Theory of Constraints Handbook - James Cox Iii [136]
The DBR System
We now discuss the specific procedures and methods that make up the DBR system for planning the flow of product through manufacturing operations. BM is the execution control portion of the DBR system. The objective of the DBR system, as for any planning and control system, is to meet Throughput expectations while efficiently managing Inventory and Operating Expense.5 The essence of the DBR approach is captured in Fig. 8-5.
The Drum
The drum considers the constraints in the system and firm customer commitments, in setting the pace for the entire system. The process of setting the drum begins by identifying the work that needs to be done at the constraint by the total output required. In the case of companies that are make-to-order (MTO), this is the work required to be performed by the CCR to meet all customer requirements that fall in a given time period (for example, all orders with customer due dates in the next 30 days). In the case of make-to-availability (MTA)6 companies, the output requirement is the total finished products required to fill the stock buffers. Once we have a list of what must be produced at the constraint, it is then simply a matter of determining the sequence of production (which product first, which product next, and so on) and the production batch size (how much will be produced once we start a specific product). Factors that should be considered in deciding the production sequence and the size of the process batch as well as detailed examples are found in Srikanth and Umble (1997, Vol. 1, Chapters 7 and 8) and Schragenheim and Dettmer (2001).
FIGURE 8-5 Illustration of the basic DBR system.
The Buffer
In a world free from disruptions, such as resource breakdowns, process yields, etc., the production lead time—the time that we allow for the raw material to be transformed to a finished part or product—can be simply equal to the sum of the process times and setup times at each step of the routing for that product. In the real world where there are many forms of disruption, the use of a planned production lead time equal to the sum of processing and setup times would be considered foolish and rightfully so. Any disruption such as a resource breakdown would make it impossible to produce the product on time. The actual production lead time will always be larger than the sum of process times and setup times. Since disruptions are unavoidable, planned lead times will have to be larger than the sum of process and setup times. This is true if we are to have any chance of making the actual production lead time equal the planned production lead time.
Whenever there is a task that is subject to variability, it is clear that the actual time the task is executed—started or finished—is going to be different from any plan that does not allow some degree of padding in the form of safety time. This is essentially the concept of the time buffer.7 What makes the application of the time buffer concept unique and powerful is the explicit recognition that the goal of a DBR planning system is not to make each task to be on time to a planned schedule, but to make the actual flow through the system sufficiently reliable to satisfy market demand. In other words, the objective is not to protect the ability of each task to be on time (to a plan) but only to make sure that the entire system is on time. This recognition allows us to provide a significantly higher degree of reliability in a DBR plan than one that tries to ensure protection for each step in the process (as in a push system or Kanban pull system). In addition, this higher degree of reliability can be accomplished at a significantly lower production lead time.
Specifically, a time buffer is defined as follows. A time buffer represents the additional lead time allowed, beyond the required setup and process times, for materials to flow between two specified points in the product flow. Two points8 commonly used in this context are material release (gating operations)