Theory of Constraints Handbook - James Cox Iii [146]
FIGURE 8-11 A detailed product flow diagram for an assembled product.
In just the part of the PFD that we have constructed thus far, two characteristics of production operations (characteristics that make it difficult to manage these operations) stand out. One factor inherent in the PFD is the dependency of operation B-020, for example, on operation B-010. This type of dependency is referred to as material dependency. Simply stated, B-020 cannot be performed unless B-010 has been completed. Every stage in a PFD depends on the preceding stage. If a box in a PFD has an incoming arrow, this indicates material dependency. The material from the box at the base of the arrow is an absolute requirement for the box at the tip of the arrow. The boxes RM-A, RM-B, and RM-C have no incoming arrows as they are the beginning of this production operation. If we were looking at the entire supply chain, then clearly these boxes would be linked to the suppliers of these materials.
A second form of dependency that is highlighted in a PFD is between steps A-010, B-010, and C-010. All of these processes require the same resource, R1. This is an example of the type of dependency referred to as resource dependency. If R1 is engaged in step A-010 and there is only one resource R1, then B-010 and C-010 cannot be performed. Another resource dependency can be seen between stages C-020 and C-040. Both require the same resource R2. In addition, R2 will have to complete C-020 and R3 complete C-030 before C-040 can be started.
In Fig. 8-11, we complete the PFD for this simple operation by adding the assembly operation. An assembly operation, by its very nature, requires more than one input material. Just as the arrow from RM-A to A-010 represents the fact that RM-A is an input to the processing step A-010, the arrows from A-040, B-030, and C-040 all converging on box D-010 indicate that all of the components A, B, and C are required to perform this assembly step. If even one of them is missing, the assembly operation cannot proceed. In Fig. 8-11, the arrow from PP1 to D-010 represents the fact that a purchased Product PP1 is required (in addition to parts A, B, and C) to perform operation D-010. We refer to assembly stages as convergence points in the PFD—multiple products/materials are assembled together to make a single product. A convergence point (a control point) represents a high degree of dependency since all materials represented at the base of the multiple arrows are necessary for this operation to be performed.
FIGURE 8-12 Product flow diagram illustrating a divergence point.
In addition to the linear and converging flows, there are cases where the flow shows a divergence. Just as convergence is characterized by the coming together of multiple materials into a single product or component, divergence (a control point) is characterized by a single material being transformed into several different output materials. Consider, for example, a case in the textile industry. Figure 8-12 shows the case of a specific type of yarn being processed at the next stage—the dye house. At the dye house, color is applied to this yarn. We know that for the same yarn, different colors can be applied (red, blue, green, etc.) and we also know that red yarn is a distinct and different product than blue yarn. In the language of the PFD, the dye house is a divergence point—the same input material (untreated yarn) can leave the dye house as any one of a multitude of colored yarns. The divergence point at the dye house shows up in the PFD as a single yarn diverging at the dye house into a multitude of different boxes.
Material dependency, resource dependency, convergence points, and divergence points are the fundamental elements of a PFD. As discussed in the next section, production operations can be classified into families based on which