Drum Buffer Rope Part 4

In this post we continue with our discussion of TOC's scheduling system known as Drum Buffer Rope, focusing on the key elements of buffers.

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.  Figure 1 defines the DBR steps and integration.

 

 Figure 1. DBR Steps and Flow.

But wait!  With a synchronized flow, and actively implementing system subordination, there is a very high probability that the performance metric of efficiency will deteriorate quickly, at least for some period of time.  It will manifest an unacceptable efficiency performance metric that is considered undesirable by most companies.  The new mantra will be to “stop the synchronization nonsense and improve the efficiency.”  Be careful what you consider to be nonsense.  In this case, the real nonsense is the efficiency metric.  When the synchronized flow is implemented, then excess capacity at non-constraints will be quickly exposed, at least for some period of time.  Based on the efficiency metrics it will appear that everything is falling apart, and you are headed in the wrong direction.  But through time, the new system reality and thinking will expose new evidence about what is actually happening in the system.  The new reality is this:

·       [PJ2] Throughput rates will increase.

·       Lead times through the system will be reduced.

·       Work-in-process inventory will go down.

·       On-time delivery will improve.

If you consider these results to be nonsense, then think of the possible consequences if you return to the “high efficiency” metric:

·       [PJ3] Throughput rates will decrease.

·       Lead times through the system will become longer.

·       Work-in process inventory will go up.

·       On-time delivery will go down.

So think carefully when answering the question about which one of these methods is really the nonsensical approach.