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before we commit to execution of the above DBR plan is “Does this plan help us complete the products in such a way as to meet customer expectations?” How can we identify when orders are planned to be completed. For this we have to extrapolate from the expected completion of products at the constraint (—Resource R1 as detailed in Table 8-3) by adding a reasonable estimate of time for the completion of the remaining steps (from R1 through assembly). In this simple case, we have chosen 8 hours (about one-third of the total planning lead time) for this estimate. This works in this simple case due to the relatively simple nature of the flow (minimal resource or material contention). Table 8-4 shows the time when different batches of Products A, D, and F are expected to be completed and be available for shipment. If we want to commit shipping times that can be met with very high confidence, then we should commit to hours 42 and 47 (which, in a 5-day, 8 hours per day workweek, corresponds to the Monday of week 2). In real-life situations, it is common practice to choose a slightly more conservative estimate and use one-half of the planning lead time. This means that the estimate of when a batch or an order can be completed (and hence available to ship) is equal to the completion of the last needed batch at a constraint plus one-half of the production lead time. In-transit lead time has to be added to this shipping date to determine when the order will be at the customer site.

Managing Flow—Controlling Execution and Buffer Management

The Need for Control and the Need for Corrective Actions

Using the DBR system described previously creates a plan that maximizes the system Throughput, by ensuring full utilization of the constraint while focusing on real customer demand. The plan is robust and protected from disruptions using time buffers and minimizes investment in Inventory by restricting inflow of material through the rope mechanism. This does not mean that the execution of the plan on the shop floor is automatic and that the execution does not have to be monitored carefully. It is true that in creating the time buffers, we have allowed for a certain level of disruption to the flow of a batch of material through the system. As long as the deviations actually being experienced by the batch are less than what was allowed for, we do not have a problem. However, when the actual deviation begins to exceed the allowable disruption, the ability of the batch to reach customers on time will be in jeopardy.

In these cases, not all is lost. In most manufacturing operations, there is opportunity for corrective action to be taken. The objective of these actions is to “make up” some of the time lost by the batch due to larger than anticipated disruptions. These actions include:

Expediting the batch by moving it to the front of the work queue at each resource

Working overtime at a resource to process this batch

Processing the batch on more than one identical resource (batch splitting)

Overlapping processing (carrying completed materials from one work center to the next to allow both work centers to work simultaneously)

Alternate routings

The use of time buffers minimizes the need for corrective actions, but it does not eliminate them. What is needed to make the DBR system deliver exceptional results in practice is a mechanism that can identify the cases where corrective action is necessary and help monitor the effectiveness of the corrective actions so that every batch can be finished on time.

Understanding Buffers: The Buffer as the Source of Information for Controlling Execution

In order to identify when a production batch is experiencing larger than “normal” disruptions, we need to go no further than understanding the time buffer in a bit more depth. When a batch of material is released one production lead time before its due date, what do we expect to happen in reality? Let us understand this by studying a sample of 100 identical batches with a production lead time of 40 days. The majority of batches experience