Theory of Constraints Handbook - James Cox Iii [149]
The general characteristics shared by A-plants include:
1. Assembly of a large number of manufactured and purchased parts into a relatively small number of end items. Each assembly point represents a decrease in the number of distinct parts and after just a few assembly steps, the number of distinct items drops dramatically.
2. The component parts are unique to specific end items. This is a key feature that distinguishes A-plants from T-plants. Consider aircraft, for example. While every aircraft has engines, the engine for each type of aircraft is unique. The engine for a Boeing 747 is completely different from the engine for a Boeing 777.
3. The production routings for the component parts are highly dissimilar. In the example of the aircraft, the routing for the manufacture of a jet engine blade is nothing like the routing for the manufacture of the compression chamber.
4. The resources and tools used in the manufacturing process tend to be general purpose. In an A-plant, the same resources are used to produce many different parts. Resources are quite flexible, in contrast to the highly specialized equipment in V-plants.
Since the major focus in traditional manufacturing environments is resource utilization and not product flow, it is not surprising that the flow through fabrication and into the finished components is erratic. In fact, the flow through all areas of an A-plant is wave-like, resulting in what is characterized as “feast or famine.” This wave-like flow means that it is highly unlikely that all of the component parts are available when needed at assembly. The missing parts must be tracked down and expedited to assembly. The feast or famine syndrome also creates the perception that bottlenecks “wander.”
The major concerns in an A-plant include:
1. Assembly is constantly complaining of shortages and expediting is a way of life in manufacturing and purchasing.
2. Unplanned overtime is excessive. Resources that were idle during the week suddenly find a wave of material that is needed urgently at assembly in their queue and this results in overtime.
3. Resource utilization is unsatisfactory.
4. Production bottlenecks appear to wander about the plant.
5. The entire operation appears to be out of control.
DBR in A-Plants
Unlike V-plants where the identification of the constraint is straightforward, the identification of the real capacity constraint is not straightforward. This is a direct result of the possibility of product flows for different component parts being different. This can create a situation in which multiple constraints appear to be present. In addition, the use of large production batches (chosen to reduce unit costs and improve resource efficiency) results in wave-like flow and the constraint appears to wander from one resource to the next. At first sight, it might appear that the resource load information from the computer planning systems would provide a simple means of identifying the constraint, especially since most of these plants have a computerized planning and control system. However, the data are unreliable to the extent that in the author’s experience resource load data from the computer system is highly suspect. As explained in Srikanth and Umble (1997), the best way to identify the constraint is to analyze resources that use the most overtime on a regular basis with the parts shortage information (the daily shortage list from assembly). The resource that uses overtime and processes parts regularly on the shortage list must be the constraint.
Two key factors should be considered in setting up the drum in an A-plant. The first is that the assembly (convergent point) operation provides an excellent place to establish the drum. Subordinating everything else to a well-constructed assembly schedule is the easiest way to achieve