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
DESIGNING ASSEMBLY WORKSTATIONS TO PRODUCE COMPUTER MOUSE ASSEMBLY
Introduction
The Microsoft Mouse Version 2.2[1] 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. Place Mouse Base (Part #8) onto Primary Fixture.
2. Place Wheel (Part #10) onto end of Spring (Part #9).
3. Assemble Wheel and Spring Subassembly with Mouse Base (Part #8).
4. Assemble Circuit Board (Part #11) with Mouse Base (Part #8).
5. Attach Plug of Cord (Part #6) to Circuit Board (Part #11)
6. 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 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:
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.
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:
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 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:
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:
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 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.