Theory of Constraints Handbook - James Cox Iii [161]
1. The time the order has to wait in queue until the signal from the rope to be released to the shop floor. This prerelease queue time depends on the prior work that the CCR has to do. Assuming the CCR has quite a lot to do, then releasing the order immediately does not add any benefit and could cause damage by creating confusion of priorities.
2. A liberal estimation of the time it takes from order release until completion (the production buffer 12). As we are going to consider the current queue for the CCR and possibly delay the actual release of the materials for that order, we do not expect a peak load within the shop floor itself. When a peak load happens, then new orders would need to wait longer before being released to the floor, thus decoupling the flow in the shop from the natural pace of the CCR.
The time buffer mentioned in Item 2 is now called the production buffer, as it describes the flow time through the shop under regular load. The production buffer does not include transportation or in-transit time to the client.
The issue of delivery date to the client requires a short discussion. The transportation time is an issue only when it is a significant part of the lead time. The question is whether the commitment of the producer includes transportation. In other words, is the transportation time part of the production planning or part of the customer’s planning?
Suppose the producer takes responsibility until the goods reach the customer. Then, the production planning should have a due date for completing the production and then have the final delivery date where the transportation time (and possible fluctuations in it) is considered.
Figure 9-1 shows time elements in lead time, but note that from now on we’ll treat the completion of the production as the due date to which we refer.
How should the production buffer be determined? In implementing S-DBR in traditional planning and control environments, the usual recommendation is to cut the current production lead time by half. The rationale is that by eliminating the large batching and huge WIP levels, the main disruption (waiting in queues at each work center) to the flow has been vastly reduced.13 Take into account that the net processing time is just a fraction of the production lead time and you realize that by cutting the wait time to half, the total production lead time is cut by half. The priority system of BM supports very high reliability within that time. Thus, cutting the standard production lead time by half is a good initial production time buffer.14
Noting the standard lead time of the industry is a good gauge for a test measurement and the time buffer to be used should not be longer than half of that number. In most cases, this reduction is not only possible but often the production time buffer can be cut even further. These further cuts should be done only after first implementing S-DBR with buffers that are equal to half the current lead times. After the shop floor has stabilized, the further reduction of the production buffers is achieved through BM with the focus of improving the flow. Recall improving flow is the main mission of operations. Marketing and Sales should then capitalize on this reduced production lead time to get higher prices and expand the market almost at will. It means Marketing and Sales have to be fully updated with the new capabilities and current status in production.
FIGURE 9-1 The elements of lead time.
Load Control
In traditional DBR, the role of the drum, the detailed schedule of the CCR, was to measure the load on the CCR and determine whether the promised due date was safe. When no detailed schedule is going to be used, then a replacement tool for measuring the load is required.
The planned load is the accumulation of the derived load on the CCR, or any other relatively loaded resource, of all the firm orders (released and awaiting release) that have to be delivered within a certain horizon of time. It is clear that more than one horizon of time might be defined. Each