Drum Buffer Rope Part 5

In this post we will discuss different types of constraints as they relate to Drum Buffer Rope, These constraints are basically internal and external constraints.

Different Types of Constraints

Constraints can exist in one of two different types.  The first type is the internal constraint— which means that the market demand for your product is higher than the capacity of the system to produce it.  Customers want much more of what you have to  offer them, but you lack the capacity to produce more.  It’s a good situation to be in, but only up to a certain point.  If you can’t figure out a way to meet market demand, then your competitors will usually figure out a way to do it for you.  In other words, they will search for other sources.  This situation is ideal for implementing traditional DBR to meet the demand and capture more market.

The second type of constraint is an external constraint.  In this case the market demand is less than your ability to produce.  The market is buying less, and in some cases much less than you can produce.  This is a less desirable situation, but one that nonetheless can exist.  This situation usually means that there is not an internal constraint to contend with. If this is the case, then it is somewhat improbable that traditional DBR will provide an acceptable solution to this conundrum.  Instead, in this situation, a modified or simplified form of DBR might be more practical.  Consider S-DBR.

Drum Buffer Rope Variants

Simplified Drum-Buffer-Rope (S-DBR)

The concept of S-DBR was developed by H.W. Dettmer and E. Schragenheim is defined in their book Manufactruing at Warp Speed¹.  The S-DBR concept assumes that the constraint is external to the system and resides in the market segment.  Customers aren’t buying as much product as you can make, or there is significant variation in market demand, which can cause the constraint to float back and forth between internal to external locations.  In this situation, the constraint becomes interactive by moving between the market constraint (external) and the production constraint (internal).  This oscillating cycle between internal and external constraints can cause its own brand of chaos in deciding which market segments should be pursued and which ones might be better left alone.  Either way it is a decision that must be dealt with.

In the scenario of an external constraint, the drum is determined and activated only when the system has firm orders in place.  The rope is now determined by the orders that actually exist, which are released based on due dates.  If the orders exceed the capacity of the system, then the constraint has become internal and different actions must be taken.  This also assumes that the internal constraint will exist only for short periods of time and can be overcome by actions like implementing additional shifts or short-term overtime.  Dettmer and Schagenheim have argued, quite successfully, that the market is the true constraint of any system.  There is much more reading available about this concept at Dettmer’s Goal Systems web site.

Multiple Drum-Buffer-Rope (M-DBR)

There is another unique situation that can require the implementation of a third type of Drum Buffer Rope, known as Multiple-Drum-Buffer-Rope (M-DBR). The situation for M-DBR is created when a single buffer location is required to supply products to more than one assembly line, and each assembly has its own drum that is keeping pace at a different rhythm. Figure 2 shows an example of an M-DBR configuration.

 Figure 2.  The M-DBR Model.

An example might be a surface mount technology (SMT) machine that is required to make different types of circuit cards for different assemblies, and each circuit card assembly flows down different assembly lines.  It’s similar to moving from the apex of an “A” plant into the base of a “V” plant, with different parts now required for a number of different “I” lines (see Appendix 8 for discussion on plant types).  The demand (drumbeat) for each “I” line will vary, but each line requires its input from a single SMT machine or a series of SMT machines.

In the configuration of multiple drums, there are also multiple ropes, and each has different requirements.  There are two signal points for the rope.  The first would be back to the buffer to release more work for that particular line.  The second rope goes back to the SMT machine to support the needs of the buffer, which in turn releases work at the front of the SMT line.

The advantage of this concept is to reduce the tendency for economic batch size quantity at the SMT machine.  Many organizations believe when they set up a machine to make parts, they should make as many parts as they can, especially if the machine is expensive, or the setup times seem especially long.  The thought that this economic batch size quantity somehow saves money is, at best, absurd.  The economic batch size only serves to slow down throughput in the system.  No money has been saved at all! In fact, it will cost additional amounts of money because throughput will have been damaged (revenue lost) and the dollars will have been spent to buy those raw materials and parts that aren’t needed yet.  Instead, you should manage the constraint, conduct the setups in the sequence and frequency required from the drumbeat in the lines and solve the problem of shorter setup times as they occur using Single Minute Exchange of Dies (SMED) techniques.  The action of the machine should be to support the buffer for the various “I” line drums, not maintain “high efficiency” at the expense of making money (remember the goal of a company?).