The good old Drum-Buffer-Rope (DBR)

• DBR was developed in the mid 80s as a relatively simple, yet effective, production planning methodology.
• DBR centers on detailed finite capacity scheduling of the capacity resource constraint (CCR). − The rest of the resources were not scheduled and their activation was based on the schedule for the CCR and/or the due-date.
• DBR was developed under the assumption of getting orders from the sales, without truly impacting the committed dates, and coming up with the best planning possible under the circumstances.

Reminder on Drum, Buffer and Rope

• Drum is the exploitation scheme for the constraint. − What to produce and the schedule for the CCR.
• Buffer is the protection scheme on the Drum.
• Rope is a schedule for the material release. Its objective is to keep the buffers intact and not allow earlier release of material than required by the buffer.

The distinction between planning and execution

• One of the tools to deal with uncertainty is to focus the planning on the truly critical areas.
• Leaving enough flexibility in the execution phase to deal with local disruptions without messing the whole planning and the expected objectives.
• This generic approach lies behind the DBR and the S-DBR planning methodologies. − The Drum is the main essence of the production planning. − Buffers belong to the planning as the mean to keep the planning intact. − Rope is critical to fully maintain both the drum and the buffers.
• Buffer Management sets the rules for the execution.

The main benefits of scheduling the CCR that we do not want to lose

• Being able to tie the Rope so that the CCR will not starve and will not waste its capacity.
• Exploiting the CCR capacity.
• Being able to know whether the delivery dates, given by Sales, are safe.
• Proper subordination to the market.
• Smooth the load on the whole shop to prevent too large temporary peaks of load.
• Enabling the full subordination of the non-constraints.

Finite capacity scheduling of the CCR Is the focus absolutely right?

• The focus of Operations is on the exploitation of the capacity of the CCR. − Many times it leads to problematic subordination to the market. − The identity of the client and the impact on the reputation are not always taken into consideration. − There is a huge difficulty to deal with urgent orders. − A policy of “adherence to the CCR schedule” makes it difficult to enter changes caused by customers who have new priorities. − The result is that the response to the market demand lacks flexibility.

Finite capacity scheduling of the CCR Is it right for make-to-stock?

• The need for better flexibility is especially noted in orders that are made to stock. − Because the true priority of a stock order is not known at the time of the scheduling. − The priority of stock orders depends on the finished goods stock versus the demand. This could change in a day.
• The current practice is to assign a date to the stock order and then base the CCR schedule on that date, like any make-to-order. − The date is based on forecast. − Many times the derived priorities of the schedule are wrong.

Finite capacity scheduling of the CCR Three buffers to maintain

• The CCR detailed schedule forces three different buffers to be implemented, without good priority mechanism between them. − The Shipping Buffer protects the due-dates. − The CCR Buffer protects the detailed schedule. − The Assembly Buffer – controls the material release of non-CCR parts. − What does it really protect? Isn’t it just an extension of the shipping buffer?

Finite capacity scheduling of not always simple

• In many cases the finite capacity scheduling of the CCR is especially complex. − dependent setups and other technical limitations that forbid certain sequences. − Time-per-batch operations, like ovens, that work on several orders together provided certain conditions apply to all orders. − Many machines, which are NOT identical, are the CCR. − CCR operations that feed other CCR operations.
• Very few, if at all, existing DBR programs can schedule the CCR considering those complexities! And, is it really needed?

When one is used to focus on the one CCR

• When the market fluctuates, even a real CCR should have periods of low load. − Thus, at those times, the constraint is just the market demand. − Should we still focus on the CCR at those times?
• Sometimes the CCR shifts from one resource to another. − It could be because Sales took T/CU too seriously. − The equilibrium is jeopardized – all the subordination processes change − A lot of data has to be checked, maintained and fed into the DBR software − Note especially that all the buffers have to be changed!

Some Key Observations

• The market demand is a major constraint even when a capacity constraint resource is active.
• Failing to subordinate to the market could lead to a significant dive down of the market demand.
• Elevating the internal constraint won’t improve anything unless the market demand is elevated as well.
• In such a case how do we exploit both constraints?
• Not disappointing the market.
• Not losing too much precious capacity of the CCR.