are identified. Optimizing machine base or workstation is also vital in Lean Manufacturing, since it can aid saving on cost and the environment, by using reconfigurable and reusable material in machines.
Principle 3: Workplace Organization. This involves everything in the work cell including tool holders and information boards. To achieve an efficient Lean work cell, it is vital to provide a smooth, uninterrupted flow of completed work pieces. Avoiding loss or misplacement of tools, and using separate tool holders for the tools in each workstation allows immediate awareness of the absence of a tool. It is also important that spare tools are stored at any automated workstation, in order to minimize downtime, and maintain a continuous flow, by replacing damaged tools quickly.
Using simple, easy to re-position and re-usable information boards at the workplace, to inform about repair procedures, work instructions, assembly processes, or even production targets, will allow workers to effectively carry out any task at hand.
Principle 4: Part Presentation. According to this concept, additional parts supplied to the workstations will minimize the number of interruptions. For instance, case lifters or bins (as shown in appendix A, figure 8 and 9 respectively) reduce interruptions because bins are ideal for small parts, whereas case lifters can be used to raise heavy and large parts to the proper height using pneumatic, electric or hydraulic power.
Principle 5: Reconfigurability. The design of the work cell must be such that it can be easily reconfigured, at times when the process needs to be altered in order to accommodate assembly of a new product. This concept also emphasizes the importance of quick changeover of a machine or workstation in order to decrease production time of the process.
Principle 6: Quality. For primary quality assurance, visual inspection by the worker or through gages (a measuring instrument or device) is required. The test fixtures and gages must be mounted onto the machine or workstation, and easily changed or replaced. The cause of poor quality may be a malfunctioning machine. This can be easily changed, if the Lean cell is designed in a way, such that there are no pneumatic or electrical connections between the machines. In this way the machine can be replaced quickly, whereas the need to disconnect lines would slow down the process. Another factor that can lower quality in Lean Manufacturing is flawed process. This can be avoided using a structural framing system, which would enable alterations to be carried out within minimum amount of time.
Principle 7: Maintainability. This refers to the ease of service. To provide optimal maintainability, a modular structural framing system is required. This would allow individual components to be reconfigured or replaced, or even, for the entire machine bases to be rebuilt, in minimum amount of time. In terms of maintaining the structure, the structural framing system provides minimum number of tools, and common components, in order to eliminate the need for a large inventory of spare parts.
Principle 8: Ease of access. This requires all necessary work components to be in easily accessible locations. To ensure all workers have access to the components, they need to be easily and quickly repositioned, and added to any workstation.
Principle 9: Ergonomics. This concept is concerned with the ergonomic problems of height, and lifting. It is required that the work cell is adequately designed to maintain, ergonomically, correct height, considering the average height of the workers. Also, the optimum height of a workstation or machine must be easily changed to accommodate the average height in different countries. In terms of lifting devices, if the weight of the parts exceeds lifting standards, then simple electric, hydraulic or pneumatic devices should be used. Finally, to prevent faulty design, and analyze ergonomic issues in the design stage, software packages that test ergonomics of a work cell can be used.
Lean Manufacturing principles can vary based on the organization. In other words, each organization develops, and uses principles, based on its requirement for improvement. Out of all of the principles illustrated above, the first six principles are more general, whereas the nine principles were specifically developed for Rexroth (Bosch Group).