Simulation Modeling of RotoBreak
Cassette Assembly Line
By
Randy Greenberg
12/7/99
Dr. Ernesto Gutierrez-Miravete
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Abstract:
This paper are the results from a sumulation run for a new cassette assembly line for
the RotoBreak molded case circuit breaker line. The assembly line flow was analyzed and
recommendations were made to balance the line to have at least 75% utilization at all
locations and meet the weekly production goal of 1000 units run on one shift. The student
version of ProModel limited the amount of locations that could be done in the model so
multiple locations were combined in order to get the simulation to run. Future
recommendations for this simulation would be to obtain a full copy of ProModel to increase
the number of locations. In addition, the model needs to be validated with actual data and this
can be done in Feb 15, 2000, when the assembly fixtures are done.
Introduction:
As part of General Electric’s New Product Introduction (NPI) and Quick Market
Introduction (QMI) and add to that Six Sigma, we are always developing new and better ways
of serving the customer. Much of the time is spent focusing on the external customer, that
customer who purchases the end product. However, little emphasis is put on the internal
customer, the manufacturing sites that make the products. With increased pressure from
global competitors, there is more and more pressure being put on controlling costs internally.
Although the whole Six Sigma initiative is to better serve the customer, manufacturing
process optimization is usually ignored and a ‘build and bust’ mentality is used. Typically it
is much easier and more cost effective to develop an optimal way for manufacturing on a new
assembly line as opposed to an existing line for the very reason that it is often hard, although
not impossible, to disrupt production in an existing line.
There is a new circuit breaker development going on, called RotoBreak, which could
benefit from the analysis of the manufacturing line using simulation since very little effort is
being put into manufacturing. As has happened many times in the past, several months have
been spent trying to debug an assembly line because of the lack of effort put in optimizing it
to any extent. The last time an in-depth analysis was done was approximately 4 years ago
when I helped simulate a sheet metal manufacturing upgrade in one of our plants. The results
of the simulation were verified to approximately 90% of what was observed when the lines
were set up and actually run. In this paper I will discuss the process I used for the simulation
as well as the results.
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Problem formulation:
The purpose of this analysis is to optimize the manufacturing layout for the RotoBreak
molded case circuit breaker being developed and ensure that the line is balanced so that the
objective of 1000 units per week can be met. The discrete event software package, ProModel
will be used to evaluate the effectiveness of the existing concept layout as well as any
proposed modifications to the assembly layout. One layout will be made to evaluate the
layout for several different assembly lines located throughout the world. There is one planned
for Puerto Rico, one planned for Europe with the current plan to be made in Poland and one
planned for Asia, which at this time is planned for Singapore. Each one of these locations is
responsible for manufacturing product at the weekly output of 1000 units.
The simulation developed here will also serve as a basis for future simulation for the
rest of the components inside the circuit breaker and will also serve as a reintroduction to the
effectiveness of the simulation tool itself.
Objectives:
I will discuss the key objectives and requirements of this project below.
1.
The overall goal of manufacturing is to be able to produce 1000 completed
cassette assemblies a week. This applies to each of the three locations: Singapore, Poland and
Puerto Rico.
2.
Although there is no prescribed, ‘right answer’, there is a requirement to use
common processes and procedures that are available at all three locations. I.e. what is done
say in Puerto Rico must be doable in Singapore and Europe as well. Manufacturing requested
this since they want to have common tools and processes so that if there is a process
improvement or design change, they can quickly and easily apply it to each one of the
locations, after initially qualifying it in one location. If they choose to go to separate type
processes for each location, then it will require that an analysis be done at each location once
a change is made to see its impact. This would require more resources and could leave the
door open for mistakes.
3.
The interchangeability of assemblies must be common. Although this mainly
deals with the design of the product, it also applies to manufacturing. Changes at one location
must be easily adapted to the other locations in the event of parts shortages at one of the key
locations
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4.
