Theory of Constraints Handbook - James Cox Iii [141]
FIGURE 8-8 Product structure and resource information for sample plant. (© E. M. Goldratt used by permission, all rights reserved. Source: Modified from E. M. Goldratt (2003, 29))
From these calculations, it is clear that this operation has a CCR in the R1 resource. Of the available time, 94 percent is required just to process the units required for the week. The remaining 6 percent is available for setups, maintenance, etc. Any consumption of time (not producing product) that exceeds 6 percent will result in missed shipments. In fact, based on the information that the capacity required for processing the needed parts is 2260 at the R1 resource, we can see that only 140 min (2400 – 2260) are available for doing setups. Each setup requires 15 min. Therefore, we can only incur nine setups. Since there are three distinct steps where we need the R1 resource (C-5, E-5, and F-5), we conclude that we can do up to three setups for each step. To be on the safe side and allow for some fluctuations, we can choose to do two setups at each step. Effectively we will run two batches of 25 units at C-5, two batches of 25 units at E-5, and two batches of 20 units at F-5. Table 8-2 shows a schedule for the R1 resource that is constructed on this premise.
TABLE 8-1 Capacity Available and Required and Percentage Load to Satisfy Weekly Demand
TABLE 8-2 Schedules for Constraint (R1 Resource) and Market for Product A
Resource
In this example, Product A does not require any time at the R1 resource. The market is the constraint for Product A.11 How do we manage the flow of this product? The simplest thing is to produce Product A using the customer orders. However, a single order of 40 units moving through the operation is not an example of smooth flow. To overcome the effects of large and lumpy flow in this case, we will divide the order into four batches of 10 units each and process them to be completed by the end of the week. The schedule for the R1 resource and the schedule of completions for Product A together represent the drums for this plant.
The next step is to establish the size of the time buffers. For this simple model, we select a constraint buffer of 24 hours (3 days, in this case). In real manufacturing plants, the production lead time currently being used provides the starting point. As indicated earlier, the first choice of the time buffer is to reduce this by 50 percent. For our example, we do not have this reference point. The choice of 24 hours reflects the need for a time buffer that is approximately 20 times the processing time for a unit (i.e., the process time is around 5 percent of the total production lead time). In addition, since all products have comparable routings, the time buffer is chosen to be the same for each. This means that raw material must be released 24 hours (3 days) before the expected completion by the constraint. Table 8-3 provides the rope or the release dates for the various raw materials into the process.
TABLE 8-3 The Rope (Material Release Schedule) for Sample Plant
TABLE 8-4 Expected Completion Times Based on the DBR Schedule
In this example, material release and the divergent point represented by operations A5 and C5 are the only schedule control points and no other information is needed for planning purposes. Table 8-2 (the drum), the choice of 24 hours as the time buffer, and Table 8-3 (the rope) provide the DBR system for this case.
An important question to answer