Different Forms of Drum Buffer Rope Part 4

In my last post, we discussed exploitation of the system constraint.  In this post, we will discuss the function of the rope and the various buffers in Drum Buffer Rope.

The rope is actually a mechanism that controls two different functions.  First, it is the mechanism that determines how much and when to release inventory into the system.  The most common practice is to tie an artificial “rope” from the constraint operation back to the front of the line.  When the constraint produces and completes one unit of work and passes it to the next operation, then the rope is pulled to signal the front end of the line to release one more unit of work into the system. Rope signaling systems can vary.  The rope signal is equal to the output of the constraint operation, no more and no less.  This release mechanism, tied to the drumbeat of the constraint, will allow for a synchronized work flow and a smooth transition of work through the system. 

The second function of the rope is to initiate and maintain subordination for all other processes in the line.  By default, following the cadence of the rope release signal causes subordination to the remaining non-constraint processes to be executed.  The non-constraints processes can only work on what has been released to work on.  By releasing work only to the drumbeat, all other operations are held in check to the rule of subordination.  Even if the non-constraints can do more work, they are restricted by subordination and only allowed to work on parts or products required by the constraint.

The systems inventory not only includes the work located at the buffer, but also the cumulative total of inventory (work) at other process locations as well.  It is possible, and recommended, that you establish an additional buffer at the shipping location.  The shipping buffer can be used to help control any system variation that occurs after the constraint.  Bad things can and do happen after processing at the constraint.  The constraint buffer provides the necessary protection in front of the constraint, and the shipping buffer provides protection after the constraint.  The shipping buffer is just a mechanism to absorb and manage the inevitable variation that will occur.  Buffer sizing at these two locations is a variable, but you do need to start with something.

Consider, as a starting point for the buffer at the drum (the constraint) location to be about one and a half for whatever units of time you are measuring.  For example, if your constraints can produce ten units in one day, then the buffer should be set at fifteen units (or 10 x 1.5 = 15).  You may decide in time that the buffer is too large or too small, so you can adjust it either up or down depending on the need.  The shipping buffer could be three or four days or less depending on the speed of product through the system.  It doesn’t need to be necessarily large in quantity or long in time.  It just needs to be sufficient to protect against variation after the constraint.  It’s also important to consider your shipping buffer time in your scheduling calculation to determine the correct release date into the system for on-time delivery.  If you watch your buffer locations carefully, you can make good decisions to increase or decrease them based on some supportive historical data.  If the buffer is constantly on the high end, then reduce it.  If it is constantly on the low end, then increase it.  Apply the rule of common sense to determine the correct buffer.

When you know and understand the constraint location, and you buffer the work activity, and you send the correct release signal to the front of the line to release more work, then you have in essence implemented a system of synchronized flow.  

In my next post we will continue our discussion of Drum Buffer Rope and will get into what a DBR system might look like.

Bob Sproull

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Different Forms of Drum Buffer Rope Part 3

Exploiting the System Constraint

Once the systems constraint is identified, it must be subjected to the red carpet treatment.  Nothing in your system is more important than the constraint— nothing!  Once you have this information, you must decide how to best manage the constraint.  If the output from your entire system depends solely on the output of the constraint, then it certainly merits special considerations.  One of those considerations is to exploit the constraint, which is Step 2 of the Five Focusing Steps.  Exploitation means that you evaluate the process to get the most out the constraint activity. 

Rarely is a constraint being utilized at, or near, the maximum that it can do.  The exploitation effort means looking for things that the constraints can stop doing.  This could be an excellent opportunity to employ the Interference Diagram (ID) to define the interferences that stop you from getting more from your constraint. You may want to implement Lean concepts to reduce waste or Six Sigma to better control variation and quality.  It might also mean taking actions as simple as keeping the machine, or process, busy during break time and lunch time, or perhaps implementing a second shift or a third shift, or even off-loading work to non-constrained processes or resources.

Exploitation does not mean buying a new machine or adding more resources, at least not yet.  It simply means finding ways to get more out of the current process than you are currently getting.  There is a very high probability that during the exploitation exercise, the constraint capacity could be improved above and beyond the capacity of the next constraint in the system.  If such is the case, then go back to Step 1 and redefine the constraint.  In a normal improvement effort, this repeating cycle between Steps 1 and 2 might be completed many times before the system is stabilized.  When the system becomes stable, then go to Step 3 in the Five Focusing Steps and ratify the subordination rule to synchronize the product flow. The end result is to stabilize and synchronize the system, and then focus on the constraint.  Let the non-constraints work as required to produce sufficient quantities to keep the constraint busy.

