تصمیم گیری جایگزینی تجهیزات و تولید ناب

Traditional manufacturing systems are built on the principle of economies of scale. Here, the large fixed costs of production are depreciation-intensive because of huge capital investments made in high-volume operations. These fixed costs are spread over large production batch sizes in an effort to minimize the total unit costs of owning and operating the manufacturing system. As an alternative to “batch-and-queue,” high-volume, and inflexible operations, the principles of the Toyota Production System (TPS) and lean manufacturing have been widely adopted in recent years in the US [1], [2], [3] and [4]. In this paper, we illustrate an equipment replacement decision problem within the context of lean manufacturing implementation. In particular, we demonstrate how the value stream mapping (VSM) suite of tools can be used to map the current state of a production line and design a desired future state. Further, we provide a roadmap for how VSM can provide necessary information for analysis of equipment replacement decision problems encountered in lean manufacturing implementation.

Traditional manufacturing systems are built on the principle of economies of scale. Here, the large fixed costs of production are depreciation-intensive because of huge capital investments made in high-volume operations. These fixed costs are spread over large production batch sizes in an effort to minimize the total unit costs of owning and operating the manufacturing system. Large work-in-process inventories are also characteristic of traditional manufacturing. The resultant “batch and queue” operation produces large numbers of a particular product and then shifts sequentially to other mass-produced products. As an alternative to batch-and-queue, high-volume, and inflexible operations, the principles of the Toyota Production System (TPS) have been widely adopted in recent years throughout the US [1], [2], [3] and [4]. Application of TPS principles have led to lean manufacturing (also called lean production, or lean thinking [4]) in which production and assembly cells consisting of product-focused resources (workers, machines, floor space, etc.) are closely linked in terms of their throughput times and inventory control. These cells are typically U-shaped or rectangular and lend themselves to (1) smooth (balanced) work flow across a wide variety of products, (2) elimination of waste, (3) high quality output, (4) flexible operation, and (5) low total unit production costs. Economic benefits attributable to lean manufacturing include reduced lead-time and higher throughput, smaller floor space requirements, and lower work-in-process [2]. In factories using lean manufacturing, large machines characteristic of batch-and-queue processes (typically referred to as “monuments”) are often no longer aligned with lean work cells and are not needed or desired. Instead, smaller more flexible machines are typically organized into work cells devoted to the production of a family of products [1], [4], [5] and [6]. Workers then operate the machines in the cell to minimize the cycle time for a family of products, minimize inventory, and maximize quality. In existing factories, eliminating monuments and investing in new, smaller machines can be troublesome to managers who were responsible for originally approving a high-volume batch-and-queue manufacturing process. Scrapping a massive piece of equipment, which still has a sizeable book value, can be viewed as admitting that a mistake was made years ago by investing in manufacturing technology that quickly became obsolete. Therefore, the decision to abandon (or replace) high-volume monolithic machines in favor of cellular manufacturing systems that employ TPS and lean manufacturing principles can be extremely difficult for managers to make, fraught with subjective factors beyond economics. The purpose of this paper is twofold: (1) to provide a roadmap to illustrate how value stream mapping (VSM) and its associated tools can be used to design a desired future state aligned with lean manufacturing principles and (2) to examine the economic aspects of replacement decisions created by lean manufacturing systems using information on anticipated cost savings from VSM. We begin with a discussion of VSM and its associated tools, how they are used to map a current state and design a future state. We then use a hypothetical example to quantify the typical economic benefits associated with lean manufacturing. Lastly, we analyze the economic trade-offs arising from a decision to invest in a future state including work cells that replace a high-volume transfer line.

Kaplan [27] identifies several tangible and intangible benefits of CIM that can be extended to the analysis of benefits in lean manufacturing. The tangible benefits proposed by Kaplan include inventory savings, reduction in floor space, and higher quality. In particular, WIP and finished goods inventory can be reduced through improved flow, increased flexibility, better quality, and improved scheduling [27]. Most financial accounting systems do not provide a good estimate for reductions in employee walking distance and travel time, and space freed up by lean manufacturing. Kaplan [27] suggests estimating the value as an opportunity cost of the space, either in terms of its rental value or annualized cost of new construction. Higher quality results from fewer defects produced before the defect is caught, equipment that stops producing when a defect is detected (autonomation) [16] and [23], and standardized work procedures that have all workers performing the operation in the same manner. The intangible benefits proposed by Kaplan [27] include greater flexibility, shorter throughput and lead-times, and increased learning. Greater flexibility is achieved by having smaller, more versatile equipment and a multi-skilled workforce. With the removal of large quantities of WIP, throughput and lead-time are reduced. Reductions of as much as 95% have been reported in some organizations [27]. There is also a marketing advantage from being able to meet customer demand within shorter lead-times. Increased learning is a result of exposure to technology and tools associated with lean manufacturing. Even if the equipment does not pay for itself, the experience gained by managers, floor associates, and maintenance may prove valuable when future technologies are implemented [27]. Although we have utilized a hypothetical example in this paper, the concepts and tools presented here can easily be applied to actual situations. This paper has illustrated a roadmap for how VSM can be an important tool to define, analyze, and quantify waste, such as excess WIP and defects, as shown in this example. Visualizing sources of waste in the current state, as well as the potential benefits that can be realized in implementing a future state for a product value stream, can help managers more easily and more objectively conduct equipment replacement analyses as they consider and pursue the adoption of lean manufacturing. In addition, this approach and analysis method can be extended to other types of capital investments, beyond traditional processing equipment, such as conveyors and other material handling systems.