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In
Value Stream-1 we looked at the Mass Production example and
attempted to rationalize the layout into a Lean system.
The summary of that exercise was as follows -
Summary
of Value Stream–
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Mass Production |
Lean System |
| Distance traveled |
214 Metres |
82 Metres |
| Value added time |
0.47 Minutes |
0.67 Minutes |
| Non-value added time |
15,306 Minutes |
5,286 Minutes |
| Value added index |
0.003% |
0.013% |
| Factory lead-time |
10.6 Days |
3.6 Days |
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To
establish the summary, we had to record a detailed step-by-step analysis
of the path through each system.
“Oh
watch out Spike, here comes Ms
PC&L
”.
“What
you did not do last time ‘Spike’
was to look at the information flow and the
customer requirement”.
To
cover these aspects of reviewing the Value Stream let’s examine ANOTHER
JOB IN A DIFFERENT PART OF THE FACTORY.
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STARTING
WITH CUSTOMER DEMAND
Assuming a Customer Demand of 125,000
units / year.
Customer Demand / day = 125,000 / (20
days/mth. x 12 mths.)
Customer Demand / day = 521 PIECES / DAY.
This means that we have to make 521 pieces each and every day
throughout the year to meet the customer demand. In Lean Manufacturing we set out to make that number each day
– we do not set out to make 1,563 pieces one day and then none for the
next couple of days. It is
521 pieces each and every day.
The next question to ask ourselves is, “Do we make all 521
pieces first thing in the morning, or after lunch or before we leave in
the evening”? The answer
is, “We make them all day”.
 “Ok,
I know that we currently operate the factory on a single shift basis
starting at 8:00am and finishing at 4:00pm with 30 minutes for lunch.
Everybody has two; 10 minute breaks during the day.
So that means we have 480 – 30 – (2 x 10) = 430 available
minutes / shift. That is 430
x 60 = 25,800 seconds / shift.”
“Now if we need to make
521 pieces / day and we have a total of 25,800 seconds available / shift
we need to make one piece every…. um……” “Hey ‘BEENIE’, come
over here!!!” |
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“If
we need to make 521 pieces / day and we have 25,800 seconds available to
us we need to make ONE PIECE EVERY 49.52 SECONDS, PRECISELY! This time is called TAKT TIME and
the formula is as follows” –
TAKT TIME = Net Operating Time / Customer Demand
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In
my terms it means -
TAKT
TIME = Available seconds per shift / Number of pieces needed per shift |
| “Not
so fast Ms PC&L.
What you’ve forgotten to tell the folks is that the 125,000 parts
per year that the customer wanted; need not be one single product.
It can comprise of a range of very similar products that use the
same process flow. If it
were a range, the TAKT TIME would apply to each product within the range.
Later on I’ll show you how we group the products and look at
other aspects of TAKT TIMES. Meanwhile
please carry on Ms PC&L.”
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“Spike,
I think we had better work together on this look at another part of the
factory. We need to arrive
at a sketch that provides an overall picture of the existing Value Stream
that includes TAKT Times, Cycle Times and Information Flow.
It may begin to look a bit congested.
Please get the process routings for the assembly being made here
and you, Spike, can sketch out the
process flow of the component parts and final assembly.”
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| “This
is the process flow of the various parts and the final assembly.
Part A is purchased and we drill a 3mm hole in the flange.
Part B and D are pressed parts, while part C is an injection-moulded
part.” |
| “The next question is what are we going to look at? We need to
find the longest path (time wise) through the process.”
If we don’t already know that we will have to check each part
through to the final assembly.”
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“Ok we can see that Part B has
the longest path through the process at 23 days.
But before we sketch this up let’s take a look at the TIME
LINE.”
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“Spike,
why did you draw a TIME LINE?
Well, take a look at the line and you will see that
Part
B
has
the longest path through the process.
When we reduce the time line of Part B through to the dispatch of
the final assembly, we need to know how this will influence the whole job.
For example if we reduce time on the assembly from operation 20
onward, the time line of all parts will reduce.
But if we reduce Part B fabrication by more than 5 days, part D
will then have the longest flow path.
