Theory of Constraints Handbook - James Cox Iii [137]
Production lead time = Sum of process times and setup times + Time buffers
The concept of time buffers is almost self-evident. Determining the proper size of a time buffer on the other hand appears to be a complex task. Since the objective of the time buffer is to protect the flow through the system from disruptions, it might appear that a detailed knowledge of these disruptions—the statistical distribution curves at each step in the flow—would help (or even be a necessary element) in calculating the size of the time buffer. While this may appear to provide a rigorous methodology, it is practically useless because the required information is not available. In the practical application of DBR, we take a more pragmatic approach to establishing the size of the time buffer. Every production operation currently uses a time buffer, whether or not it is explicitly understood. By this, we mean that the production lead times used—informally or in a computerized ERP system—is many times larger than the process and setup times. All of this additional time is a time buffer. We also know that most often this currently used time buffer is much too large. This is because buffers are used to protect each step in the flow and not just the system flow as a whole and, more importantly, larger time buffers make it possible to minimize the situations in which resources simply run out of work. Since the traditional view is that a resource standing idle is a waste, lead times must be large enough to minimize idle time at each resource. In effect, one can view the current lead time as giving us an upper limit—the point where current lead time provides too large of a time buffer.
If current lead times establish one extreme for the time buffers, another extreme—a time buffer that is too small—is provided when the production lead time is close to the sum of process and setup times. In fact, in almost all production operations a production lead time that is even just three times the process time would be considered unrealistically aggressive. At each extreme end, the time buffers are actually ineffective in providing protection and promoting smooth reliable product flow. When the time buffer is too small, the cumulative disruptions that every batch of product is subject to quickly overwhelm and consume the available buffer. When the time buffer is too large, the shop floor is clogged with too much material, making it difficult to manage the flow. Each operation will have plenty of work from which to choose, and the chance that they will all be coordinated to choose the right work to promote a smooth organized flow is slim. The results are piles of inventory everywhere, long lead times, poor due date performance, and chaos on the shop floor. Between the two extremes is a range of options. Based on vast experience, we believe that Fig. 8-6 captures the essence of the effectiveness of time buffers as we increase buffer size from very small to very large. The key observation is that the curve in the region where the time buffer has a high degree of effectiveness is relatively flat. This means that there is no real benefit to complex calculations that yield precise buffer values. Being in the right ballpark is sufficient. Again, based on vast experience, a good value for the time buffer in most production environments is one-half of their current production lead time.
FIGURE 8-6 Graphical representation of the effort required to maintain a smooth flow as the buffer is increased.
The time buffer established here becomes the time element that will be used to implement the second principle of flow management (prevent overproduction). If we want to prevent production ahead of time, then we should not make the material available ahead