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Planning Vs. Execution - Part 4 of 5

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THE GENERIC TOC SOLUTION-A SUMMARY

 

The theory of constraints makes a clear distinction be­tween the decision taken at the planning stage and those taken at the execution stage.

The decisions at the planning stage:

1. Decide upon the objectives of the planning—what do
we commit to our market?

2.      Identify the critical elements that if impacted by
Murphy might disrupt the objectives. For instance,
any bottleneck is vulnerable to any delays, because it
doesn't have the capacity to catch up.

3.      Plan the critical elements as needed.

4.      Plan the buffers that protect the objectives and the
critical elements in the plan.

The decisions at the execution stage:

1.      Follow the critical elements of the plan as is.

2.      Monitor the state of the buffers.

3.      Prioritize the activities according to state of the buff­
ers they feed.

4.      Expedite only those activities that feed buffers that
are at risk.

EXAMPLE NO. 1: SCHEDULING THE SHOP FLOOR

TOC has a methodology for planning the shop floor called drum-buffer-rope (DBR).

 

The "drum" is the master production schedule plus the detailed schedule of the capacity constraint resource (CCR). All resources that have excess capacity are not scheduled in detail, as they have enough excess capacity to overcome Murphy.

"Buffer" is the protection mechanism. TOC uses mainly time buffers to protect the on-time performance. The "time buffer" is a similar concept to MRP's fixed lead time that is used between levels in the bill-of-material (BOM), with one critical difference. In DBR the "buffer" covers the whole way from the raw materials release up to the constraint or the shipping. Like "lead time," it allows enough time for queue (wait time), setup, run, and move. But it does so for much longer than MRP. The rationale is to protect only the single capacity constraint and the shipping dates.

Rope is the part of the planning that schedules the release of material.

 

TOC assumes that only one critical resource may truly constrain the whole system. Hence, all the rest of the resources need to have excess capacity. That excess ca­pacity allows the non-constraints to be much more flex­ible. Hence, there is no need to detail the schedule for the non-constraints. Certainly there is no need for pro­tecting the non-constraints.

The DBR methodology dictates the rules for the planning, which includes the buffers at the critical places. The planning objectives are shipping every firm order on time. In order to ensure on-time delivery, the capacity of the most loaded resource (called CCR— capacity constraint resource) should be carefully checked. When the market demand exceeds the CCR's capacity, we like to load the CCR to its limits, and to commit delivery based on the CCR schedule. We need to allow enough time after the CCR for the order to be completed SAFELY. That time, which is the esti­mated lead time plus safety, is called the shipping buffer. When we load the CCR to its limit and com­mit accordingly to the market, the detailed schedule of the CCR is a critical element of the planning. We need to ensure that the CCR will be able to follow the schedule. That means buffering the CCR with a CCR buffer. The CCR buffer allows enough time for the materials to reach the CCR before it needs the parts. This is done by the rope part of the planning. Of course, we also need to ensure the availability of mate­rials to support the release schedule.

Buffer management monitors the buffers. If the CCR buffer in Figure 4 is 10 days, then 7 days after the release, the parts are expected to be at the CCR's site. If not, mean­ing 70 percent of the buffer has been consumed and the

parts are not where they should be, expediting actions should push the parts to the CCR. So, the schedule on the CCR should be followed as is, except for probable delays on the CCR itself. The shipping buffer should take care of these delays—unless more than 70 percent of that buffer is consumed, which also triggers expediting.

The combination of planning that incorporates buff­ers to protect the objectives and the critical elements of the planning produces a much less detailed planning that strives to do the most the system can do (TOC ter­minology of that is: exploiting the system constraint), and yet it is a robust planning.

The part that is the execution leaves quite a lot of deci­sions that either are not crucial for meeting the due dates OR concentrate on the real threats to the plan as viewed by the buffer management methodology. Every exception is treated so the original plan will still be carried out.

Continued

Part 1  Part 2   Part 3  Part 4  Part 5


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