2.875 – Fall 2001
Annabel Flores, James Katzen
2.875 - Fall 2001
Mechanical Assembly and Its Role in Product Development
Term Project: Report #4
DESIGNING ASSEMBLY WORKSTATIONS TO PRODUCE COMPUTER MOUSE
ASSEMBLY
November 21, 2001
Annabel Flores
James Katzen
Photos taken from:
http://www.petergof.com/x-ray/Mouse.htm
http://www.Mousemorf.com/images/Mouse2b.jpg
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Annabel Flores, James Katzen
DESIGNING ASSEMBLY WORKSTATIONS TO PRODUCE COMPUTER MOUSE
ASSEMBLY
Introduction
The Microsoft Mouse Version 2.21 is an ergonomic, dual-button Mouse. The simple, eleven-part
design provides an opportunity to analyze the product’s assembly characteristics. Previous
reports developed for this class have analyzed either individual parts or the interface between a
subset of parts of the mouse. The most recent report consisted of an in-depth analysis of the
assembly of the product. This report will focus on the design of a particular workstation for
assembly. A specific workstation, involved in the assembly of components critical to the
function of the mouse, will be examined in detail. The layout of this station, the movements of
the actuator and support personnel, the placement of subassemblies, and the movement of the
parts through the station will be addressed. In addition, the cost for procurement, and the time
required to complete the needed assembly steps will be estimated.
Our assembly analysis is based on one major assumption; the product is assembled using robot
assembly. The product’s large production volume, as well as the flexibility in adjusting the
assembly process for the scope of the product, led us to believe that robotic assembly would be
the most likely assembly method. In addition, this assumption allowed us to develop our
understanding of the additional complications that surface in assembly using automation.
An earlier report listed the primary steps involved in assembly of the mouse as:
1.
2.
3.
4.
5.
6.
Place Mouse Base (Part #8) onto Primary Fixture.
Place Wheel (Part #10) onto end of Spring (Part #9).
Assemble Wheel and Spring Subassembly with Mouse Base (Part #8).
Assemble Circuit Board (Part #11) with Mouse Base (Part #8).
Attach Plug of Cord (Part #6) to Circuit Board (Part #11)
Route Cord (Part #6) through slot in Mouse Base (Part #8) and secure Strain Relief to
Mouse Base (Part #8).
7. Attach Horizontal Gear (Part #7) to Mouse Base (Part #8).
8. Attach Vertical Gear (Part #7) to Mouse Base (Part #8)
9. Attach Mouse Cover (Part #5) to Mouse Base (Part #8).
10. Invert Assembly and place into Secondary Fixture.
11. Place Ball (Part #2) into Ball Holder (Part #1).
12. Place Ball and Ball Holder Subassembly into Mouse Base (Part #8).
13. Secure Mouse Base (Part #8) and Mouse Cover (Part #5) by inserting and tightening
Screw (Part #4).
14. Attach top Sticker Pad (Part #3).
15. Attach bottom Sticker Pad (Part #3)
This list did not address the material handling needs of the assembly process, or the need to
allocate and balance assembly steps among different stations to meet cycle time targets and
machine complexity and cost constraints. In planning an complete assembly line to produce the
1
Refer to Appendix A and Appendix B, respectively, for Bill of Materials and Exploded View for part naming and
numbering conventions.
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Annabel Flores, James Katzen
Mouse assembly, the above list must now be expanded to include support operations, such as
those involved in the loading of parts into the initial fixture, the testing and inspecting of parts,
subassemblies, and the finished product, and finally, the packing of finished assemblies. It was
decided that these operations would logically be grouped into five workstations: one manual
station, followed by three robotic stations, followed by one final manual station. Using this
allocation, the complete list of the stages required for assembly the mouse is shown below:
 Station 1 (Manual):
o Step 1: Place Mouse Base (Part #8) onto Primary Fixture
o Step 2: Place Wheel (Part #10) onto end of Spring (Part #9)
o Step 3: Assemble Wheel and Spring Subassembly with Mouse Base (Part #8)
o Step 4: Transfer pallet between Station 1 and Station 2
 Station 2 (Automatic):
o Step 5: Locate Pallet on Locating Pins
o Step 6: Assemble Circuit Board (Part #11) with Mouse Base (Part #8).
o Step 7: Attach Plug of Cord (Part #6) to Circuit Board (Part #11) and attach strain
relief of Cord to Mouse Base (Part #8)
o Step 8: Attach Horizontal Gear (Part #7) and Vertical Gear (Part #7) to Mouse
Base (Part #8)
o Step 9: Attach Mouse Cover (Part #5) to Mouse Base (Part #8)
o Step 10: Transfer between Station 2 and Station 3
 Station 3 (Automatic):
o Step 11: Invert Assembly and place into Secondary Fixture.
