Theory of Constraints Handbook - James Cox Iii [156]
The DBR methodology has clearly defined the critical points in the product structure that must be planned carefully. Three major control points are:
1. The due dates for all orders after careful validation that these dates are quite safe.
2. The detailed schedule of the CCR.
3. The schedule for the material release.
The criticality of the first control point is self-evident—we should not commit to dates we cannot meet. The second one is simply the essence of Step 2 in the 5FS—exploiting the system constraint. The criticality of the third control point is not self-evident. We usually see in many environments, particularly in manufacturing, that there is a lot of work-in-progress (WIP) that just sits and waits for resources. The immediate cause is the release of work too early to the shop because the first resources are available. The assumption is that the earlier work is released and starts, the higher the probability of finishing the work on time. However, once the first resources finish processing those orders they simply join the queue for the subsequent resources. The damage of having too much work without a clear and rigorous priority mechanism is enormous. While the resources upstream might be looking for work, some of the other resources might be flooded with work. When this happens, the resource under pressure tries hard to optimize its own efficiency, often at the expense of orders that are truly urgent. Actually, in most cases, the operators do not have any idea what is urgent and what is not. Many manufacturing orders are comprised of large batches. When the manufacturing orders are large, they often contain urgent customer orders and much less urgent stock orders all in one manufacturing order. Thus, too many manufacturing orders have a certain quantity that is very urgent to customers and some other quantity that is not. The loaded resource cannot do all of the manufacturing orders at the same time. Therefore, while a work center is working on a large manufacturing order with several urgent customer orders buried within it, other manufacturing orders that may also have urgent customer orders wait their turn.
The rope in the DBR planning methodology is the mechanism to ensure release of only orders that are soon required by the detailed schedules of the CCR and shipping buffers. This mechanism also forces the minimization of batching. The rope ensures against work that is not truly required being released to the shop.
The Implications for the Execution Phase
We have shown the general idea behind “minimum planning.” Let’s now describe the execution decision-making rules. When planning is not detailed, much more is left to the execution phase.
Including buffers in the planning has a special meaning for the people in the execution phase. The local objective in execution is to be able to execute the critical planning instructions. The state of the buffer is an excellent signal of whether things are going according to plan.
BM is the control mechanism on the progress of executing the plan. First, let’s introduce my definition of the term control:
Control is a proactive mechanism to handle uncertainty by monitoring information that points to a threatening situation and taking corrective actions accordingly.
The definition makes it clear that any control system is targeted to identify the actual emergence of a known threat, and it clearly belongs to the execution phase. It must have the most current and accurate information that the execution people need to carry out their jobs.
The need in the execution phase is to validate that everything is ready on time for the next critical directive based on our planning. The obvious possible threat is either being late to the CCR and thus starving the constraint or being late to the due date of the whole manufacturing order and thus making the customer order late. These two areas are protected by time buffers according to the DBR methodology.
Let’s define the state of the buffer as the percentage of the time the buffer has