Theory of Constraints Handbook - James Cox Iii [150]
1. Meet market commitments.
2. Be within the constraint’s capabilities.
3. Achieve a smooth flow through all of the operation.
The second factor is that the batch sizes that are being used are often too large and should be significantly reduced. Small batches are key to achieving a smooth flow and they should be aggressively reduced. Keep in mind that a batch is too small only when it creates a capacity constraint due to the increased number of setups that might be caused.
The schedule control points are material release, assembly, shipping, and the physical resource constraint (if one exists).
T-Plants
The critical feature of a T-plant is that the final products are assembled using a number of component parts and these component parts are common to many different end items (in contrast to an A-plant). Because of this sharing of components, the assembly part of the product flow has the structure shown in Fig. 8-16. Note that the number of end items is larger (much larger) than the number of component parts. This creates the sudden explosion of the PFD to create the T-shape. To illustrate the magnitude of this explosion, consider a case where there are six component parts and each part has four variations, giving a total of 24 different components. The number of possible end products is 4 × 4 × 4 × 4 × 4 × 4 = 4096!
Most manufacturers of consumer products are T-plants. Consider the production of personal computers. The basic elements—hard drive, processor, memory, display, etc., are available in a few variations each. For example, the hard drive is available in 40, 60, and 80 G sizes. The processor might be available with speeds of 1.8, 2.0, or 2.4 GHZ. As illustrated above with just a few such variations, the number of distinct computers that the manufacturer produces can be very large indeed.
The characteristics of a T-plant are:
1. A number of common manufactured and purchased parts are assembled together to produce the final product.
2. The component parts are common to many different end items.
3. The production routings for the fabricated component parts are usually quite dissimilar.
FIGURE 8-16 Product flow diagram for a typical T-plant.
The dominant characteristic of a T-plant is that the assembly point is actually a divergence point. The same component part (80 G hard drive) can be assembled into a very large number of different end units. Unlike a V-plant where the divergence points are spread through the operation, the divergence in a T-plant is concentrated in the assembly area. The impact of this is devastating. We have seen the impact of a simple divergence point in the case of V-plants. In a T-plant, the divergence is assembly and this means that not one but all components are diverted to the wrong product if assembly produces the wrong item. This significantly magnifies the impact and spreads through the whole system like wildfire. This is illustrated by the simple case shown in Fig. 8-17 involving four component parts A, B, C, and D, and four assembled Products E, F, G, and H. The arrows show how the products are made and the figure indicates the inventory available for each part. Now suppose that an order for 100 parts of Product E is due to be assembled and shipped. The assembly of Product E requires 100 units of part A and 100 units of part B and is next on the assembly schedule. However, as shown in Fig. 8-17, part A has zero inventory. An expediter will have to be dispatched to expedite 100 units of part A. In the meantime, the assembly operation is going to be idle. However, it is possible to make 100 units of Product H, which requires part B and part C. In most cases, the assembly will not be left idle. Product H will be produced, since it is an active part and might very well have an order due next week. Note that this action consumes the available stock of part B, while at the same time creating finished inventory of Product H. Alternately, Product