o Step 12: Transfer between Station 3 and Station 4
 Station 4:
o Step 13: Place Ball (Part #2) into Ball Holder (Part #1)
o Step 14: Place Ball and Ball Holder Subassembly into Mouse Base (Part #8)
o Step 15: Secure Mouse Base (Part #8) and Mouse Cover (Part #5) by inserting
and tightening Screw (Part #4)
o Step 16: Attach top Sticker Pad (Part #3)
o Step 17: Attach bottom Sticker Pad (Part #3)
o Step 18: Plug in Cord’s connector into Test Fixture Connector Block
o Step 19: Functional Test
o Step 20: Remove Cord’s connector from Test Fixture Connector Block
o Step 21: Attach Hologram Sticker
o Step 22: Transfer between Station 4 and Station 5
 Station 5:
o Step 23: Bundle Mouse Assembly with Product Documentation and Software
o Step 24a: Pack into OEM packaging OR
o Step 24b: Pack into aftermarket packaging
The middle assembly station (Station 2) was chosen for in-depth analysis. This workstation was
selected because the assembly stages completed at this station predominantly determine the
proper function of the finished product. Therefore, proper completion of each assembly step of
this station must be achieved, and therefore close analysis must be used in the design and layout
of this station. A preliminary design for this workstation is shown below:
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Annabel Flores, James Katzen
Hopper for
Mouse
Covers
Pick And
Place
Robot
Conveyor Belt
Direction of
Rotation
Direction of Rotation
Gear Hopper
Figure 1: Proposed Layout of Workstation #2
The remainder of this report will focus on the necessary product redesigns to facilitate robotic
assembly and a thorough description of the workstation including material flow and required
motions. In addition, a first attempt is made to estimate the cost of the workstation.
PRODUCT REDESIGNS
In the previous report, we identified a number of assembly problems in the product design. In
order to design a workstation, and consequently an entire product assembly layout, we needed to
identify the product feature redesigns we believe are needed for efficient robot assembly.
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Spring and Wheel
Previously, we recognized that the Spring and Wheel presented the most difficult components to
assembly. We proposed a linear spring design to replace the need for a Spring and Wheel
manual assembly. However, additional investigations are needed to evaluate the feasibility of
the Linear Spring. Until a better design is finalized, the Spring and Wheel subassembly design in
the 1999 model will continue to be used.
Circuit Board and Cord
A number of additional problems were identified in the assembly of the Circuit Board and the
Cord and the Mouse Base. Recall that attaching the Circuit Board to the Mouse Base requires a
number of adjustments, translations, and rotation in various different directions in order to clear
assembly features used by the Mouse Base to mate to the Mouse Cover and to retain the Circuit
Board. Currently, undercuts in the Mouse Base serve to secure the vertical position of the
Circuit Board. However, it is these undercuts that force the reorientation of the Circuit Board.
The decision to assemble the product using robotic assembly leads to a necessary simplification
of the process. Primarily, the Mouse Base can be simplified to eliminate the undercuts as well as
other unnecessary features. The previous report identified a mystery part in the vicinity of the
gears. This mystery part is one of the protrusions that prevent the Circuit Board from simply
dropping into the Mouse Base. Unless the purpose of these features is discovered, they should
be eliminated to simplify the assembly process. In addition, the cutouts on the Circuit Board that
provide clearance between the Circuit Board and Mouse Base should be enlarged to be able to
position and lower the Circuit Board in its nominal position to the Mouse Base.
In the current design, routing the Cord around the side of the Mouse Base from the connector in
the Circuit Board to the front of the Mouse Base and then inserting the strain relief feature would
be difficult to automate given the flexibility of the part. Instead, we will take Professor
Whitney’s suggestion to relocate the Plug from the rear of the Circuit Board to the front as
shown below:
Switches
Reoriented Gear
Relocated
Electronics
New Location
Of Connector Block
Figure 2: Circuit Board Redesign
Shortening the length of the Cord that resides within the mouse facilitates assembly. In order to
clear up the necessary real estate in the front of the Mouse Base assembly, the Gears and Sensors
must be reoriented. The strain relief feature on the Cord should also be redesigned so that
assembly can occur from the vertical direction. Currently, the strain relief feature must be
positioned inside the mouse and then pushed out through the corresponding hole. Using the
suggested redesigns, a short Cord and strain relief could be inserted into the Mouse Base in one
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Annabel Flores, James Katzen
motion. As mentioned in an earlier report, the Plug should be redesigned to prevent the robot
from incorrectly inserting the connection rendering the mouse unusable.
In our original assembly sequence, we assumed the Circuit Board and Cord would enter the
assembly area as a subassembly to allow testing of the Circuit Board and Cord during its
production stage. However, inserting the Circuit Board into the Mouse Base becomes
significantly more challenging if the robot must also control the motion of the Cord during the
same step. The Circuit Board can still be tested after its production stage through the use of a
test cord that mimics the interface between the Circuit Board and the Cord. This allows the two
components to be assembled individually, greatly simplifying the process.
Cover
The Mouse Cover and Mouse Base interface will also need to be redesigned to facilitate robotic
assembly. The current interface design has one function, to fasten the front end of the Mouse
Cover to the Mouse Base. The rear portion of the Mouse Cover is fastened to the Mouse Base
through a single threaded Screw. In an earlier analysis, we mentioned the possibility of
eliminating the Screw from the Assembly, efficiently eliminating the need to invert the product.
However, until an evaluation of the product redesign is completed and all of the product
requirements are met, the removal of the Screw will be postponed. Therefore, the front end of
the Mouse Cover and the Mouse Base interface must still be able to fasten the components
together.
The current design of the Mouse Cover necessitates a complicated assembly process to engage
the Mouse Base fastening features. This process requires a rotation as shown, as well as several
independent translations to properly align and mate assembly features.
Figure 3: Current Mouse Base and Mouse Cover Interface
Ideally, redesigning the interface should allow the Mouse Cover to be assembled onto the Mouse
Base by a single translation along the vertical axis. A viable option is to modify the interface to
include spring tab snap-fit fasteners as shown below:
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Annabel Flores, James Katzen
Figure 4: Spring Tab Redesign for Mouse Cover to Mouse Base Interface
The above product redesigns greatly simplify the assembly steps and motions required to achieve
complete assembly. The remainder of this report will assume that these redesigns have been
incorporated.
