Theory of Constraints Handbook - James Cox Iii [153]
Simplified Drum-Buffer-Rope (S-DBR) is a variation on the original DBR methodology. It was suggested by Schragenheim and Dettmer (2000) in Manufacturing at Warp Speed as a valid, simplified replacement especially suited when the implementation has to use the common material requirements planning (MRP)/enterprise resource planning (ERP) software. Since then, the basic principles of S-DBR were adopted by Dr. Goldratt. Important improvements were added and dedicated software for S-DBR has been developed by Inherent Simplicity Ltd. under the close supervision of Dr. Goldratt. S-DBR has now replaced the older DBR as the preferred planning method with one exception, which will be explained later in this chapter.
Another important realization concerning production planning has to be mentioned. Both S-DBR and DBR were assuming a make-to-order (MTO) environment. During the rethinking of the TOC-focused planning methodology, it was recognized that the make-to-stock (MTS) production environment should be based on different principles. The author dedicates Chapter 10 to MTS, or rather to make-to-availability (MTA)1 environments, to emphasis the clear distinction.
Another comment should be made. While DBR and S-DBR are planning methods, they are not stand-alone methods. Buffer Management (BM), the TOC control mechanism, should be viewed as inseparable from the planning method. Thus, Chapters 9 and 10 deal with both the DBR/S-DBR planning as well as with BM as an absolutely necessary part of both planning methodologies.
The purpose of this chapter is to explain the S-DBR/BM concepts, logic, and procedures through the development of the ideas over time. Thus, the emphasis is on the historical development, which is critical to the full realization of the continual paradigm shift we have gone through during the last 25 years since the introduction of DBR.
A Historical Background and Perspective
In the mid-1980s, DBR represented a huge advancement in providing a robust plan for the production floor. DBR was developed as a major departure from the concept, created by its own developer, of very sophisticated and detailed planning of the shop floor. In the late 1970s through the first half of the 1980s, Dr. Eliyahu M. Goldratt led a software company, Creative Output Ltd., in developing a sophisticated program called OPT® (Optimized Technology) to plan manufacturing orders in great detail for any kind of production shop floor. OPT® was a true advanced planning and scheduling (APS) program even though the term was coined years later. At the time, the name given to such programs was “finite-capacity scheduling system” and that name hinted at a contrast with the MRP II programs of the time, which were known as “infinite-capacity scheduling systems.”
DBR came as an antithesis to the OPT® concept and it came from the author of OPT® development—Dr. Goldratt himself. Instead of ultra-sophistication in trying to solve a complicated net of links between the processing steps and resources, of which several might have limited capacity (bottlenecks), a vastly simplified concept emerged: in any chain, there is one link that is the weakest. That link determines the strength of the whole chain; thus, detailed planning of that specific link should be the kernel of the overall production plan. The name given to the core planning—scheduling the one bottleneck ensuring its smooth and effective utilization—was the drum. The resulting understanding was that the bottleneck is the only resource whose efficiency really counts. However, planning the bottleneck does not ensure that the plan would be executed as is. Murphy, the symbol for everything that might go wrong, could mess things up and the bottleneck might face a situation where it has to stop processing because parts are missing. Instead of sophisticated synchronization of all resources, the concept of providing a buffer to protect