If we forgot about this we will have told Mr Boss we have reduced
the process flow lead-time by “X” amount of time, but in reality we
did not. When Mr Boss realizes
this, I will get the blame, I always get the blame around here!”
Oh no,
Spike, I
think you are clever! “Well, gee
thanks!”
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Spike,
you’ve drawn in the process flow, cycle times and up-time etc. and
I’ve put in the information flow. I
have not put in the delivery times from the supplier and to the customer.
In some cases where long distances are involved these factors
become very important.”
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Well
we have drawn a sketch of the current system and have found out that the Production
Lead Time through the system is 23 days.
We also know that the value added time is 102.6 seconds for the
critical flow, but it is not the total value added time for the
complete assembly. We have
also calculated that the TAKT TIME to satisfy
the customer demand of 125,000 assemblies per year is approximately 50
seconds.
Is
that all we need to know about TAKT TIME?
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“NO!” “When we
looked at the TAKT time we assumed that all the machines were available to
work on the assembly that we needed to make.
In this example, the assembly machines only work on that assembly
but the presses and moulder also work on other products.
Therefore the TAKT time of 50 seconds for the presses and moulder
is not valid. Where machines
are shared with other products or product ranges it is necessary to set-up
a machine matrix and check its specific TAKT Time e.g.
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| Press Number 4801 |
Product A |
105,000 / year |
Cycle time 10 seconds |
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Product B |
100,000 / year |
Cycle time 14 seconds |
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Product C |
110,000 / year |
Cycle time 11 seconds |
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Product D |
120,000 / year |
Cycle time 13 seconds |
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Total |
435,000 / year |
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| Assuming one shift operation as previous calculation, the TAKT TIME would
be
TAKT
TIME = Available seconds per piece / Number of pieces needed per shift
= 25,800 seconds / { 435,000 / [20 x 12] } = 14.23 seconds |
| This means that that press would have to produce a part every
14.23 seconds to meet the customer demand.
From the cycle time shown it would appear that the press could cope
because all parts can be made faster than the TAKT Time.
In practice although some of the parts can run even faster than the
TAKT TIME you would expect to have to run this on part of a second shift
due to UPTIME and CHANGEOVER factors.”
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“OK, we now
realize in this case the presses and moulder do not relate to the TAKT
TIME that we originally calculated.
Ideally we would like to set all the processes to the TAKT TIME but
this is not always possible. So let’s look at the assembly processes
instead.
Remember,
it is always a good idea to start at the end of the process flow and work
backwards when trying to reduce the process flow lead-time.
At the moment all the assembly processes are batched.
Before we decide to flow the job lets study each process.
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| Returning
to “People and Machines” in the first section called “Before
Lean”, we can recall that an operation can comprise of the persons
time and the machine time. We
looked at various examples of combinations of these two elements. We will probably have to add a third element, walk time from
one process to the next if we decide to flow the processes.
Remember the TAKT TIME for the assembly was 49 Seconds. We can
see below that each process cycle is considerably less than the TAKT TIME
so in this example we may not have to improve each process. Changeover
time is not critical for now because the machines only work on this job at
the moment.
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| Can we set this up as a one-piece flow system? In
summary the persons work content is 44 seconds; the machine content
is 35 seconds; walk time is 0 seconds and uptime is 8
seconds. Making a grand
total of 87 seconds. Remember
our TAKT TIME is 49 seconds. The
uptime allowance was calculated from the sub total by applying uptime % to
it.
It
would appear that we have no problem with each process cycle because in
total they do not exceed the TAKT TIME.
In fact they are so low that the processes are obviously idle for
much of the week. The base
machine cycles are even lower. It
would appear we do not need to tamper with them.
The question facing us is can we flow these
processes using just one person. The
total time needed to perform the manual work is 44 seconds, but added to
that would be the walk time from process to process and the effect of the
uptime allowance. If the walk
time is 10 seconds (allowing about 1 second or less per metre) and the
uptime allowance is 8 seconds, the total cycle time would be 62 seconds. This is above the 49 seconds but if we could automate some of
the manual unloads it may be possible to reach the TAKT target. |
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| I think I will join you Spike, it is time for a pint! We will take another look in Value Stream-3.
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