Theory of Constraints Handbook - James Cox Iii [164]
The rule for determining a safe due date is the current planned load date (the first opening in the planned load) plus half of the production buffer time.
In Fig. 9-3 we see a graphic illustration of the time segments involved in computing both the safe date for committing orders to customers and for determining the release date for orders to production. Looking at the top right of the figure, we see that a full production buffer is placed at the safe date committed to the customer. Continuing at the top, we notice that we back off one full production buffer length in time to determine the release date for the order. In the lower segments of the figure, we deal with the elements that go into the computation of the safe date. (Determine the date where the planned load ends on the CCR and add half of the production buffer.) We see that the production buffer (a liberal estimation of the production time) is logically divided in half. It can be seen that the point in time when processing is to occur on the CCR (depicted using a minimized picture of the CCR planned load profile) is the dividing point (in time) for splitting the production buffer in half. One half of the production buffer time is added to the planned completion date for the last order loaded in the CCR planned load, approximating when this next order will start processing on the CCR. This effectively adds the required time for completion on the CCR and mainly the end of the production process for the new order. Adding this half of the production buffer to the planned CCR date then gives us the safe date for commitment of the new order to the shipping dock. From that same point on the load chart, the other half of the production buffer (in effect the upstream half) is subtracted from the time when processing is to occur on the CCR. It is worth noting that in these calculations, the actual processing time on the CCR (touch time) is ignored in computing the release of raw materials and estimated shipping dates because CCR touch time versus queue time in the production process is usually negligible.
FIGURE 9-3 The timelines for safe date, order release, and buffer placement in S-DBR.
Taking this in another sequence, we see that from the point of order release to processing on the CCR we provide half the production buffer to get the order to the CCR, and the other half of the production buffer to get the order from the CCR to order completion. (Both of these time components are implied in releasing the order one full production buffer time ahead of the safe date.) Taking the time of processing on the CCR plus half the production buffer, we get the safe date for customer commitment.
The rule does not specify the location of the CCR operations within the routing. So, fixing the required time between the planned load and the safe completion date to be half the production buffer for the product requested by the order seems arbitrary. This works well except where the CCR is very close (measured in time) to the material release or shipping in the product structure and in such an extreme case, some other division (of the production buffer other than half before and after the CCR) should be used as will be noted later.
Another question is whether the size of the manufacturing order has a major impact on the size of the production buffer and on the planned load. Suppose the production buffer for a “normal” order size of 50 units is 10 days. If the order is for 200 units, then it seems the processing time at the CCR would take significantly longer than usual, and it would impact the time the order requires from the downstream operations. Wouldn’t it?
The rationale behind the rule of using the default buffer for orders with different quantities is to have a simple and straightforward method to monitor