Wednesday, June 12, 2013

Focus and Leverage Part 226

This will be the final posting on the Constraints Management series and I hope you have enjoyed it.  In this posting we'll take a look at TOC's most famous scheduling system, Drum Buffer Rope. 

4.  Drum Buffer Rope Scheduling – One of the lessons learned from those who have read The Goal is that TOC’s scheduling system known as Drum Buffer Rope (DBR) is very effective for synchronizing the flow of work.  It got its name from a troop of boy scouts hiking across country, but were continually slowed by an over-weight Boy Scout named Herbie.  DBR was used to describe how a typical system has dependencies and statistical fluctuations that must be accounted for is work is to flow smoothly.  In The Goal, Herbie was the Drum (constraint) because he walked at the slowest pace.  The Buffer and Rope were techniques used to synchronize the Boy Scout’s walking pace as they traversed the path on their hike.  Let’s look at each of these three components in a bit more detail.

The Drum – In a manufacturing (or service business) the drum is actually a schedule for the resource with the most limited capacity and is referred to as a Capacity Constrained Resource (CCR).  Because this resource has the least amount of capacity, it determines the maximum output of the entire business.  Doesn’t it make sense that since the drum determines the maximum output of the business, it should be protected against upstream disruptions?  This is where the buffer and the rope come into play.

The Buffer – Because starving the CCR would delay the completion of work, a protective buffer is created to protect it from inevitable disruptions and variability from upstream processes feeding the CCR.  The buffer is a period of time (not feeder products) in advance of the time scheduled to begin work on the CCR, to ensure that the non-constraints complete their work in time so that the CCR is never starved for work to do.

The Rope - The purpose of the rope is to guard against over-production by the non-constraints.  Over-production on these resources effectively creates pockets of excess WIP which negatively impacts overall cycle times and delays deliveries to customers.  The rope is a signaling process which tells the gating process when to release raw materials to the front end of the process.  It is, in effect, a raw material release schedule that will presnt the early release of materials into the system.

In DBR there are three types of protective buffers to protect various parts of the process.  As we just mentioned, there is a buffer in front of the CCR, but we also have a feeding buffer in front of any assembly process that combines with the CCR's product.  Finally, we insert a shipping buffer to protect customer deliveries.

By using the drum, the buffer and the rope effectively it is not uncommon to see on-time deliveries approaching 100% while overall lead times for products being reduced from 50-70%.

At the beginning of this series of posting I said I wanted to present my thoughts on Constraints Management (CM) and what it means to me.  The fact is, Constraints Management is not simply a collection of tools used to solve problems….it’s much more than that.  CM is even more than, as the name Theory of Constraints might suggest, just a theoretical basis for how a business enterprise should be run.  Constraints Management, for me has become a way of life.  It defines how I think and act both as a consultant and as a human being.

As I began looking through the Constraints Management lens years ago, I began to see the world differently than I ever had before.  I realized that I had been looking at improvement in the wrong vein.  By that I mean that I had fallen into the "fix everything" trap rather than focusing on the system's leverage point, the system constraint.  So for me, what Constraints Management is, is the same name as my's all about focus and leverage.  By being able to identify the leverage point of the system I'm working in, I can focus my improvement work on it to gain maximum effectiveness.
As a final thought on this subject, there seems to be a "power struggle" of sorts going on between the "believers" of the individual improvement components, Lean, Six Sigma and the Theory of Constraints.  It's as if they are saying, my way is the best....forget the other two!  My view is very simple and it's why I didn't name my second book TLS or some other title combining the individual components.  I named my second book, The Ultimate Improvement Cycle for a very good reason.  The true ultimate improvement method is a combination of best practices, with each individual component bringing something important to the mix.  From a ten thousand foot view, Constraints Management provides the laser-like focus on what to change and where in the system you should focus your efforts.  Lean provides the tools required to identify and remove or at least reduce existing waste.  Six Sigma's tools help us stabilize our processes and make them much more predictable.  But when you combine all three of these methodologies, you are able to optimize the system in which you are working.  This symbiosis truly is the ultimate improvement cycle.

 Bob Sproull


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