The figure below is a simple four-step diagram of a
manufacturing process used to produce some kind of product. Based upon what you
have learned from seeing the piping system, ask yourself which step is limiting
the production of this product through this process and why it is the limiting process
step. Let’s look at some different scenarios to help answer this question in
more depth.
The first question we should ask is, based upon the
processing times of each step, how long does it take to process a single part
through this process? For the first part of this process, the processing time
for one part would be the sum total of each of the individual processing times
as follows:
7
Days +14 Days + 21Days + 7 Days = 49 Days
The next question to answer is, once the production
line is full, what is the output rate of this simple process? The answer to
this question is that because Step C, at 21 days, limits the output rate, then
the rate of this process, as it currently exists, is one part every 21 days. The
figure below summarizes this process with the constraint highlighted. Let’s now
look at some additional scenarios that will have an impact on this
manufacturing process.
Suppose that Step A has problems and goes down for 7
days, what would happen to the output rate of this process? The simple answer
is, nothing changes because it only takes 7 days to complete, so there are
still 7 days of buffer time left over to supply Step B. Now suppose Step B goes
down for 7 days. Again, nothing changes, because it only takes 14 days to
complete, so it should still be able to supply Step C in time before it is
starved. Finally, if Step D goes down for 7 days, throughput remains the same
because it has a time buffer of 14 days due to Step C’s extended processing
time.
Unfortunately, if Step C goes down for 7 days, you
will have lost 7 days of throughput that is lost forever! Now let’s look at how
to increase the throughput of this process.
If you are able to reduce the processing time on Step A from 7 days to 4
days, what would happen to the output of the process? The simple fact is that
if you reduce the processing time on Step A from 7 days to 4 days, throughput
remains the same because of Step C’s longer processing time. Likewise, if you
reduce the processing time on Step B from 14 days to 7 days, what happens to
the output of this process? If you reduce the processing time on Step B from 14
days to 7 days, no throughput improvement will occur, again because of Step C’s
longer processing time. If you reduce the processing time on Step D from 7 days
to 4 days, what happens to the output of the process?
Just like the other examples, if you reduce the
processing time on Step D from 7 days to 4 days, not much happens, again
because of Step C’s longer processing time. So, based on all of this, what is
the only way to increase the throughput of this process? The simple answer to
this question is that if you want to increase the throughput of this process,
you must focus all improvements on the constraint operation and reduce its
processing time!
For example, what happens to the throughput of this
process if you reduce the processing time in the constraint from 21 days to 18
days as in the figure below? The immediate effect of this time reduction is
that you improve the throughput of the process from one part every 21 Days to one
part every 18 Days, or a 17 percent increase! Because of the impact of your
constraining step on the output, doesn’t it make sense to focus most of your
improvement efforts on the constraint? Exceptions to this would be if there are
quality issues causing scrap or excessive rework with Step D or prolonged
delays within Step B. This, of course, assumes that the demand for your product
is high enough to be able to sell the additional product.
The next, most obvious question you might ask is, how do you reduce the processing
time in the constraint? The answer to this question is the essence of this
book. By reducing waste (through Lean tools and techniques) and variation
(through Six Sigma tools and techniques), focusing primarily on the system
constraint. In other words, by doing things like off-loading work from the
constraint to non-constraints, or by eliminating scrap or rework conditions in
the constraint and non-constraint process steps after the constraint. The key
factor to remember is that if you want to maximize the output of your
manufacturing process, you should never allow the constraining operation to sit
idle because every minute lost on your constraining operation is lost forever!
In my next post, we will discuss a different subject.
Bob Sproull
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