ASSEMBLY STEP ANALYSIS
Each assembly step will now be described in detail. For each step, the required operation time
will be calculated. In addition to the time calculations, the in-flow and out-flows of all
assemblies and parts will be discussed. Finally, the required motions of the robot, as well as the
human operators that are needed to load parts, will be choreographed.
Based on recommendations from lecture, it was decided that batch sizes of four components
would be used to produce the Mouse assemblies. This was decided upon to minimize the impact
that tool changes would have on overall cycle time. The drawing below shows the planned pallet
that will be used in the assembly process:
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Cavities
Boss to Fit into
Mouse Base Hole
Figure 5: Proposed Pallet Design That Will Fixture Four Assemblies At Once
Step 5: Locate Pallet on Locating Pins
Step Description
Before assembly steps can commence, it is critical that the pallet containing the prior assembled
parts (Mouse Base, Wheel and Spring) be oriented and located in a fixed and known position.
This requires that all degrees of freedom of the pallet be constrained. It is assumed that
automated conveyors will be used to move the pallet from Station 1 into Station 2. Therefore, as
is typically done in industrial automation, a pneumatically-actuated lift cylinder should be used
to raise the pallet slightly off the conveyor. The upwards movement should be sufficient enough
so the moving belt no longer touches the pallet, and therefore does not act to shake the pallet as
the parts contained are being worked on. The air cylinder should act to push the pallet up against
alignment features, and these locating pins should be used to orient the pallet. A typical pin-hole
and pin-slot combination could be used to constrain the pallet in all directions, while avoiding an
over-constrained condition.
In-flow and out-flows of all assemblies and parts
A pallet, containing four sets of Mouse Base/Wheel/Spring sub-assemblies will enter this step by
being moved along a conveyor belt. Time-controlled gates will hold one pallet on the conveyor
belt above the pallet lift while other time-controlled gates meter the flow of pallets into the
station. The pallet lift will then raise the pallet into position. No other in-flows or out-flows of
assemblies and parts will be present.
Required Motion of Robot
A simple vertical motion of an air cylinder is required in this step. Since the required motion is
quite small, (the clearance between the pallet and the conveyor belt does not need to be any more
than one inch), a small air cylinder, with a stroke of approximately two inches is all that is
needed at this step. Hard stops, rather than the end of the air cylinder’s stroke, should be used to
achieve a precise vertical positioning of the pallet. The air cylinder must be capable of applying
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a resistive force at least equal to the force needed to press the four sets of Horizontal and Vertical
Gears into their position in their respective Mouse Base. This ensures that the pallet will not be
pushed down, which could result in jarring contact with the conveyor belt. This resistive force
could be achieved by using a coil spring.
Required Motion of Human Operators
Since this is an automatic station, no human operators will be required under normal conditions.
However, since the potential for mis-positioned parts and pallets exists, it is important that an
access panel be included to allow a human operator to clear parts and restart the operation.
Electrical interlocks must be included on this access panel, so the assembly station is
immediately shut down when the panel is opened, in order to limit safety risks to the operator.
Time Calculations
As no assembly operations are being conducted in this step, operation time will be relatively
short. However, care must be taken to ensure that the starting acceleration and the finishing
deceleration, as well as the steady state speed of the upwards motion is not so great that it jars the
held parts out of position. If this occurs, there is a risk of damage to the parts, the pallet, the
fixtures, as well as the tooling. The locating of the Pallet on Locating Pins is estimated to take
0.75 - 1.5 seconds. Note that experiments could easily be conducted to determine the minimum
time that can be used to position the parts, while ensuring the parts remain properly seated in the
pallet and fixtures.
Step 6: Assemble Circuit Board (Part #11) with Mouse Base (Part #8).
Step Description
It is assumed that the electronic Circuit Boards will be supplied to the Mouse assembly location
in pallet form. These pallets will contain multiple circuits boards, placed in a specific orientation
and a fixed spacing. In this step, one Circuit Board will be picked up from a pallet of parts and
placed into the Mouse Base. This will be achieved with a standard pick and place robot. The
redesigned assembly features on the Mouse Base will properly locate and constrain the Circuit
Board.
In-flow and out-flows of all assemblies and parts
The prior assembly step will have properly positioned the fixture containing four sets of Mouse
Bases, Wheels, and Springs. There will be no additional movement of these parts during this
operation. Since the Circuit Boards are assumed to be packaged on a pallet, a complete pallet of
Circuit Boards will be automatically moved into place once all Circuit Boards have been
removed from the cavities of one pallet. The moving of the pallets will be achieved by using a
standard pallet feed system. The frequency of pallet cycling will be dependent on the number of
Circuit Boards that are held by each pallet.
Required Motion of Robot
At the beginning of this assembly step, the robot end-effector must select the proper gripper set
from its magazine of tools. Once this has been completed, the robot should move using gross
motions, from its default tool-change position, in the horizontal plane until the gripper is above
the Circuit Board pallet, directly above a Circuit Board in that pallet. A simple counting
algorithm can be utilized to keep track of which cavities of the pallet contain a Circuit Board.
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Gripper Motion Direction
Once in position, the robot should move downward in a gross motion, so that the gripper fingers
(shown below as hatched gray rectangles) would be aligned with the internal edges of the Circuit
Board, as shown in the picture below:
Figure 6: Gripper Finger Placement For Circuit Board
Note that if the workspace if free from obstacles, the two motions can occur simultaneously,
which would save valuable cycle time. The gripper should then be opened, and sufficient
spreading force should be applied to securely hold the Circuit Board, but damage to the delicate
Circuit Board must be avoided. Once the Circuit Board has been gripped, the robot end-effector
should move upward in the vertical axis, and then in the horizontal plane, until the end-effector is
above the Mouse Base. Fine motions should be used for this motion, as there is some risk of
damage to the Circuit Board, even if the robot trajectory is properly planned. The end-effector
should then be moved downward so that the hole and slot in the Circuit Board engage the peg
features of the Mouse Base. Once this motion is complete, the gripper fingers should close,
releasing the Circuit Board. Then, the end-effector can move in a gross motion, in the vertical
axis, as well as in the horizontal plane, so that the gripper fingers are again place at the part
presentation area, this time directly above the next occupied cavity of the Circuit Board pallet.
