Drum Buffer Rope Part 3

In my last post we began our discussion on managing variation especially as it applies to Drum Buffer Rope. In this post we will continue our discussion by continuing on with Goldratt's 5 Focusing Steps.  You will recall that Goldratt's first step tells us to identify our 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.

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.