The boundaries of DBR/SDBR

DBR, Drum-Buffer-Rope, is a TOC planning methodology for manufacturing companies. In this post I focus on DBR/SDBR for make-to-order (MTO), when client orders specify the products, the quantities and the delivery time.  I’ll dedicate another post on the boundaries of managing stock. On top of the planning, there is Buffer-Management, which is used to guide the priorities in the execution.

I’m not going to discuss here the details of DBR and the differences between DBR and SDBR. I wish to outline the situations where the rationale of DBR/SDBR is valid.  For instance, Goldratt created CCPM because it became evident that DBR would NOT work for projects.  Is it clear to YOU, the reader, what basic assumption(s) behind DBR are not valid in multi-project environment and vice-versa?

The basic situation is manufacturing or service organizations getting orders from clients who ask for certain packages of well-defined products or services to be delivered within a certain lead-time. The organization does not usually hold these products in stock, so there is a need to produce them. The organization might need materials purchased from suppliers and then produce the specific orders for those clients.

Key basic assumptions where using DBR/SDBR would yield reliable delivery performance:

1. The net touch-time of any production order is very low relative to the lead-time, the time from accepting an order until delivery.
   1. Goldratt assumed less than 10% of the time-buffer used for such an order.
   2. The rest of the time the production order waits for resources to become available, which means that the utilization level of many resources is not very low.
2. The level of statistical fluctuations within the shop-floor itself is not too large.
   1. For instance, when the average setup is two hours it is unlikely to take 20 hours.
   2. Overall such an environment is exposed to much less fluctuations than in projects!
   3. This assumption also addresses the level of scrap.  It is unlikely, or very rare, that a whole production order is scrapped.
   4. It means that the shop-floor maintains good enough quality in all stages.
3. The organization maintains acceptable control on our supply and outsourcing.
4. All the operations are at the same location, or close enough, so transportation time between the facilities is short relative to the time-buffer.
   1. This assumption could be viewed as an extension of assumption #1.  We need to understand that transportation, also dry-time, is part of the “touch time”.

One key assumption for Buffer-Management:

1. Most orders either finish in Yellow or finish very close to their penetration into Red.

When one assumption, or more, is not valid it does not mean that DBR/SDBR is useless, but it does mean certain changes are mandatory.

I like to explain more the impact of touch-time on the methodology. Actually the assumption about touch-time is the main difference between production and multi-projects. In projects we expect that the time it takes to process the tasks along the critical chain is equal to the time to complete the whole project.  This is in sharp contrast to production where orders wait between operations for quite long time.  Because of this key difference the term “critical path” or “critical chain” is not relevant for production.

What happens when the touch-time of a certain operation is about 30% of the time-buffer?

In SDBR the time-buffer covers the whole production process, from the release of the materials to completion. When one operation takes 30% of that time we have to ask two critical questions:

1. Is the time-buffer enough to protect the due-date from fluctuations along the whole production process?  The 30% touch-time is not part of the protection time, so, do the rest 70% provide enough protection?
2. In buffer management for DBR/SDBR the exact location of the production order is not reflected in the status of the buffer, because when the touch-time is negligible then it does not truly matter.  However, when one specific operation takes 30% of the buffer it matters a lot whether the production order has still to go through it or already passed it. When the order is still behind the long-operation the remaining effective buffer is significantly shorter than the remaining time until the due-date.

A new application of  buffer management within SDBR implementation facing high touch-time was developed and presented at the TOCICO conference in 2012 (Lisa Scheinkopf, Yuji Kishira, and Amir Schragenheim). This is an example of understanding the boundaries of the basic assumptions and the level of required changes when a specific assumption is invalid.

It is important to emphasis again that the above assumptions refer to make-to-order production. The critical distinction between MTO and MTA (make-to-availability) was recognized much later than DBR and also SDBR.  The assumption that we have a specific quantity to deliver at a specific date was not clearly verbalized in both MRP and DBR.  Once the role of that assumption became clear a different methodology for MTA had been developed.  The lesson is to try our best to verbalize the underlining assumptions, which together define the boundaries where a specific methodology is valid.  Then we can identify the situations that lie beyond those boundaries and look for an appropriate solution, which can be close or very different from the original methodology.