The second consideration is to make sure the constraints are busy all the time.  Never let the constraint run out of work to do.  If the constraint stops or slows down, then the entire system will stop or slow down.    The best way to accomplish this objective is to make sure there is always work in the queue in front of the constraint.  In other words, create a buffer of work in front of the constraint.  The entire system output has total dependency on the constraint output, and constraint output is directly proportional to system output.  Think in terms of the right amount of work, in the right location and at the right time.

The system constraint not only determines the amount of throughput you can achieve, but it also determines the correct amount of work-in-process (WIP) inventory that should be maintained in the system.  The correct inventory level will be reached almost by default when system subordination is actively pursued and implemented.

In my next post, we'll discuss the "rope' of Drum Buffer Rope.

Bob Sproull 

Different Forms of Drum Buffer Rope Part 2

In my last post, we discussed the basics of systems.  In this post we will discuss the basics of variation and how variation can negatively impact all systems.  We will then discuss how variation can be managed by employing Drum Buffer Rope.

Variation

For years, if not decades, people and organizations have dedicated considerable time and effort to remove variation from systems.  The utopian goal is to remove as much variation as possible from the system.  No matter how much planning is employed, no matter how much effort is extended, variation will still exist!  If you were asked; how long it takes you to get to work every day, your response might be something like, “about thirty minutes.”  The instant you answer with the word “about,” you have introduced variation into the system.  You  know that historically speaking, some days you get to work in twenty-five minutes and yet others days it can take thirty-five or forty minutes.  In your “get to work” system, things can happen that will either speed up the process or slow it down.

Variation exists in everything, especially within a system. You understand that some processes will produce at a faster or slower rate than others, and this is the premise behind variation. Because of variation, the output from a system will not be linear, but rather it will operate within a range that changes.  This variable range is known as statistical fluctuation, and it exists in every system.  It’s important to understand that you cannot make variation go away.  The theory and practice of Six Sigma has pioneered the race to variation reduction.  But even with the most valiant efforts of time and money, not all variation can be removed.  You can reduce the amount and severity of variation, but it will still exist.  Once you understand that variation is a constant variable in any system, it’s easier to understand that at some point you will reach the minimum variation that is controllable in the system, and any efforts to reduce variation beyond that point are fruitless.  Perhaps, instead of spending so much time and effort on techniques to remove variation, the focus should really be on techniques to manage variation.   

 Managing Variation with Drum Buffer Rope

When viewing a system through the eyes of DBR, it becomes quickly apparent that improving every step in the process is not required, nor will the sum total of all of those discrete system improvements equal an improved overall system.  When conducting a full systems analysis, with the intent of implementing DBR, an important consideration to know and understand is the location of the system constraint, or slowest operation.  In Goldratt’s Five Focusing Steps, this is Step 1—Find the constraint.  Once you know where the slowest operation resides, you now have the information necessary to know where to focus your attention within the system.  Why is it important to understand where the slowest operation is?  Because this is the location that controls and determines the output for your entire system.  In essence, the entire system will produce no faster than the slowest operation can produce.  (The system can produce less, but it won’t produce more.)

With the constraining operation identified, you have collectively quarantined the “drum” beat for your system.  Knowing the drumbeat is of strategic importance to implement and gain any system improvements.  The drum provides you with the necessary information of knowing where to focus your improvement efforts.  Historically, many organizations can and do conduct many improvement projects on a yearly basis.  The mantra seems to be that every organization and every process should strive for improvement.  The thought is that each organization is improving at some level of frequency to make the whole system better.  However, the sum of many efforts does not always equal what is good for the whole.

The problem with this type of thinking is it is a totally unfocused shotgun approach to solve the problem.  In effect, it presents an improvement policy that states: if I select a wide enough range, then I should hit the target, or at least come close to the target.  When you take the shotgun approach you might hit everything a little bit, but miss the full impact required to make real change and improvements.  If your shotgun approach includes trying to improve non-constraints, and most do, then the system as a whole gains nothing!  The improvement of non-constraints in isolation of the entire system, without a comprehensive analysis, is just a way of dealing with symptoms and not the real issue (constraint).  Without the ability and the accurate information necessary to focus on the real issues, the disease goes merrily on.  Improvement of non-constraints is a noble gesture, but one that yields little, if any, real improvements.  Every process within a system does not need to be improved at the same time!  Some system processes are more important than others.  Without knowing where your constraint resides, your efforts to improve will be unfocused and consequently worthless, serving only to consume large amounts of money, resources, and time.

