Theory of Constraints Handbook - James Cox Iii [172]
When the basic routing of every order is relatively simple, like a sequence of operations without parallel legs, S-DBR is still a valid option. However, having one or several very long operations poses some problems to BM to reflect the true state of urgency of the order. Let’s use an example to demonstrate the problem.
Suppose the appropriate production buffer of an order is three weeks. The last operation is a long test that takes a whole week to go through. That one week is a fixed length of time. The sum of the previous net processing times is short, taking at most eight hours. If the test determines a problem, it usually requires the replacement of a purchased component, which is done in minutes. Therefore, all in all the touch time is approximately 35 percent of the production buffer, but the vast majority of it is accumulated at the very end. What is the appropriate priority of an order after two weeks? The regular priority would show an order just entered into the red. But, if the order testing has not started yet then actually the order is already late.21 On the other hand, if the order is already three days into the testing then it’d most certainly be on time.
In the previous example, only one operation is truly long, while the rest are normal manufacturing where the net processing time per piece (also per order) is very short. In this case, it is possible to introduce some changes to the way S-DBR and BM rules are implemented that would work. In the particular case of the example, all we need is to model the requirement to reach the testing no later than a week earlier than the customer’s due date. By generating a fictional safe date not for the full completion of the order, but for entering the last operation one week before the due date, we force the right priorities in the system prior to the final testing.
In cases where the long operation is in the middle of the routing, there might be a need to implement back-to-back time buffers, creating a somewhat less simple solution, but still not requiring scheduling the CCR in detail, and most of the S-DBR procedures are in place.22
In the extreme case (such as firms in the process industry) where an order has several long operations that are spread through the routing, yet the routing itself is a simple ‘I’-shaped structure,23 changes to the buffer management algorithm would still provide the right priorities. This feature, developed by Inherent Simplicity Ltd., is beyond the scope of this chapter.
Implementation Issues and Processes
One of the primary advantages of S-DBR over the traditional DBR is in the speed of implementation and results. Implementing S-DBR should always start with choking the material release so that only orders to be delivered in the horizon of the production time buffer would be found in the shop floor. We’ve already mentioned that a good initial estimation of the time buffer is one-half the current production lead time. If it is not clear what the current production lead time is, then take the standard lead time in this industry and cut it to half.
A few exceptions to the half-the-production-lead-time rule exist. The first is an environment of a dedicated assembly line, where all the WIP in the line is restricted to several hours. The other exception is where real effective Lean methods have vastly reduced the WIP and lead time. In those cases, the production buffer can be based on the current production lead time for implementation.
Choking the material release must include dealing with the current batching policies by either abolishing them by making the customer order quantity the batch size or at least reducing the batch size.
The next mandatory move is to establish BM. This move can be done manually or be supported