drum-buffer-rope
TRANSCRIPT
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Drum-Buffer-Rope: Using Theory of Constraints in Workflow and Facility Design
Several years ago, I was called into a
client’s production facility which was
experiencing a major production problem.
Turn-Around-Time (TAT) had increased to
over 100% of what had been guaranteed to
their customers. Even with the addition of
25% more printing capacity, daily
production had not increased to utilize this
added potential.
As I began to analyze the situation, I
realized the extent, as well as the simplicity
of the problem. The difficulty was not with the efficiency of the printing
or binding equipment – although those efficiencies were not as high as
could be expected. They were not low enough to cause the drastic
increase in TAT, and the low improvement in production.
The identified problem was simply this: Work-In-Process inventory was
extremely high.
The impact of WIP on throughput and TAT
Eli Goldratt, in his book “The Goal,” describes the impact of WIP
inventory on throughput. His methodology, called the Theory of
Constraints (TOC) reduces the effect of WIP on overall system
performance.
Solving the Problem – Workflow
Solving the problem requires use of what Goldratt terms “Drum – Buffer
– Rope.” In this method, an operation is selected to control the tempo of
work flowing through the production system (the “drum”). In Godratt’s
methodology, this controlling operation is the constraint, or bottleneck of the production system. Since the output
of the production system cannot be faster than its slowest member, it makes sense for this operation to be the
drum.
One of the key concepts in TOC is that the constraint should never be idle. In other words, the constraint should
never run out of work. This makes sense since if the production system can only yield the amount that the
constraint produces, we must maximize its production.
Case Study
THE CLIENT
A large digital book printer
THE CHALLENGE
Turn-around-time exceeding customer
commitments
THE SOLUTION
•Identify the bottleneck in the
manufacturing system
•Determine a strategy for limiting WIP
in the system
THE RESULTS
200% decrease in TAT; 50% increase
in throughput
Craig Paxson
www.newsalemconsulting.com
Creating Order at the Edge of Chaos
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Drum-Buffer-Rope
But how can we ensure the constraint never runs out of work to process? This is the concept of
the “buffer”. The buffer is a predetermined amount of WIP. If that amount is chosen carefully, the
constraint is protected from being idle, while not increasing TAT.
The rope is the mechanism that pulls new work into the system. Work should be pulled into the
system at the same rate that it leaves, with small adjustments as necessary. .
Workflow Applications
The above discussion seems like common sense. But how is it applied? Looking at most multi-
step operations, there are piles of WIP everywhere - WIP between the printers and the binders;
between the inserters and the mailers. Even more devastating than the WIP is that the amount is
uncontrolled. It is simply a result of the mismatch in output of the preceding and succeeding
operations. Figure 1 showing the client’s workflow illustrates this situation.
Figure 1 – Print Shop Flow without Buffers
The first step in applying TOC to the situation is to determine where and how large the WIP buffers should be.
Although the preferred method is to put the buffer in front of the constraint, we may need more than one buffer; or
Craig Paxson
www.newsalemconsulting.com
Creating Order at the Edge of Chaos
3
Drum-Buffer-Rope
the constraint may not be an appropriate place to insert a buffer. An analysis of the production
system (routes that parts take through the system, the production speed of equipment, how
equipment is manned, setup times, etc.) is necessary to determine where the buffers need to be.
This analysis will also determine the size of the buffers.
Workflow Redesign
In this case, two buffers were created - one prior to perfect binding, and one prior to case-binding
(reference Figure 2). These were the appropriate places for these buffers because multiple pieces
of work were put together at those points. This made it possible for the production personnel to
determine easily which pieces were missing and needed to be run. The size of the buffers also
allowed production personnel to determine where to put their people. If the buffer is too large, more
resources must be assigned to the downstream processes (overtime, people, etc.). If the buffer is
too small, more resources must be assigned to the upstream resources (i.e. running the material
for that buffer in upstream processes for example).
Figure 2 - Print Shop Flow with Buffers
Craig Paxson
www.newsalemconsulting.com
Creating Order at the Edge of Chaos
4
Drum-Buffer-Rope
One of the keys in using this workflow is to only have WIP in the buffers. Between the binder and
cutter, for example, there should be only a small amount of WIP. This makes the production line
behave like one continuous machine, reducing turnaround time. We use the methods proposed in
“Factory Physics” to determine the proper size for the buffers.
Results
In this case, limiting WIP storage to the buffers and the amount of WIP in the system decreased
TAT by over 200% and increased throughput by over 50%, with no additional manpower or
machinery required.
Facility Design
Generally, facilities are designed to “shoehorn” the most equipment possible, or to minimize the
travel of parts or people. As we have seen from our discussion above, we also need to provide
space for WIP buffers; and the facility should discourage the accumulation of WIP anyplace other
than the designated buffers.
Summary
Workflow and facility design should take into account the Theory of Constraints. Analysis must be
done to determine the location and size of WIP buffers and facilities must be designed with these
buffers in mind.
For more information on using Theory of Constraints in your operation, contact V3Synergy Consulting.
V3Synergy Consulting specializes in helping small and digital printers to improve profitability through the
application of workflow process improvement and waste management.
Craig Paxson
www.newsalemconsulting.com
Creating Order at the Edge of Chaos