The line must be able to produce the prescribed quantity of units on a single shift,
40-hour week. Because second shifts and overtime are more expensive, in order to keep
production costs as low as possible, it is required to keep to the 40-hour week. Above 40
hours, at least in Puerto Rico and the Singapore locations, the second shift costs
approximately 25% more to run than the first shift
5.
Inventory should be kept to a minimum so as to minimize carrying costs.
Although it varies based on component to component, the basic requirement is to not carry
more than one weeks worth of inventory at a given time. This also requires that the
simulation be run with zero inventories at the beginning of the week and end up with zero
inventories at the end.
6.
Raw material parts shipments for parts manufactured internally will occur in
weekly quantities at the beginning of each workday. Normally these would be factored into
the simulation, but because of the limitations of the student version of the software, these are
viewed as shipments. The molded cassettes however will arrive daily.
7.
Employee/location utilization is expected to be above 75%. Anything below this
is considered to be inefficient. Although it would be desired for 100% employee utilization,
the reality is that the assumptions made in cycle times and delivery and annual productions
are about 75% accurate, due to fluctuations in demand that vary +/-20%.
Evaluation of effectiveness:
Several key criteria must be met to evaluate the effectiveness of one simulation over
the next. These are:
1.
Meeting the 75% or better employee utilization per location
2.
Meeting the 1000 finished units per week output with zero inventory of work in
process.
3.
Maintaining a single shift, 40-hour week.
Limitations on the layout alternatives are as follows:
1.
No new processes. This would require training and development and these skills
are not currently available. Some improvements may be made in the stations such as
modifications to welding processes and staking fixtures.
2.
The resource assigned to the particular machine must be with the machine for the
duration of the cycle, since the processes are labor intensive
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3.
Space is assumed to be limited to 2000 sq ft for this part of the assembly process.
Several machines can be utilized for each process with the only limitation on the amount of
floor space. The floor space issues will be resolved with the rest of the manufacturing layout.
Buffer size is unlimited for the weekly runs since the part size is small enough that 1000 parts
could easily fit inside a tub and this would be in sync with the arrivals of parts made inside
and outside which arrive weekly. (Except the cassette which is daily)
4.
As mentioned before, % Utilization must be above 75% for any one of the
locations and the goal of meeting 1000 units per week must be met within a 40 hour week
5.
For this analysis, cycle time data will be used based on existing, like processes.
Although no real information exists on distributions, the triangular distribution will be
assumed for all processes with the upper end of the distribution being more spread out for the
only reason that variations due to operators not correctly assembling the parts or adjustments
that need to be made are not that uncommon.
It appears that because this assembly line uses locations, discrete time simulation
would be appropriate here. Each location has a finite processes time and each location runs
independently from one to the next with only requirement that demand from one location be
used to feed the next location. It is for this reason that ProModel will be used here to do the
simulation.
Schedule:
Because of an aggressive implementation schedule, the following schedule has been
put together. The key deliverables are a working model with recommendations by 12/7/99.
The validation of the model will occur when the assembly fixtures and manufactured parts are
available and will begin on 2/15/00. Further enhancements to the model will be made on
3/15/00 based on the output of the model with a finalized recommendation to be presented on
4/15/99.
Model conceptualization:
Component description:
The product being analyzed here is a cassette assembly for a molded case circuit
breaker. This is one of the four key components of the circuit breaker. This component is
responsible for making and breaking the circuit. It consists of a two-piece plastic housing,
called the cassette. To allow for current to flow through, there is a conductor path. The
conductor path consists of two straps with silver contacts welded to them. The part that
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makes and breaks the circuit is the contact arm. It, too, has contacts brazed to it. During the
normal operation of the breaker, there is wear on these contacts. Therefore, a spring
adjustment system is used to compensate for the wear of the contacts. This is the rotor
assembly, which houses the contacts and springs. In addition to providing the compensation
for wear, this rotor also rotates the contact arm on and off to close and open the circuit. The
completed rotor assembly consists of springs and pins and stamped parts. The rotor is put
inside cassette housing and is positioned between the two straps. The cassette housing is a
thermoset-molded housing. Because it is compression molded, it requires a secondary
operation called deflash to remove the flash from edges of the part. The cassette maintains
the proper distances for the parts and provides the structural integrity required for the proper
operation of the circuit breaker. Another key component of the circuit breaker is the arc chute
assembly. This consists of a set of stacked steel plates with a gap between each one. To
maintain the gap between the plates, there is a molded arc chute side that is mounted to the
plates. This is used to quench the arc during interruption of the circuit.
