New Book Part 14

In my last post I explained that in DBR, the drum is the constraining resource, and the pace of the drum sets both the priority and work schedule for your process. I also explained that 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.

• Green Orders result from the buffer status being greater than 67%, which means that there is still plenty of time left to complete the order, so no need to expedite.

• Yellow Orders are observed if the buffer status falls between 33 and 67%, which means that disruptions have taken place somewhere in the process, and there is a high risk that any other disruptions will probably result in a late order.  However, there should still enough time to complete the order, without the need to expedite.

• Red Orders happen when the buffer status falls below 33%.  The inevitable assumption here is that, if there are more disruptions to the order, the order will definitely be late without expediting.

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 this post, I will talk about how we calculate these buffers.

Although this may sound overly simplistic, the correct buffer size is only known by monitoring your process. Like I explained earlier, if parts are always arriving in the green and yellow zones, then the buffer is most likely too big. Equally, if the parts are always in the yellow and red zones, then the buffer is usually too small. Goldratt suggested that the initial buffer size can be calculated by taking one-half of the current lead time and dividing that time between the constraint buffer and the shipping buffer. This initial buffer size can then be adjusted up or down, depending upon when the parts are arriving. For example, if you find yourself always in the expediting mode, then your buffer is too small, so more time must be added. If you never or rarely experience expediting, then your buffer can be reduced. This attention to buffers is the essence of what we call buffer management.

Each open work order or production batch, will have a buffer status that we can calculate. For example, based upon the buffer status we can relate the color code to a percentage of buffer consumed, according to the following guidelines:

You must always remember that buffer status can change at any time. Because of this, it is recommended that buffer status be checked every shift, and that a First-In-First-Out (FIFO) priority system be followed.  This means that the priority order for working on orders should be red, then yellow and then green.  There should always be a documented record of why orders might consistently fall into yellow and red zones, because they represent an opportunity for improvement in lead times and on-time delivery rates. These improvements could result in a distinctive competitive advantage for your company.

The above figure, taken from Debra Smith’s fantastic book, Measurements for Effective Decision Making, is a graphic of what the DBR system looks like, with the Drum, Buffer and Rope identified appropriately.  Smith explains that in addition to the constraint and shipping buffer, there is a third buffer, the assembly buffer. We included this, in the event that your process involves an assembly that requires the part produced at the constraint before it is completed. That must be accounted for as well, if part of your process includes assembly.

In my next post, I will continue the discussion on the UIC by looking at steps 3a, 3b, and 3c. 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.

Bob Sproull