Theory of Constraints Handbook - James Cox Iii [159]
If the market demand is the major constraint and we see the need to have some protective capacity provided on the CCR, then there is no need to have a special CCR buffer that would protect the sequence of the production orders on the CCR. We still need to carefully monitor the load on the CCR, but not necessarily to schedule the CCR in detail. Of course, we’ll expand on this point later when we describe the S-DBR process.
How Is the Planning and Execution Viewpoint Addressing the Issue of Scheduling and Buffering the CCR?
This viewpoint requires validating that whatever is included in the planning is a must. Thus, the question is, “Do we need to schedule the CCR?” Is detailed scheduling the only way to ensure a good enough exploitation of the system constraints?
Once we recognize that even the CCR has to subordinate to the commitments made to the market, then we have to conclude that scheduling the CCR in detail is not required in most cases (later we deal with the one exception). This means also that the CCR buffer, used to protect the schedule of the CCR, is not required and the only buffer that is truly necessary is the one aimed to protect the commitment to the market. The CCR should prioritize its sequencing decisions according to BM at the shipping buffer. However, the load on the CCR still requires monitoring. The insight here is that there is a difference between monitoring the load on a CCR resource and dictating a sequence on that resource.
How Does Refraining From a Detailed Schedule of the CCR Affect the Execution?
The traditional DBR requires three different buffers: the constraint, the shipping, and the assembly buffers (detailed in Goldratt, 1990), but if one concentrates just on the due dates of the firm orders at hand, then only one buffer, the shipping buffer (now called the production buffer in S-DBR), covers the whole production time from material release until order completion, is required. Following the green-yellow-red buffer priorities8 as they emerge from the use of having one-time buffer per order is much simpler than deciding between a red assembly buffer versus a red CCR buffer or a red shipping buffer.
What Does the Emphasis on Flow Add to the Challenge to Traditional DBR?
This view is fully concentrated on the trigger of the flow—the customer order. The point of the flow is to be able to commit to the client as fast as possible. With this as the main objective, then the challenge to DBR is obvious: Do we really need the constraint buffer or is it a disruption of the flow? After all, the constraint buffer initiates early release of the parts, so on average they reach the constraint and then wait for their schedule to be processed. Having that planned waiting time at the CCR is a disruption to the flow.
It is obvious that there is a need to choke the release to only what is truly required now. This would prevent the tendency in traditional DBR to release certain orders very early in order to exploit the CCR’s capacity.9 Looking on the CCR as “disruptive to the flow” also highlights the need to be able to quickly elevate the CCR capacity whenever necessary, because even the unplanned wait time at the CCR could be significant due to its relative lack of capacity and any wait time represents a certain disruption to the flow. Of course, this is not always practical, but the basic thinking is right. We now recognize in TOC that the ultimate goal is to both grow constantly and remain stable at all times. Thus, the constraint should not be the capacity of a resource that can be easily elevated because whenever such a resource cannot fully subordinate to the demand (forces too long wait time, thus blocking the flow) it should be elevated. The underlying assumption is that the financial worth of the additional demand that would not be possible to maintain with such