The new Circuit Board should be gripped and inserted in the second Mouse Base, using the
motions just described for the first set. This operation should then occur two more times, so that
in all, four Circuit Boards have been placed. Upon completion of this, the end-effector should
then be moved in a gross motion back to the tool change location.
Required Motion of Human Operators
Again, since this is an automatic station, little human motion or intervention will be needed
under normal conditions. Human operators will be needed to load pallets of Circuit Boards. The
frequency of human interaction will depend on the number of Circuit Boards that are placed on
each pallet by the Circuit Board supplier. In addition, since the potential for mis-positioned parts
and pallets exists at this station as well, it is important that an access panel be included to allow a
human operator to clear parts and restart the operation. Electrical interlocks must also be
included on this access panel, so the assembly station is immediately shut down when the panel
is opened, in order to limit safety risks to the operator.
Time Calculations
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Annabel Flores, James Katzen
Using the Boothroyd and Dewhurst2 data charts for robotic assembly, we were able to estimate
the cycle time for this assembly step. At this step, the Circuit Board requires a special tool for
handling and assembly. Since the decision was made to assemble four products at the same time,
the time required for tool changeovers can be effectively neglected. Since the Circuit Board is
not easily aligned, but is added and secured immediately using motion along the vertical axis
requiring simple manipulation, the Boothroyd-Dewhurst chart estimates that 3.3 seconds is
required to assemble the Circuit Board and the Mouse Base. Four Circuit Boards inserted into
four Mouse Bases results in a cycle time of 13.2 seconds. Note that this time is based on
Boothroyd and Dewhurst’s empirical studies of typical robotic assembly times, rather than based
on speed capabilities of a particular robot. Since fine motions are suggested (see below), due to
the fragileness of the Circuit Board, the actual assembly time for this step may be greater than
that estimated by Boothroyd and Dewhurst.
Step 7: Attach Plug of Cord (Part #6) to Circuit Board (Part #11) and attach strain relief of
Cord to Mouse Base (Part #8)
Step Description
In this step, two distinct operations will be completed. The first operation is the connection of
the Cord to the Circuit Board. This is achieved by mating the connector block halves on the
Cord and the Circuit Board. The second operation is the attachment of the Cord’s strain relief to
the Mouse Base. Note that due to the product redesigns mentioned at the beginning of this
report, these two operations are now much simpler. The need to route the Cord around the side
of the Mouse Base is eliminated, and the strain relief no longer has to be pushed through a small
hole in the Mouse Base. Both of these operations will be completed simultaneously by using a
two-part gripper, mounted to a pick and place robot.
In-flow and out-flows of all assemblies and parts
The first assembly step in this workstation will have properly positioned the fixture containing
four sets of Mouse Bases, Wheels, and Springs. The prior assembly step will have added the
Circuit Board. Since all these parts are completely constrained, there will be no additional
movement of these parts during this operation. It is assumed that the Cords will be produced by
a subcontractor. Because of this, it is reasonable to assume that the subcontractor can be
required to place the Cords carefully on a pallet, in a fixed orientation. If this is the case, a pick
and place robot can easily pick off one Cord at a time. Since we will be using pre-filled pallets, a
complete pallet of Cords will be automatically moved into place once all Cords have been
removed from the cavities of one pallet. The moving of the pallets will be achieved by using a
standard pallet feed system. The frequency of pallet cycling will be dependent on the number of
Cords that are held by each pallet.
Required Motion of Robot
At the beginning of this assembly step, the robot end-effector must select the proper gripper set
from its magazine of tools. Once this has been completed, the robot should move using gross
motions, from its default tool-change position, in the horizontal plane until the gripper is above
the Cord pallet, directly above a Cord in that pallet. A simple counting algorithm can be utilized
to keep track of which cavities of the pallet actually contain a Cord. Once in position, the robot
should move downward in a gross motion, so that one set of gripper fingers would be aligned
2
Product Design For Assembly, G. Boothroyd & P. Dewhurst, Boothroyd Dewhurst Inc., Wakefield, RI, 1991
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Gripper Motion
Direction
Gripper Motion Direction
with the edges of the Cord’s connector block and one set of fingers would be aligned with the
strain relief, as shown in the picture below:
Figure 7: Gripper Finger Placement For Cord
Note that if the workspace if free from obstacles, the two motions can occur simultaneously,
which would save valuable cycle time. The grippers should then be closed, and sufficient
clamping force should be applied to securely hold the connector and the strain relief, but damage
to the delicate connector must be avoided. Once the parts have been gripped, the robot endeffector should move upward in the vertical axis, and then in the horizontal plane, until the endeffector is above the Mouse Base. Fine motions should be used for this motion, as there is some
risk of damage to the Cord, even if the robot trajectory is properly planned. The end-effector
should then be moved downward so that the connector block halves mate and the strain relief
slides into the Mouse Base. Once this motion is complete, the gripper fingers should open,
releasing the Cord. Then, the end-effector can move in a gross motion, in the vertical axis, as
well as in the horizontal plane, so that the gripper fingers are again place at the part presentation
area, this time directly above the next occupied cavity of the Cord pallet. The new Cord should
be gripped and inserted in the second Mouse Base, using the motions just described for the first
set. This operation should then occur two more times, so that in all, four Cords have been
placed. Upon completion of this, the end-effector should then be moved in a gross motion back
to the tool change location.
