Theory of Constraints Handbook - James Cox Iii [168]
The color code for order A1 is
(8 – 5)/8 × 100% = 37.5% → Yellow
The color code for order B1 is
(6 – 3)/6 × 100% = 50% → Yellow
The color code for order C1 is
(8 – 10)/8 × 100% = –25% → Green (actually above the green)
Note that this order may have been released early to keep the CCR from going idle.
The color code for order D1 is
(35 – 8)/35 × 100% = 77.14% → RED!
Certainly, the resource should immediately process the D1 order. What will be the next one? We don’t know at this time. More orders could show up and one or two of them could be red. Buffers and buffer penetration percentages for each of these orders are seen in Fig. 9-4.
Several points regarding this example are worth discussion. Order D1 has the farthest due date from the current four candidates for immediate processing. However, because D1 has that long buffer, the assumption is that it needs that length of time as a buffer and thus the remaining 8 days signal the order is under pressure.
However, it is clearly written that the manual work in processing D1, which justified the especially long-time buffer, has already been dealt with. In such a situation, should we still look on order D1 as a “red order?”
It could be that at that point in the routing D1 is not truly urgent. However, we put a process in place that should yield good results in the vast majority of the cases. Can we really optimize the priority procedure to such a degree that in spite of the fluctuations in the shop and mistakes people make that better results would be achieved? Trying too hard to optimize within the “noise” (level of uncertainty) of the environment will actually increase the impact of the noise. This is one of the insights we have learned from Deming’s funnel experiment.19
Suppose that D1 is stuck upstream of the relevant resource. Therefore, only three orders are at the site at this time. Which order should the operator choose? The buffer status of B1 is higher than A1, but the rule is to decide based on the color. The color of both orders is the same, yellow, and thus any choice of one of the two is acceptable. We assume the operator is aware of the buffer status, but might consider saving a setup, which is relevant only for orders with the same color code.
FIGURE 9-4 Buffer penetration shows priorities for processing.
Short-Term Planned Load
The main need for load control is for establishing the synchronization between Production and Sales, mainly by providing the estimates for safe dates as the earliest dates to be used by Sales for quotation. Thus, the horizon for that planned load is the expectation of the clients for response time. The planned load for this main horizon also provides signals for when to press Sales to bring more sales or restrain the sales to a degree.
What about the more immediate horizon, like the production buffer time? Every order that is now on the shop floor has to be delivered within the production buffer time (unless that order was released early to keep the CCR busy). The point of load control at this time is to ascertain that the capacity of the CCR is more than enough to deliver everything on time.
How could it be that S-DBR would find itself in a situation where it ran out of capacity and there is not much hope to deliver all the orders on time? After all, every single order was given a due date that was in line with the available capacity as assessed by the planned load.
There are several possible causes for such a case of capacity shortage in the very short-term. One is that enough capacity is available, but it is in the shape of overtime or outsourcing, which means that capacity costs extra money. That extra capacity was probably considered by the planned load (depending on how the planned load was modeled), but now a clear decision is required whether to actually utilize the overtime and how