Layout development:
The line is considered to be a flow line as opposed to a job shop. This decision was
based on fact that the units from day to day manufactured on this particular line are the same.
Because of this, the locations will be dedicated locations to this assembly and will not require
changeover times to work on one part to the next. This is in line with other similar breakers that
are made at other facilities.
The simplified conceptual layout is show in figure 1. It consists of the following key
work areas:
1. Welding
2. Staking
3. Molding
4. Arc chute assembly
5. Assembly
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Figure 1-Conceptual layout
Because of the limitations of the simulation package, that limits the total number of
locations to 20, each location in the simulation is actually a combination of several different
locations. In the description of each stage of the assembly process discussed below, I will
describe all the steps that are involved in the assembly line as well as what I am able to
simulate. Times for each location were based on standards that were obtained from
manufacturing processes using similar type operations. As discussed before, a triangular
distribution was assumed. No real data exists for breakdowns, since most of these processes
are well defined and that breakdowns are not that common so they were eliminated from the
simulation.
Welding area:
The welding are consists of two welding locations. The line and load strap welder
location and the contact arm welder location. Both use similar processes.
1. Line and load strap welder:
At the line and load strap welder location, the two key parts, the straps and the
contacts come from other processes. The silver contacts come in from an outside vendor in
weekly quantities. The straps are made inside the plant and are delivered to the welding
station in weekly quantities. The contacts are brazed to the straps via resistance welding. The
contacts are placed inside a fixture mounted on the welder and the strap is placed on top. The
machine is cycled and the part is removed. The part is then put on the conveyor to go to the
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next station, which is the strap to cassette location where the straps are placed inside the
breaker. The assumed cycle time is triangular (59,63,72), all in seconds.
2.
Contact arm welder:
At the contact arm welder, the two key parts are the contact arms and the contacts. As
before the contacts come from an outside vendor in weekly quantities and the contact arms are
made inside the plant and are delivered in weekly quantities. The contacts are brazed to the
straps via resistance welding. The contacts are placed inside a fixture mounted on the welder
and the arm is placed on top. The machine is cycled and the part is removed. The part is then
put on the conveyor to go to the next station, which is the secondary link to contact arm staker
location where a linkage is staked to it. The assumed cycle time is triangular (95,110,130), all
in seconds.
Arc chute assembly
1.
Arc chute assy:
This is where the arc chute is assembled. The key components here are the arc chute
plates and the arc chute sides. The process is set up that both the arc chute sides and the arc
plates arrive from inside the plant on a weekly basis. The arc chute sides are placed on the arc
chute fixture, one on each side. Then 7 plates are loaded into the slide in the proper
orientation and the machine is cycled. The arc chute is then removed and the cycle is started
over. Once the arc chute assembly is complete, it is put on the conveyor where it goes to the
arc chute insulation and rotor to cassette location. The assumed cycle time is triangular
(57,64,75), all in seconds.
Molding
1.
Cassette deflasher location
This is where the cassette is deflashed. The key component here is the cassette that is
deflashed as well as the ablative inserts that are installed after deflashing it. For the
simulation the installation of the ablative parts was eliminated and was included in the deflash
time. The normal process is that the cassettes are run inside the plant on a daily basis since
the molding time is long. The parts come off the molding machine where they are transferred
to the deflash location daily. Then two pieces of ablative material are installed and they are
sent to the strap to cassette location where parts are assembled inside it. The assumed cycle
time is triangular (310,345,400), all in seconds.
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Staking area
1.
