Friday, June 1, 2012

Drum, Buffer, Rope... and other scheduling systems


Very recently I was asked how to do 'Drum, Buffer, Rope' in SAP. The person asking was interested on how to work with constraints; mainly the bottleneck work center. There are many theories out there and each one has their on scheduling heuristics. 'Lean' has heijunka, 'agile' has FIFO and the 'Theory of Constraints' has Drum, Buffer, Rope (DBR). They may all be used interchangeably and SAP has them all pre-configured. You just have to know where to find them.

Heijunka is a leveling, sequencing and scheduling method, which can be employed when using the tools of repetitive manufacturing. There is a strategy profile which is called Heijunka.



As you can see, there is also a strategy called FIFO in the scquencing board. 


These strategies are not limited to repetitive manufacturing. They can also be used with discrete orders which are scheduled, capacity leveled and sequenced in CM25 - the graphical planning table.

Drum, Buffer, Rope is a similar thing in that it is employing algorithms in order to come up with a paced production program in a certain sequence, and it is based on the Theory of Constraints. This theory is concerned with running the bottleneck work center and the following gives a bit of an explanation on how this could be achieved. 

Managing the constraint is mostly about managing the non-bottleneck systems and making them “aware” how fast they should work — when they should slow down, when they should stop, or when they should increase pace and by how much. The Drum-Buffer-Rope system allows for such a systems-wide awareness.
The Drum is the bottleneck or constraint and acts as a drum — it sets the rhythm that the whole system should follow. In Lean Manufacturing, this is also called “Takt Time.”

The Buffer. There are situations when when upstream processes can’t produce as much as is needed by the bottleneck; the result is that the constraint is starved and the overall system output is compromised. So, we must have a buffer of inventory that is the size of the accounted-for variation in demand. This will help to level-out variation. A buffer will assure that the constraint never has to wait and, waiting is a form of waste. Similarly, if upstream processes are producing more than the constraint has the capacity to handle, then there’s going to be excess inventory sitting in front of the constraint and, hence, another form of waste. Put another way, the buffer is the inventory and inventory is directly related to lead time (remember my blog about Little's Law?)

DBR is used to avoid either of these scenarios — the Feast or the Famine — by dictating the batch size and frequency of the inputs into the Buffer.
The Rope is a method by which the constraint can signal to the upstream processes (non-bottleneck processes) when to slow down, when to stop, or when to produce faster. This is called “Pull Scheduling” in Lean Manufacturing terms.

so in SAP terms one could say that we schedule the bottleneck (drum) with a takt-based method like heijunka or FIFO in MF50 or LAS2. The buffer may be a consumption based replenishment method like a reorder point or material forecast method, an external forecast on component level or a kanban control cycle. In the end the buffer inventory in front of the bottleneck needs to be designed exogenous to the DBR system. The rope then is some sort of a pull system that may come from another kanban, conWIP or a range of cover profile.

There are many ways on how to design a production line so that it follows a Drum, Buffer, Rope philosophy... the point is: you can do it with standard SAP-ERP.