Required Motion of Human Operators
Once again, since this is an automatic workstation, little human motion or intervention will be
needed under normal conditions. Human operators will be needed to load pallets of Cords. The
frequency of human interaction will depend on the number of Cords that are placed on each
pallet by the Cord supplier. In addition, since the potential for mis-positioned parts and pallets
exists at this station as well, it is important that an access panel be included to allow a human
operator to clear parts and restart the operation. Electrical interlocks must also be included on
this access panel, so the assembly station is immediately shut down when the panel is opened, in
order to limit safety risks to the operator.
Time Calculations
Using the Boothroyd and Dewhurst3 data charts for robotic assembly, we were able to estimate
the cycle time for this assembly step. At this step, the Cord requires a special tool for handling
and assembly. Since the decision was made to assemble four products at the same time, the time
3
Product Design For Assembly, G. Boothroyd & P. Dewhurst, Boothroyd Dewhurst Inc., Wakefield, RI, 1991
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Annabel Flores, James Katzen
required for tool changeovers can be effectively neglected. Since the Cord is not easily aligned,
but is added and secured immediately using motion along the vertical axis requiring simple
manipulation, the Boothroyd-Dewhurst chart estimates that 3.3 seconds is required to assemble
the Cord and the Circuit Board. Four Cords inserted into four products results in a cycle time of
13.2 seconds. Note that this time is based on Boothroyd and Dewhurst’s empirical studies of
typical robotic assembly times, rather than based on speed capabilities of a particular robot.
Gripper Motion Direction
Gripper Motion Direction
Step 8: Attach Horizontal Gear (Part #7) and Vertical Gear (Part #7) to Mouse Base (Part
#8)
Step Description
In this step, the Horizontal Gear and the Vertical Gear are gripped at the same time by a robot
gripper. The Horizontal Gear and Vertical Gear are carefully oriented. After being gripped, the
Gears are pressed into their respectively assembly features in the Mouse Base. To facilitate
simultaneous insertion of the two Gears, the Gears must be carefully oriented with respect to the
robot gripper and the Mouse Bases held in the pallet. This orientation is shown below:
Final Orientation in Mouse Assembly
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This enables the Gears to be picked up from the part feeder and placed into the Mouse Base
without the need of a rotational orientation (about the vertical axis).
In-flow and out-flows of all assemblies and parts
Since specific orientation of the Gears are required, it is suggested that vibratory bowl feeders
which feed self-aligning tracks be used. A material handler is required to simply dump a large
quantity of Gears into a hopper. One bowl feeder would supply Gears for the Horizontal Gear.
Another identical bowl feeder, rotated at 90 degrees with respect to the first bowl feeder, would
supply Gears for the Vertical Gear. Because the Vertical Gear and the Horizontal Gear are
identical, the hopper can feed each bowl feeder. The self-aligning tracks would allow a small
inventory of parts to be present, in case the vibratory bowl feeder jams. The layout of the bowl
feeders, and the self-aligning tracks is shown below:
Hopper
Direction of Rotation
Direction of
Rotation
Required Motion of Robot
At the beginning of this assembly step, the robot end-effector must select the proper gripper set
from its magazine of tools. Once this has been completed, the robot should move using gross
motions, from its default tool-change position, in the horizontal plane until the gripper is above
the part presentation location. Then, the robot should move downward in a gross motion, so that
the gripper fingers would be aligned with the shafts of the Gears. Note that if the workspace if
free from obstacles, the two motions can occur simultaneously, which would save valuable cycle
time. The gripper should then be closed, and sufficient clamping force should be applied to
securely grip the Gears, but damage to the Gears must be avoided. Once the Gears have been
gripped, the robot end-effector should move upward in the vertical axis, and then in the
horizontal plane, until the end-effector is above the Mouse Base. Gross motions can be used,
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since there is little risk of damage to the Gears, if the robot trajectory is properly planned. The
end-effector should then be moved downward so that the Gear shafts engage the assembly
features of the Mouse Base. At this point, fine motion in the vertical axis should be used to fully
seat each shaft and ensure each snap fit has been completed. Once complete, the gripper fingers
should open, releasing the Gears. Then, the end-effector can move in a gross motion, in the
vertical axis, as well as in the horizontal plane, so that the gripper fingers are again place at the
part presentation area, with the gripper fingers aligned with the shafts of another set of the Gears.
The new set of Gears should be gripped and inserted in the second Mouse Base, using the
motions just described for the first set. This operation should then occur two more times, so that
in all, four sets of Gears have been placed. Upon completion of this, the end-effector should then
be moved in a gross motion back to the tool change location.
Required Motion of Human Operators
There is little required motion of human operators at this assembly step. Humans would only be
needed for two tasks. The first task is the periodic dumping of Gears into the part-feeding
hopper. This is a simple task, as the parts would likely be delivered by the supplier in cardboard
boxes or plastic bags, with the Gears simply randomly piled in the container. The second task,
which would again be periodic, would be the clearing of part jams in the vibratory bowl feeders.
Part jamming is an inherent limitation of bowl feeders, yet the occurrence of jamming can
largely be reduced by adjusting the vibration profile of the feeder. However, as part jams will
never be completely eliminated, occasional human interaction will be required.