Second link to contact arm staker
This station is simplified for the simulation. The key components here are the contact
arm, the pin and links. Normally two operations happen here. The first is that a pin is staked
to a link. The pins come in on a weekly basis and the link is manufactured in house. It then
goes on a conveyor to where the second link is staked to the pin and contact arm. This next
location is what is referred to as the contact arm to link assembly location. This is where the
first assembly of link and pin is slide through a hole on the contact arm and then another link
is put on the opposite end of the pin. This is then staked as well and the resulting assembly
goes to rotor assembly location. For this simulation the input is a pin and rotor from the
brazing station and the output is a rotor, which goes to the rotor assy location. The assumed
cycle time is triangular (40,50,61), all in seconds.
Assembly
Normally there are several stations here as well, but because of the limitations on the
model, we will simplify it to three locations: the rotor assembly, the strap to cassette and the
arc chute insulation and rotor to cassette. I will discuss the operations that happen at each
station
1.
Rotor assembly
The key components here are rotor pins, springs, rotor, contact arm assembly and
center pin. Normally, the pins and springs are purchased and the remaining components are
manufactured in the plant. For the simulation, the simplified component is the rotor. At this
location, the rotor pins, springs, rotor, contact arm assembly and center pin are put on a
fixture and assembled by stretching the springs over the pins. The resulting assembly is then
placed on the conveyor where it goes to the arc chute insulation and rotor to cassette location,
where it is assembled in the cassette along with the insulation at the strap to cassette location.
The assumed cycle time is triangular (65,71,82), all in seconds.
2.
Strap to cassette
The key components here are the cassettes and strap assembly. One cassette half is
placed on an assembly fixture and two straps are placed inside the cassette. They are pressed
into the cassette by the second cassette put over then. This assembly is then put on the
conveyor to the arc chute insulation and rotor to cassette location where it meets up with the
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rotor, and arc chute and some other miscellaneous parts. The assumed cycle time is triangular
(67,71,90), all in seconds.
3.
Arc chute insulation and rotor to cassette
This is the final assembly station where the remaining parts are installed and the
cassette is riveted together. The key components for this are the rotor assembly, the cassette
halves with straps installed, insulation disks and rivets. For this simulation, the rivets and
insulation disks have been eliminated. The process goes by where one insulation disk is put
in the cassette and the rotor is then put over that. Two arc chutes are installed and the second
insulation disk is put on the other side of the rotor. The second half of the cassette is put over
the assembly. Then 9 rivets are put in the rivet holes and the part is put in a press where the
rivets are staked over. This completes the assembly. The assumed cycle time is triangular
(95,105,110), all in seconds. The resulting model is in Appendix A.
Results, verifications and validation:
To verify the model was correctly built, I first started simple and then started adding
logic such as more locations and more complicated times. I started with the processing times
equal as well as the quantities of parts going through the system equal. To ensure the model
was operating properly when I added more complex logic to it, I increased the time by 1000%
at one station just to ensure that flow of the model was working. Once I was comfortable, I
ran the simulation with the initial cycle times. Complete results are in Appendix A. As part
of analysis of the output, I looked at three things. One was if any location was blocked,
indicating a backup of parts. Another thing was percent utilization to see if one location was
being used 100%. If it was, then it most likely could not supply the proper number of parts to
the remaining locations. The last factor I considered was final output to see if I was meeting
the goal of 1000 units. 10 replications were run to evaluate the variation of the model. The
results are discussed below.
When the initial layout was debugged and run, the output was approximately 383 units
in a 37.5 hour run (a 40 hour run with ½ hour break for each day). When I examined percent
utilization, I determined that the cassette deflasher location was the bottleneck since buffer
size was unlimited and this station was the only one where it was being utilized 100%. Being
that this had limited output, it also would affect the processes outputs and utilizations
downstream from it. I increased the number of cassette deflashing stations from one to two
and evaluated the results in the next run. This initial result is shown in figure 2.
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Percent utilization-inital run
Cassette deflasher station
location
Arc chute insulation and rotor to cassette
Strap to cassette
Rotor assy
Second link to contact arm staker
Arc chute assy
contact arm welder
Line and load strap welder
0
20
40
60
80
100
% util.
