New Book Part 13

In my last post I discussed the role non-constraints play in the flow of products through your system and then presented the basics of TOC's scheduling system, Drum Buffer Rope (DBR).  In this post I  will continue the discussion on DBR. As a reminder, this material is taken from my newest book, The Focus and Leverage Improvement Book - Locating and Eliminating the Constraining Factor of Your Lean Six Sigma Initiative, published by Routledge/Productivity Press.

DBR Continued

In my last post I told you there were three very important buffers which were:

• A buffer in front of the constraint to avoid starvation in the event that Murphy (and Thompson) strikes any resource in front of the constraint.

• A buffer in front of assembly, if a constraint part is required to complete the assembly.

• A buffer in front of shipping, to assure on-time delivery, in the event that Murphy (and Thompson) strike upstream of shipping.

I also explained that It is important to understand that these three buffers are generally in the form of time, rather than physical products.  The management of these buffers is critical to your success using DBR, so the question becomes, if time is the buffer, how do you know how much time is required?

• The buffer must be large enough to handle all of the uncertainties associated with Murphy’s (and Thompson’s) Law, that seem to always occur and eat up buffer time, so that your schedule is achieved.

• The release of raw materials must be done on time, to assure that the constraint is never starved and that the shipment and customer delivery dates are never compromised.

The above figure is meant to illustrate any of the three buffers listed above. The basic purpose of these buffers, is to provide a signal to everyone involved as to when and when not to require expediting. That is, when a part is late arriving at the buffer, it creates what is referred to as a “hole” in the buffer. Typically, for each type of buffer, there are three zones, a safe zone (green), a caution zone (yellow) and an expedite zone (red).  Each zone is equal to one third of the total calculated time in which the product must arrive at the buffer location. As you might have guessed, if the part arrives at the constraint on time, it is said to be in the green zone time, so there is no cause for alarm.  On the other hand, if a hole is formed in the yellow or red zones, there is reason to be concerned and immediate action must be taken to prevent constraint starvation.  In the event that parts fail to arrive in the red zone on time, this naturally means that extreme and immediate actions must be taken (i.e. expediting the part).  When this happens the part will definitely arrive late at shipping and will ultimately be late arriving at the customer location.  I can’t emphasize enough that the correct use of these three buffer zones is absolutely critical to DBR’s success.

In addition to these buffers, you can also use improvement data for additional improvements.  For example, if your parts always arrive in the green zone or yellow zones, then there’s a good chance that your calculated buffer time is too large and could be reduced. On the other hand, if your parts are consistently arriving in the red zone, then you must increase the size of your buffer times to avoid continually having late orders to you customers.  It is also possible that what you have identified as the constraint, is not actually the true constraint and that the real constraint lies elsewhere.

In DBR, the drum is the constraining resource, and the pace of the drum sets both the priority and work schedule for your process. The buffer is typically the time schedule in front of the constraint, shipping and any assembly operation needed at the constraint to protect the due dates for all three areas. Your ability to meet these due dates is dependent upon completing two things:

• The buffer must be large enough to handle all of the uncertainties associated with Murphy’s (and Thompson’s) Law, that seem to always occur and eat up buffer time, so that your schedule is achieved.
• The release of raw materials must be done on time, to assure that the constraint is never starved and that the shipment and customer delivery dates are never compromised.

The rope is simply the length of time needed to complete the processes that are upstream from the drum. It is a signal sent to the beginning of the process, to make sure that raw materials are released on time. In my next post, we will talk about how we calculate these buffers.

Bob Sproull