Theory of Constraints Handbook - James Cox Iii [151]
FIGURE 8-17 Example of the phenomena of “stealing.”
The concerns or issues that are shared by T-plants in general are:
1. Large finished goods and component inventories.
2. Poor due date performance (30 to 40 percent of the orders early and 30 to 40 percent of the orders late).
3. Excessive fabrication lead times.
4. Unsatisfactory resource utilization in fabrication.
5. Fabrication and assembly act as separate unsynchronized plants.
DBR in T-Plants
In a T-plant, we have two situations. The most common situation is that most T-plants tend to belong to the MTS environment. Typically, buffer stocks are maintained at both the component level (just prior to assembly) and at the finished goods level. In this case, there are no real constraints (see discussion of MTS in Chapter 10) and the proper system to implement is the S-DBR system discussed in Chapter 9.
If this is not the case and there are capacity constraints, then the key factor is that the assembly operation must be managed properly. As long as stealing occurs at assembly, T-plants will be chaotic and flow will be difficult to manage. However, once stealing is eliminated through tight control of the assembly operations, then a T-plant becomes an A-plant and the DBR system discussion in A-plants should be followed.
The schedule control points are materials release, divergence, convergence, and the physical resource constraint (if one exists).
I-Plants
I-plants are the simplest of the production flows. The major issue with I-plants is the sharing of resources between the different products. Each product follows the same sequence of operations. There is little or no assembly and there are no divergence points.
The characteristics of an I-plant are:
1. All parts have similar routings.
2. Resources are shared between different parts, while raw materials are not.
3. There is very little assembly involved.
The typical I-plant product flow is shown in Fig. 8-18 and the shape makes the name obvious.
I-plants are the simplest of plants to manage. Nevertheless, traditional focus on resource utilization results in the use of production batches that are much larger than required to maintain a smooth flow. As a result, WIP piles can be created and the wave-like flow of A-plants can be observed. Consequently, I-plants have the following concerns/issues:
1. Low due date performance.
2. High WIP inventories.
3. Level of output below theoretical line rates.
FIGURE 8-18 Product flow diagram for a typical I-plant.
DBR in I-Plants
I-plants are straightforward to manage from a product flow standpoint. The DBR system as described in the previous sections can be designed and implemented with little complication. Identification of the constraint is simple—all personnel will be aware of this resource and the inventory buildup should confirm the section of this resource. Simple steps to improve productive use of this resource (see the section on Step 2—exploiting the constraint) should be followed by the implementation of the DBR system.
Most academic research has been conducted on I-plants (primarily on lines of 10 or less work centers) as indicated in Chapter 7. It is by far the simplest to simulate and study. In contrast, most plants are V, A, T, or combinations of these structures.
Summary
This chapter covered the basic terminology and concepts related to the TOC production solution. As such, it provides the foundation for a deeper understanding of DBR in an MTO environment, S-DBR in an MTA environment, and supply chains linking manufacturing to the downstream links. The various types of buffers are defined and illustrated, as are the various types of plants with their control points. A discussion of implementing DBR in each environment