In my next post, we'll dive deeper into Goldratt's second step.....Exploitation

Bob Sproull

Different Forms of Drum Buffer Rope Part 1

In this series of posts, I'm going to lay out the basics of the Theory of Constraint's scheduling tool known as Drum Buffer Rope.  I will start with the traditional version of Drum Buffer Rope and then introduce different versions of it.

In his book The Goal, Eliyahu Goldratt effectively uses a story written in a novel format to walk the reader through the steps necessary to move a manufacturing organization from the traditional manufacturing concepts to a facility managed using the concepts of Drum-Buffer-Rope (DBR).  This nontraditional approach through logical thinking is masterminded by a character named Jonah.  Jonah is able to help Alex Rogo understand the invalid thinking and assumptions being used to manage his plant and the negative consequences associated with that type of thinking.  By helping Alex focus his thinking on how the plant is being managed, Jonah helps Alex logically discover a new and better way. And Drum-Buffer-Rope (DBR) is the centerpiece of this process. 

 The thinking behind DBR is really quite simple, but mostly just logical.  Thinking logically is nothing new, but it is not the way most people think.  The fundamental view of DBR is to focus on the system as a whole rather than only a single segment of the system—at least until you have clearly identified the constraint.  This idea of looking at the global system is a major shift in the way systems have previously been viewed and managed.  Prior to global-systems thinking, the pervasive point of view was (and still is) that any systems improvement, at any location, would improve the overall system.  The idea being that the sum total of several isolated improvements would somehow equal an improved to the overall system.  But such is not the case.  The effects of employing the “shotgun” approach to systems management can cause a series of devastating systemic effects.

A system can be defined as a sequence of steps or processes that are linked together to produce something as an end result.  With that definition in mind, it’s easy to understand how virtually everything can be linked to some kind of a system.

Engineering organizations have systems, banks have systems and grocery stores have systems.  Almost anything you can think of is the product of a system.  By design, a system can be as small and unique as two processes linked together, where the output of one process becomes the input for the next process. Or systems can be very complex, with many processes linked together, maybe even hundreds or more.  Just because a system is complex does not mean it can’t be improved—it just means it’s complex, and that’s OK.  Even in a system as simple as two linked processes, one of those two processes will constrain the other.  It’s just the nature of how things work.  If a systems constraint did not exist, then the system should, at least theoretically, be able to produce at infinite capacity.  But infinite capacity is not a level that is ever achieved from a system.  All systems are restricted, at some point in time, by some type of output limitation.  This limitation is usually determined by the presence of some kind of system-capacity limit.  No matter how good the system is, there is still only so much it can do.  Sooner or later whatever kind of system is being analyzed, it is will reach its maximum system capacity and be unable to produce more.  If higher system outputs are required beyond the current capacity, then the system must be changed.

In my next post we will discuss the impact of variation.

Bob Sproull

The New Beginning

It's been a while since I last posted an update for a number of reasons, but I plan to change this going forward.  I have been working on a sequel to my last book, The Secret to Maximizing Profitability, which I will be sending to my publisher in the next several months.  For those of you who have read The Secret to Maximizing Profitability, you will remember how Tom Mahanan was able to transform his company, Tires for All, into a highly profitable company.  As a result of his work, Tom was promoted to a permanent seat on his Board of Director's board with one caveat.  He had to continue his current improvement efforts on other portfolio companies owned by the Board of Directors.  He accepts this new role and immediately begins his new improvement efforts.  I will keep everyone posted on the status of this book.  The title of this new book will be entitled, The New Beginning, and as I mentioned, it will be a sequel to The Secret to Maximizing Profitability.

Going forward with my blog, I will be posting on a more regular basis, so please bear with me.  And if you have any subjects you'd like me to write about, please respond to my blog and let me know the subject matter you're interested in me writing about.  I have actually missed posting on a more regular basis, but I have been busy with other things in my life, both personal and professional "things" that stole time away from my blog writing.

Thanks for your patience everyone,

Bob Sproull