Theory of Constraints Handbook - James Cox Iii [129]
An alternate viewpoint of the same factory floor is to look at how materials flow. Materials move through the factory, flowing from raw material to finished product. Along the way, they are transformed or worked on by resources. The material thus flows from raw material to one resource to another until it is fully transformed and the finished product leaves the factory or operation. We call this viewpoint a flow centric viewpoint. From a flow centric viewpoint, the same factory in Fig. 8-1 would be represented as shown in Fig. 8-2. Since there are three separate materials, there are three separate flows. The transformation of any product, such as Product A (represented by the solid line), from raw material (RM 1) to finished product can be represented by a unique sequence of operations—resource R1 performing operation 010, then resource R3 performing operation 030, etc. We have chosen the vertical direction to depict the time sequence of these steps. In this particular case, the three separate products are produced in very similar fashion; they follow identical paths. Figure 8-3 shows the resource centric and flow centric views when the three products have very dissimilar routings. These diagrams are referred to as Product Flow Diagrams or PFDs.
The essence of the manufacturing operation—the transformation from raw material to a finished product—is reflected in the flow centric view. It is not surprising that the management of production/manufacturing operations should be based on a flow centric view and not a resource centric view. In his article, “Standing on the Shoulders of Giants,” Goldratt (2009) presents the core argument that Henry Ford’s assembly line process and Dr. Taichi Ohno’s Toyota Production System (TPS) originate from a focus on flow.
By a flow centric view, we mean much more than looking at production operations in the format of Fig. 8-2. The primary role of production management is recognized to be to manage flow. Effective management of flow implies that the movement of all material through the factory will be smooth and fast with no stoppages. In any flow, obstacles to the flow result in buildup of material—a traffic jam—and are considered highly undesirable. Resource centric methods are interested in keeping resources busy and consider buildup of material unavoidable.
FIGURE 8-2 A flow centric representation of the plant in Fig. 8-1.
FIGURE 8-3 A comparison between the resource centric representation and the flow centric representation of a plant where the routings for the products are dissimilar. [© E. M. Goldratt used by permission, all rights reserved. Source: Modified from E. M. Goldratt (2003, 29)]
To understand the key difference between the two views, think of it this way. When walking through the factory you are bound to see idle resources and idle batches of material. Which bothers you more in the pit of your stomach? If the resource standing idle bothers you more, then you are exhibiting a resource centric view. If the batch of material sitting idle bothers you more, then you are exhibiting a flow centric view. What we have learned from Henry Ford and Taichi Ohno is that the flow centric view is the proper point of view for effective management of the system.
Traditional cost accounting-based management methods, unfortunately, are resource centric in nature. Operators typically describe themselves in terms of the resource or resources they operate—press operator, furnace operator, etc. Managers also describe themselves in terms of the resources they control—Press Department, Heat Treat Department, etc. The entire management control system is geared to track the activities of the resources and, in particular, to track, understand, and hence eliminate non-production or idle time on the resource.
Ford and Toyota Production Systems—A New Perspective
In a groundbreaking article in 2009, Goldratt provided