Time Calculations
Using the Boothroyd and Dewhurst data charts for robotic assembly, we were able to estimate
the cycle time for this assembly step. At this step, the Gears require a special tool for handling
and assembly. Since the decision was made to assemble four products at the same time, the time
required for tool changeovers can be effectively neglected. Since the two Gears are not easily
aligned, but are added and secured immediately using motion along the vertical axis requiring
simple manipulation, the Boothroyd-Dewhurst chart estimates that 3.3 seconds are required to
assemble each set of gears. One set of Gears inserted into four products results in a cycle time of
13.2 seconds. Note that this time is based on Boothroyd and Dewhurst’s empirical studies of
typical robotic assembly times, rather than based on speed capabilities of a particular robot.
Step 9: Attach Mouse Cover (Part #5) to Mouse Base (Part #8)
Step Description
In this step, one Mouse Cover will be picked up from a table and placed onto the Mouse Base.
This will be achieved with a standard pick and place robot. The redesigned snap-fit assembly
features on the Mouse Base will properly locate and constrain the Mouse Cover. The complex
shape of the Mouse Cover complicates the part presentation at this assembly stage. First, the part
is asymmetrical. Next, protruding snap-fit features cause the parts to hook onto each other,
making separation more difficult. Finally, the rounded edges of the top surface makes it difficult
to positively locate the Mouse Cover in a fixture. Therefore, a more complicated system,
incorporating a vision system, will be needed to position and orient Mouse Covers correctly.
Numerous systems that perform pattern recognition are commercially available, such as the
Adept® version shown below:
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Figure 8: Commercially Available Pick and Place Robot That Performs Pattern
Recognition4
In-flow and out-flows of all assemblies and parts
To address the complexity issue of the Mouse Cover part presentation, the use of a vision system
with pattern recognition and a parts recirculator is suggested. This system would operate in the
following way. First, a hopper of parts would be filled periodically by a human operator. Then,
the hopper would occasionally spill a few parts onto a table. An overhead camera would the
scan the parts on the table, looking for any parts that are oriented with the top of the Mouse
Cover facing up, and any of those parts that are not touching other parts or edges of the table. If
any of these parts are present, the position and orientation of an acceptable part is sent to the
control strategy for the pick and place robot. The robot will then move to that location, rotate its
gripper about the vertical axis and the grip the part. The assembly steps will then be completed.
Once the steps are completed, another acceptable Mouse Cover will be chosen for assembly.
This cycle will continue until there are no more acceptable parts on the table. Once this occurs,
the parts on the table will be automatically cleared and redeposited in the hopper. New parts will
then be deposited on the table, and the cycle repeats.
Required Motion of Robot
At the beginning of this assembly step, the robot end-effector must select the proper gripper set
from its magazine of tools. Once this has been completed, the robot should move using gross
motions, from its default tool-change position, in the horizontal plane until the gripper is above
the Mouse Cover table, directly above a Mouse Cover that is in an acceptable orientation. The
vision system controller would instruct the robot gripper to rotate about the vertical axis so that
the gripper will be aligned with the Mouse Cover of interest. Once aligned with the Mouse
Cover, the robot should move downward in a gross motion. Note that if the workspace if free
from obstacles, the two motions can occur simultaneously, which would save valuable cycle
time. The gripper should then be closed, and sufficient clamping force should be applied to
securely hold the Mouse Cover, but damage to the Mouse Cover must be avoided. Once the
Mouse Cover has been gripped, the robot end-effector should move upward in the vertical axis,
and then in the horizontal plane, until the end-effector is above the Mouse Base. Gross motions
4
Photo taken from http://www.adept.com/Main/products/machine_vision/vis_guid.html
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would be used for this motion, as there is little risk of damage to the Mouse Cover, even if the
robot trajectory is properly planned. The end-effector should then be moved downward so that
the snap-fit assembly features of the Mouse Base engage the snap-fit features of the Mouse
Cover. Once this motion is complete, the gripper fingers should open, releasing the Mouse
Cover. Then, the end-effector can move in a gross motion, in the vertical axis, as well as in the
horizontal plane, so that the gripper fingers are again place at the part presentation area, this time
directly above another Mouse Cover that is in an acceptable orientation. The new Mouse Cover
should be gripped and attached to the second Mouse Base, using the motions just described for
the first set. This operation should then occur two more times, so that in all, four Mouse Covers
have been placed. Upon completion of this, the end-effector should then be moved in a gross
motion back to the tool change location.
Required Motion of Human Operators
There is little required motion of human operators at this assembly step. Humans would be
needed for periodic dumping of Mouse Covers into the part-feeding hopper. This is a simple
task, as the parts would likely be delivered by the supplier in cardboard boxes or plastic bags,
with the Mouse Covers simply randomly piled in the container. The frequency of human
intervention depends on the number of Mouse Covers that are contained in each container, and
on the size of the part-feeding hopper. Since the size of the Mouse Cover is relatively large
(compared to other parts of the Mouse), it is expected that either the part-feeding hopper for this
part will be quite large in order to balance the levels of line side inventories.
Time Calculations
Using the Boothroyd and Dewhurst data charts for robotic assembly, we were able to estimate
the cycle time for this assembly step. At this step, the Mouse Cover requires a special tool for
handling and assembly. Since the decision was made to assemble four products at the same time,
the time required for tool changeovers can be effectively neglected. Since the Mouse Cover is
not easily aligned, but is added and secured immediately using motion along the vertical axis
requiring simple manipulation, the Boothroyd-Dewhurst chart estimates that 3.3 seconds is
required to attach the Mouse Cover to the Mouse Base. Four Mouse Covers attached to four
Mouse Bases results in a cycle time of 13.2 seconds. Note that this time is based on Boothroyd
and Dewhurst’s empirical studies of typical robotic assembly times, rather than based on speed
capabilities of a particular robot.
