Theory of Constraints Handbook - James Cox Iii [139]
The basic challenge is to ensure that all work centers perform the right tasks in the right sequence and at the right time. With computers becoming ubiquitous in manufacturing, it is very tempting to accomplish this objective by providing each work center with detailed schedules that are constantly updated (hopefully in real time). DBR takes a counterintuitive but far simpler approach to accomplishing this goal. The simplest and most effective way to make sure that a work center does the right job is to have only the material for the right job available. Eliminate unnecessary WIP and you eliminate opportunities for working on the wrong stuff. With this approach, the emphasis of control is shifted to strictly limiting material available at a work center to what is immediately needed. In production operations, the availability of material in the shop is controlled by the actions at the material release points—the points where raw material is released to fabrication, finished parts are released to assembly, purchased parts are released to assembly, etc.
To implement the rope, the material release points are provided with a detailed schedule that lists what materials need to be released, in what time frame, and in what sequence. If this task is managed properly, then access to unnecessary work is denied to most work centers, thereby forcing them to work on the right products. Most of the work centers that are non-constraints will simply process material when it becomes available. When a work center (a non-CCR) finds itself with more than one batch of material, what are the rules for determining the priority sequence? The real question to ask here is whether sequence really matters. In the majority of production operations, the processing time for a batch of products at any single work center is a very small fraction of the total production lead time or the total time buffer. This being the case, the difference between working on one batch before another is insignificant. It should be remembered that we are talking about very few cases where multiple batches will be available to choose from and even here, the number of batches is small. Thus, a simple rule will suffice to ensure major distortions are avoided. The priority rule can be a simple “first-in, first-out” or FIFO rule.
In simple linear flows, simply controlling the release of material will be sufficient to control the execution through the whole system. The basic principle we have followed is that we can make sure that a work center cannot work on the wrong product if material is not available. In other words, the mere fact that material is available is sufficient information to give the green light to that work center for processing. In complex flows, this basic fact is not always true. For example, at divergence points (see V-plant discussion later in this chapter), the same incoming material can be processed into different outgoing materials. It is obvious that when such a work center can be activated by material availability, we have to specify what the output products should be and how much of each product we want. While the timing of the jobs is controlled by the availability of material, workers at each divergence point (a control point) need to be provided with a detailed list of what and how much of each product to produce, as well as the priority sequence for the products.
Similar to divergence points, assembly or convergent point s may also need to be controlled. Purchased parts may be obtained in quantities larger than required for specific orders; fabrication may also have combined different orders to reduce setups at CCRs; and in T-plants (see the discussion later in the chapter), the same basic component parts can be assembled in different combinations to create different end items.