Theory of Constraints Handbook - James Cox Iii [128]
In this chapter, we first show the application of the TOC approach to managing production environments—known as Drum-Buffer-Rope (DBR) and Buffer Management (BM). DBR and BM are the systems that emerge from the application of the Five Focusing Steps (5FS). DBR is the TOC methodology for planning and BM is the TOC methodology for execution control. The term planning is used for those activities that start with known market demand and generate the plans for managing the flow of material through the factory including identifying what purchased materials will be needed and when. Execution control refers to the actions that are taken during the execution phase of the plan developed previously. These actions are necessary to ensure that the plans are followed and include the corrective actions that must be taken when deviations from the plan threaten to compromise delivery dates and Throughput of the system.
FIGURE 8-1 Resource centric representation of a plant producing three products (A, B, C) with four resources (R1, R2, R3, R4).
After explaining these systems and their logic with simple examples, we next move to a discussion of complex, real-life flows. Real-life production environments are characterized by high levels of detail complexity and high levels of dynamic complexity.2 Many of these elements, especially the ones in detail complexity, are specific to the individual environment and make each one appear to be different and unique. However, the behavior of these systems as a whole are characterized more by the way their dynamic complexity relates one to the other. These relationships and their many apparently different operations exhibit similar behaviors with respect to operational performance as measured by on-time deliveries, system inventories, production lead times, and so on. Third, we present a classification of production operations based on the structure of the product as contained in the bill-of-material and routing or process information. We classify the product flows into four major types—V, A, T, and I—or a combination of these four types. The real power of this classification is that operations that belong to a particular V, A, T, or I type will share similar performance characteristics and business problems to others in the same group. The application of DBR in each type of plant is also discussed.
Managing Flow—Planning and DBR
The Need for a Focus on Flow
A production operation is characterized by a number of resources that typically occupy fixed spots on the factory floor. Materials move from one resource to another in accordance with the rules specified in the routing sheet for the specific material/product. Typically, we think of the factory or production operation from this spatial or static perspective. We will call this a resource centric view of the operation. For the simple case of a factory that has four resources R1, R2, R3, and R4 and makes three products—identified as Product A, Product B, and Product C—the resource centric view of the operation is depicted in Fig. 8-1. The solid black line (—) represents the path in which Product A moves from RM 1 (raw material 1) through the various resources as it is converted from raw material to a finished product. The dotted line (. . .) represents the path in which Product B moves from RM 2 through the various resources, and the dash-dot line (-.-.-.) shows the path followed by Product C from RM 3 through the various resources. This resource centric viewpoint is also the viewpoint of traditional management methods. Cost control is the primary goal of operations management and the traditional view is that resources drain or consume costs. The way to manage cost is to manage the efficiency of each resource and to make sure that no time is wasted at any resource. Goldratt (2003) has aptly captured this viewpoint of traditional operations management in the phrase: “A resource standing idle is a waste.” Consistent with this view, most measurements in operations are resource centric