Step 10: Transfer between Station 2 and Station 3 (Automatic)
Step Description
Once the assembly steps at this workstation are completed, the pallet lift will lower to it bottom
position. This will allow the bottom surface of the pallet to rest on the conveyor belt. The timecontrolled gates that held the pallet on the conveyor belt above the pallet lift will now be opened,
allowing the pallet to move along the conveyor and onto the next workstation. The upstream
time-controlled gates will then open, allowing another pallet into this workstation, and the
assembly sequence is repeated.
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In-flow and out-flows of all assemblies and parts
This step will commence with the lowering of the pallet onto the conveyor belt. The pallet will
then exit this step by being moved along the conveyor belt. No other in-flows or out-flows of
assemblies and parts will be present.
Required Motion of Robot
The same air cylinder that was used to lift the pallet into position will be used to lower the pallet
onto the conveyor, thereby allowing the pallet to move to the next station. Refer to the earlier
description of the required motion of this air cylinder for more details.
Required Motion of Human Operators
As was stated before, since this is an automatic station, no human operators will be required
under normal conditions. However, since the potential for mis-positioned parts and pallets
exists, it is important that an access panel be included to allow a human operator to clear parts
and restart the operation.
Time Calculations
Similar with the raising of the pallet lift at the start of this workstation, no assembly operations
are being conducted in this step. Therefore, operation time will be relatively short. However,
care must still be taken to ensure that the starting acceleration and the finishing deceleration, as
well as the steady state speed of the downwards motion is not so great that it jars the held parts
out of position. If this occurs, there is a risk of damage to the parts, the pallet, the fixtures, as
well as the tooling. The lowering of the Pallet lift is estimated to take 0.75 - 1.5 seconds. Note
that experiments could easily be conducted to determine the minimum time that can be used to
lower the pallet, while ensuring the parts remain properly seated in the pallet and fixtures.
PRODUCT TESTING
There are two types of tests that need to be performed for functionality, electrical performance
and mechanical interface. The electrical performance test consists of verifying the functionality
of the Circuit Board and its individual components. The mechanical interface tests verify that
the Mouse Cover made contact with the Circuit Board switches and that the movement of the
Ball is properly translated through the Gears and to the Circuit Board optical encoder sensors.
In our earlier analysis of the product assembly, we imbedded most of the testing into the
assembly process. The electrical performance would be tested off line while the mechanical
interface would be tested immediately after the components involved in the interface were
assembled. A reevaluation of the assembly process led to a restructuring of the assembly
sequence. As mentioned earlier, the Circuit Board will still be tested off line through the use of a
test cord, however a full functional test will now be conducted after assembly is completed.
The mechanical interface tests present a more difficult decision. Testing the interface between
the Mouse Cover and the Circuit Board switches can be done at the end of Workstation 2 since
all the components are in place. However, doing so would mean the testing responsibility would
fall on the robot and would add additional complexity and cost in the workstation. More
critically, the testing process could impact the ability to balance the lines and potentially cause
additional delays during assembly.
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Testing the interface of the Ball and Gears requires that the Ball be inserted into the Mouse Base.
Currently, this is not accomplished until the end of Workstation 4. Quality principles
recommend that tests should be performed as early in the assembly process as possible to
minimize the wasted assembly effort on defective parts. The assembly sequence could be
reorganized to insert the Ball and Ball Holder into the Mouse Base earlier in the process.
Unfortunately, this effort to ensure quality early in the process is a direct tradeoff with Design
for Assembly principles that dictate product reorientations should be eliminated or at least
minimized in the assembly process.
Utilizing a robot to initiate, control and perform the test would be difficult. Specifically,
plugging in the Cord to a simulation tool would require human interaction. However, an
automated simulation would provide a base test to accurately calibrate the performance of the
Mouse. The mouse could be fixed onto a testing table that would fix the lateral position of the
product while a conveyor belt underneath could activate the Ball to test the interface. It is
important to note that the product must be inverted once again to allow gravity to push the ball
onto the conveyor belt. The operator that plugged the Mouse into the simulation could also
position it onto the test surface in the proper orientation.
In addition to functional tests, it is suggested that sensors be used to ensure that by the each part
required in this operation is actually present and properly gripped. Sensors that monitor either
the clamping force or position of the grippers’ fingers can achieve this inspection capability.
Additional sensors, such as a vision system, can be used to ensure the presence and proper
positioning of the parts after they have been mated to the Mouse subassembly. However, as each
of the parts involved in this assembly station (Circuit Board, Cord, Horizontal and Vertical
Gears) are crucial to the function of the Mouse, the improper placement, or the absence of these
parts will very likely be caught at the functional test station in Station 4. Therefore, the added
cost of a vision system could be eliminated. But, since each assembly stage should not rely on
downstream operations to catch its mistakes, the vision system is recommended.
CYCLE TIMES
Using the Boothroyd-Dewhurst data charts for robotic assembly, we were able to estimate the
cycle time for assembly at Workstation 2. Each component that enters the workstation, Mouse
Base, Circuit Board, Cord and Gears, requires a special tool for handling and assembly. The
robot tooling head will need to accommodate all four tools. In order to minimize tool
changeover, we propose assembling four products at the same time, effectively negating the time
required for tool changeovers. Since all four parts are not easily aligned, and are added and
secured immediately using motion along the vertical axis requiring simple manipulation, the
chart estimates that 3.3 seconds are required to assemble each part. Four parts inserted into four
products results in a cycle time of 52.8 seconds. (3.3 seconds*4 parts*4 products). If we assume
that between 0.75 and 1.5 seconds will be needed to have the pallets enter the workstation and be
properly positioned, and that another 0.75 to 1.5 seconds will be need to have the pallets leave
the workstation, an overall cycle time of 54.3 to 55.8 seconds, or simply 55 seconds, is
estimated. This equates to roughly 14 seconds per Mouse assembly.
