Theory of Constraints Handbook - James Cox Iii [270]
FIGURE 14-6a Stratification of a time buffer.
With regard to time buffers, this commonly means that early, expedite, and late zone penetrations into the time buffer are noted and managed at two levels—first, to direct execution actions to keep the constraint and delivery schedule stable; and second, to capture information that identifies the source of variation for future improvement activities to increase system stability. Figure 14-6a shows the stratification of a time buffer into different zones ranging from an early (far left, sometimes referred to as the light blue (LB) or white buffer zone) to expedite (red), to late delivery (far right, sometimes referred to as the dark red (DR) or black zone). The general zone color designations are provided in the figure. Notice there is a released work order (48709-01) that has not entered the time horizon that the buffer represents in front of the drum. One of these work orders will enter 9 hours out. If this facility works 24 hours a day, then that entry will occur at 10:00 A.M. on Wednesday.
We will have to reconcile the work order’s actual presence in the buffer by recording when it entered the buffer and judge that against its scheduled entry to create a view about what, if any, corrective actions need to taken. When a work order is not ready and in the buffer at the start of the green zone (scheduled buffer entry), a penetration is created in the buffer. This hole can be caused, for example, by missing materials, tooling, specs, etc. The severity of this penetration will ultimately determine when we have to act and the priority of work orders on which we have to act.
This means that we have to think about the five zones from two perspectives, “Yet to Be Received” (at the drum) and “Received” (at the drum). These are the two situations that can occur. When something is “Yet to be received” the clock is still ticking on the time it has to travel to the constraint. When something has been “Received,” the hole has been filled. In Fig. 14-6b, a real-time buffer board that reconciles a released work order against their buffer status is shown.
Notice that when we account for the same time horizon from the two different perspectives, it actually creates 10 status zones. Those zones are:
1. Early—Yet to Be Received (LB). This zone actually represents all released work orders that are on the way to the buffer.
2. Green—Yet to Be Received (G). This is a hole in the buffer. Not a serious hole, but a hole nonetheless.
FIGURE 14-6b Reconciling released orders against buffer status.
3. Yellow—Yet to Be Received (Y). This is a deeper hole in the buffer that should now be getting the attention of the personnel responsible for managing the buffer.
4. Red—Yet to Be Received (R). This is the deepest hole that we can dig without affecting the drum schedule. This zone alerts the appropriate personnel that if corrective actions are not taken, the drum schedule will be disrupted.
5. Late—Yet to Be Received (DR). The drum schedule has already been disrupted by this work order and it is still not present.
6. Early—Received (LB). The work order is physically present at the buffer resource and ready to be worked on by the drum ahead of the time horizon for which we are scheduled. This usually means that the standards we are using to generate the schedule may be over-estimated (very common since most companies’ standards are highly inflated to try to combat Murphy and disruptions everywhere) or the work order was released ahead of schedule.
7. Green—Received (G). The work order was received within the scheduled time horizon with a relatively large amount of time to spare.
8. Yellow—Received (Y). The work order was received within the scheduled time horizon with moderate time to spare.
9. Red—Received (R). The work order was received at the constraint resource within the scheduled time horizon with little time remaining before it is scheduled on the