Figure 2-Initial run results
The results from the second run were that the output doubled to 766 units. Again
though, the cassette deflasher location was the bottleneck and this was then increased to 3
stations. This result is shown in figure 3.
Percent utilization-first modification
Cassette deflasher station
location
Arc chute insulation and rotor to cassette
Strap to cassette
Rotor assy
Second link to contact arm staker
Arc chute assy
contact arm welder
Line and load strap welder
0
20
40
60
80
% util.
Figure 3-first modification
The result from this third run of the simulation was that the output increased to the
1000 units. This is shown in figure 4.
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100
Percent utilization-second modification
Cassette deflasher station
location
Arc chute insulation and rotor to cassette
Strap to cassette
Rotor assy
Second link to contact arm staker
Arc chute assy
contact arm welder
Line and load strap welder
0
20
40
60
80
% util.
Figure 4 –Second modification
The only issue is that the strap to cassette location was under utilized at only 60%
utilization. My recommendations is to take the ablative material that is inserted at the cassette
deflasher location and possibly have the operator install the ablative material in the cassette at
the strap to cassette location. The average time to put the ablative material in is
approximately 10 seconds per cassette so this results in 20 seconds. By doing this, it should
be able to bring the utilization up to around 75%-80%. I did this running an extra simulation
where made these modifications. These results are shown in figure 5.
Percent utilization-third modification
Cassette deflasher station
Arc chute insulation and rotor to cassette
location
Strap to cassette
Rotor assy
Second link to contact arm staker
Arc chute assy
contact arm welder
Line and load strap welder
0
20
40
60
80
100
% util.
Figure 5-Line balance results
This helped better balance the system to the requirement for utilization of above 75%
for the various stations. In addition, I would combine the second link to contact arm staker
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100
location to the rotor assembly location. This would also help bring the utilization above the
75% for these two locations as well.
Future Steps
The next step of the simulation is to validate the assumptions that were made.
Unfortunately due to time constraints, I will not be able to do this at this point, but I will at
least outline the next steps for validation stage of this project. The model seems to indicate a
fairly balanced line. To start with, a mini production line should be created to evaluate
process flow per these recommendations. This line could start out as distinct locations, which
would at least validate the cycle times for the equipment. As was stated earlier, the times
selected for the processes were obtained from similar processes that are done in the past.
With this, I believe these times should be close. This validation step could be done quite
easily and can be done during the planned date of 2/15/00. Note that it is a good idea to
revalidate this system anyways at this time since modifications to the design could be done
that could affect assembly times. This will require coordination with the design team to
ensure that drastic design changes are not reflected in the simulation. I would also look into
getting the full version of ProModel since several locations were combined due to the
limitations of the student version.
The final step in the analysis is to take the recommendations after the validation
analysis and rerun the model with any enhancements required based on it. After all this is
done, then the line could then be implemented.
Final Conclusion:
This analysis was done to simulate a new cassette assembly line for the RotoBreak
circuit breaker development program. The goal was to take the assumptions made to meet the
1000 unit production and to validate them or to make the necessary recommendations. There
were three main recommendations that came out of this simulation to meet the requirements
of the simulation. The first recommendation was that there should be 3 cassette deflashing
locations. The second recommendation is that the deflash location which included both the
deflashing as well as the installation of the ablative material be broken up and that the
installation of the ablative material be done at the same location as where the straps are
installed. In addition, the same operator could be used for both the staking location as well as
the rotor assembly locations.
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The next step in the simulation is to validate the model by building a mini assembly
line to confirm the location cycle times and make the necessary modifications to the
simulation. I would look at getting a full version of this program to complete the analysis
since several locations were combined into one so that the program could run and this does
not deliver the full benefits of running the simulation could be realized for each of the
simulation. Overall I thought the results from this simulation were beneficial and help stress
the importance of using such a tool.
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Appendix A-Setup and output files
a.) Initial case
b.) First modification
c.) Second modification
d.) Final modification
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