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The Gantt chart included below outlines each assembly step and the time required for each step
for a complete assembly cycle. The blue shaded boxes represent the time during which a
specific operation is taking place. The green shaded box represents the next commencement of
the next cycle. Note that all tasks are sequential, and no tasks can be done in parallel.
Elapsed Time (seconds)
0.0 1.5 4.8 8.1 11.4 14.7 18.0 21.3 24.6 27.9 31.2 34.5 37.8 41.1 44.4 47.7 51.0 54.3 57.6
Operation
Step 5
Tray Enters
Workstation #2
Step 6
Pick & Place Circuit
Board in Mouse 1
Pick & Place Circuit
Board in Mouse 2
Pick & Place Circuit
Board in Mouse 3
Pick & Place Circuit
Board in Mouse 4
Step 7
Pick & Place Cord
in Mouse 1
Pick & Place Cord
in Mouse 2
Pick & Place Cord
in Mouse 3
Pick & Place Cord
in Mouse 4
Step 8
Pick & Place Gears
in Mouse 1
Pick & Place Gears
in Mouse 2
Pick & Place Gears
in Mouse 3
Pick & Place Gears
in Mouse 4
Step 9
Pick & Place
Mouse Cover in
Mouse 1
Pick & Place
Mouse Cover in
Mouse 2
Pick & Place
Mouse Cover in
Mouse 3
Pick & Place
Mouse Cover in
Mouse 4
Tray Leaves
Step 10 Workstation
WORKSTATION COST ESTIMATES
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One of the most important aspects of a manufacturing system, particularly in robotic assembly, is
to determine the cost associated with each workstation. Each assembly robot is priced
differently depending on its performance requirements and capabilities. In order to determine
baseline costs for the robot, the general performance requirements were defined. The conceptual
design of the workstation require the robot to lift a small payload, have pick and place
capabilities, and have the ability to carry and use four different gripper tools with minimal
changeover. Since these are not advanced requirements, it is believed that a standard robot could
be purchased from a major robot manufacturer, such as Denso® or Adept®. After researching
these and a number of other different companies on-line, we were unable to find a book price for
robotic assembly.
However, outlining the different costs associated with installing, purchasing and operating the
robot can still be used to develop cost estimates of the workstation. The largest percentage of
costs is associated with the physical equipment needed such as the robot, tooling (special
grippers), and material handling equipment. In order to automate the assembly process, material
flow is also accomplished through automation. Primarily, a conveyor-type belt is needed to
transfer completed work between workstations. Pallets, trays and shakers will also be needed to
control the orientation and the flow of the individual components into the workstation. In
addition, a system, potentially an automated one, will need to replace empty pallets and trays
with new, filled pallets into the workstation as needed. In addition to the cost of the robot, a
significant portion of time and money will have to be devoted to developing software that
effectively runs the workstation. It is important to note that installation costs should also be
considered in determining the feasibility of a new workstation. If either the equipment or
software is new, operators that interface with the workstation will have to be trained in use and
maintenance. The cost of equipment was estimated to be between $100,000 and $125,000, the
cost of installation at approximately $15,000 and between $25,000 and $35,000 for software
development.
In order to determine the cost of running the workstation for one full cycle, the fixed costs and
the variable costs must be separated. All of the capital required to create one workstation is a
fixed cost. The variable costs associated with one cycle are the raw material costs (estimated at
$2.00) and labor costs. However, there are no direct labor costs associated with this workstation.
There is some indirect labor costs associated with this workstation, such as the operator needed
to fill part hoppers with components. However, it is assumed that this operator will have
responsibilities other than material handling, and his/her labor costs can be allocated
proportionally among those responsibilities.
The chart below outlines the labor requirements to meet the production volume of 800,000
products in one year. (The production volume was based on Microsoft’s claim that 2 million
products were in the market and our assumption that a production run lasted 2.5 years.)
7 hours/shift
25200 seconds/shift
477.27 products/shift
2 shift/day
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250 days/year
238636 products/year
800000 products/year
3.35 lines required
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TABLE OF FIGURES
Figure 1: Proposed Layout of Workstation #2................................................................................ 4
Figure 2: Circuit Board Redesign ................................................................................................... 5
Figure 3: Current Mouse Base and Mouse Cover Interface ........................................................... 6
Figure 4: Spring Tab Redesign for Mouse Cover to Mouse Base Interface ................................... 7
Figure 5: Proposed Pallet Design That Will Fixture Four Assemblies At Once ............................ 8
Figure 6: Gripper Finger Placement For Circuit Board ................................................................ 10
Figure 7: Gripper Finger Placement For Cord .............................................................................. 12
Figure 8: Commercially Available Pick and Place Robot That Performs Pattern Recognition ... 16
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APPENDIX A: Bill of Materials
Bill of Materials
Mouse Assembly
Mouse Base Subassembly
Mouse Base
Spring/Wheel Subassembly
Spring
Wheel
Circuit board
Gears (2)
Cord
Ball Holder Subassembly
Ball Holder
Ball
Mouse Cover
Screw
Sticker Pads (2)
Part No.
8
9
10
11
7
6
1
2
5
3
4
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