A Guide to Implementing the Theory of
More Definitional Subtleties
Time doesn’t stand still, and not all definitions remain constant. It would be nice if they did, but they don’t. The introduction of simplified drum-buffer-rope has resulted in a need to redefine buffers to some extent. So now we have two definitions in the literature and also in active use. Let’s explain the older definitions and then explain the more recent changes.
In traditional drum-buffer-rope with an internal constraint (the birth-place of Theory of Constraints) the constraint buffer and the shipping buffer are discrete parts of the whole manufacturing lead time. They don’t sum to the manufacturing lead time, there is a gap, the gap is the duration that is scheduled on the constraint.
There is another buffer that we have addressed in passing; an assembly buffer. Traditionally this is viewed as an extension of the shipping buffer. We will return to this in a moment.
So on these pages we have used the older buffer approach consistently across both make-to-order and make-to-stock and both traditional drum-buffer-rope and simplified drum-buffer-rope. There is nothing wrong with this.
Recently, however, in two sources the constraint buffer has been defined as a part of the shipping buffer (1, 2). Strictly, I guess we should call the constraint buffer here a “control point buffer,” but let’s not get too particular. The shipping buffer in-turn is the whole of the manufacturing lead time. There are very good reasons for doing this.
The most important reason is that the constraint should be in the market not in the firm, thus most often there should not be a control point or constraint that we need to schedule in detail. In which case the shipping buffer is perfectly adequate. If there is a constraint or control point that needs to be scheduled in detail then it’s buffer duration simply becomes a subset of the shipping buffer duration. Note that this only applies to make-to-order, or make-to-stock under the transitional arrangement that we described earlier, after all we can’t schedule in detail in make-to-replenish because the priorities will continue to change after material release.
What we are seeing is a change in emphasis from an internal business constraint to an external market constraint. It is subtle but significant.
Room for confusion arises when people exposed to the Insights material go back to the existing literature. Room for confusion exists in using the revised edition of the production simulator (new definitions) and the MICSS simulator (prior definitions).
Many people will however recognize that adding a “free product” shipping buffer to the first diagram and the difference between the shipping and constraint buffer to the second diagram and we would have, apart from the difference in the “gap” for the constraint consumption, essentially the same. OK, for clarity lets show that.
Finally, assembly buffers are no longer required. The rationale for this is outlined in Warp Speed (3).
(1) Goldratt, E. M., (2003) Production the TOC way (revised edition). North River Press.
(2) Goldratt, E. M., and Goldratt A., (2003) TOC insights into operations. Goldratt’s Marketing Group.
(3) Schragenheim, E., and Dettmer, H. W., (2000) Manufacturing at warp speed: optimizing supply chain financial performance. The St. Lucie Press, pp 151-152.
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