2.875 – Fall 2001

Annabel Flores, James Katzen

2.875 - Fall 2001

Mechanical Assembly and Its Role in Product Development

Term Project: Report #5

DESIGNING AN ASSEMBLY PROCESS TO PRODUCE COMPUTER MOUSE ASSEMBLY

November 28, 2001

Annabel Flores

James Katzen

Photos taken from:

DESIGNING AN ASSEMBLY PROCESS 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 analyzed individual parts’ design, the interface between a subset of parts of the mouse, the key characteristics and the functional requirements of the product, the proper assembly of the Mouse, and the design and layout of a particular Workstation for assembly of the Mouse.

This report will present the chosen assembly sequence and its advantages and drawbacks in comparison to other assembly sequences. In addition, the floor layout of the assembly process will be addressed, including process equipment, material handling/storage, and operator placement. Production rate, production volume, and cycle times for each operation will be estimated. Finally, the flow of parts and operators involved in the assembly sequence will be choreographed. Unfortunately, computer simulation tools were not successfully used to study the chosen assembly sequence. Efforts will be made to find a suitable computer that supports the recommended programs, and if found, a simulation analysis will be included in the next project report.

Assembly Sequences

As in the previous report, we assume that specific product redesigns would have been made to the Mouse to facilitate assembly. These product redesigns include:

  • Relocation of connector block of Circuit Board to side closest to where strain relief of Cord passes through Mouse Base.
  • Shortening of Cord to accommodate relocated connector block and eliminate need to route Cord around edge of Mouse Base.
  • Redesign of strain relief of Cord to permit top-down assembly mating of strain relief into Mouse Base.
  • Redesign of snap fit features on Mouse Base and Mouse Cover to permit top-down assembly mating of these two parts.
  • Redesign of Ball Holder to a simpler disk design that eliminates a subassembly.
  • Elimination of “mystery feature” on Mouse Base to permit top-down assembly mating of Circuit Board into Mouse Base.

Figure 1: The "Mystery Feature" on Mouse Base that complicates necessitates reorientation of Circuit Board during assembly.

As has been described in previous reports, there are a number of different assembly sequences that may be used to assemble the Mouse. At the extremes, the product could be assembled using fully automated assembly, fully manual assembly or a hybrid combination of the two. The following section outlines the assembly sequences with a comparison of these three methods.

Selected Assembly Sequence – Hybrid Design

A previous report described a number of potential hybrid assembly sequences for this product. The optimum choice resulted in better efficiencies as it minimized unnecessary operations. It was decided that these operations would logically be grouped into five Workstations: one manual station, followed by three robotic stations, and then followed with one final manual station. The complete list of the stages required to assemble the mouse is shown below:

  • Station 1 (Manual):
  • Step 1: Place Mouse Base onto Primary Fixture
  • Step 2: Place Wheel onto end of Spring
  • Step 3: Assemble Wheel and Spring Subassembly with Mouse
  • Step 4: Transfer pallet between Station 1 and Station 2
  • Station 2 (Automatic):
  • Step 5: Locate Pallet on Locating Pins
  • Step 6: Assemble Circuit Board with Mouse Base.
  • Step 7: Attach Plug of Cord to Circuit Board and attach strain relief of Cord to Mouse Base
  • Step 8: Attach Horizontal Gear and Vertical Gear to Mouse Base
  • Step 9: Attach Mouse Cover to Mouse Base
  • Step 10: Transfer between Station 2 and Station 3
  • Station 3 (Automatic):
  • Step 11: Invert Assembly and place into Secondary Fixture.
  • Step 12: Transfer between Station 3 and Station 4
  • Station 4 (Automatic):
  • Step 13: Place Ball into Mouse Base
  • Step 14: Secure Ball and Mouse Base by inserting and twisting Ball Holder
  • Step 15: Secure Mouse Base and Mouse Cover by inserting and tightening Screw
  • Step 16: Attach top Sticker Pad
  • Step 17: Attach bottom Sticker Pad
  • Step 18: Attach Hologram Sticker
  • Step 19: Transfer between Station 4 and Station 5
  • Station 5 (Manual):
  • Step 20: Plug in Cord’s connector into Test Fixture Connector Block
  • Step 21: Functional Test (computer controlled with automatic data collection)
  • Step 22: Remove Cord’s connector from Test Fixture Connector Block
  • Step 23: Bundle Mouse Assembly with Product Documentation and Software
  • Step 24a: Pack into OEM packaging OR
  • Step 24b: Pack into aftermarket packaging

Appendix C shows the proposed floor layout for the hybrid assembly sequence.

Fully Automated Assembly Sequence

Successful incorporation of a full robotic assembly process would require a significant amount of product redesign. These redesigns would be required to ensure adequate delivery of all key characteristics, especially the KC between the Mouse Ball and the Gears (as this has largest effect on proper mouse functionality).

It was found in prior analysis that the KC between the Mouse Ball and the Gears is met through the use of a spring loaded Wheel that pushes the Ball against the Vertical and Horizontal Gears. The wheel must contact the Ball in a specific location to minimize friction. The current design uses a long thin steel Spring and a small plastic Wheel to achieve this function. The current design must be manually assembled first into a subassembly then onto the Mouse Base, because of the asymmetrical properties of the Spring. To allow for automated assembly, which includes robot feeding, handling and assembling, the Spring and Wheel would need to be modified or eliminated.

In addition, design changes would have to be made to the Cord. Since we believe that there is a need to test at least a representative sample of the assemblies prior to shipment, it is necessary to plug the Cord into a test fixture at the functional test station. Since the current Cord is quite flexible, it would prove very difficult to program a robot to deal with the Cord’s flexibility while the Cord’s connector is manipulated into and out of the test fixture.

As can be seen, there are drastic changes that need to be made to the current design of the mouse to allow for full robotic assembly. As such, this assembly sequence was judged to be being infeasible at this time and was discarded from further consideration.

Fully Manual Assembly Sequence

A possible assembly alternative is to assemble the product using only manual assembly. This assembly sequence would allow a cellular design to be used as detailed below:


Figure 2: Full Manual Assembly, Cellular Design

This layout requires that the individual components to be combined in a kit outside of the work cell and then fed into the cell next to Workstation 1. At each station, the operator would take parts from the kit and perform the required assembly operation(s). The subassembly would then be placed onto a tray. The racks and trays would then be moved to the next Workstation, with kits traveling on the racks above the work surface and trays being slid along simple roller type conveyors. At the completion of Workstation 3, where all components have been assembled and the kit is thus empty, the kit container would then exit the cell and be returned to the kitting area for a new kit to be made. This full manual assembly would still use the same assembly sequence as other assembly designs, however in this case, there would be fewer Workstations and the work would be divided slightly differently.
  • Station 1:
  • Step 1: Transfer kit from delivery area onto rack
  • Step 2: Transfer tray from empty tray storage area onto conveyor
  • Step 3: Place Mouse Base onto Primary Fixture
  • Step 4: Place Wheel onto end of Spring
  • Step 5: Assemble Wheel and Spring Subassembly with Mouse Base
  • Step 6: Assemble Circuit Board with Mouse Base
  • Step 7: Transfer tray out of Station 1
  • Step 8: Transfer kit out of Station 1
  • Station 2:
  • Step 9: Transfer tray into Station 2
  • Step 10: Transfer kit into of Station 2
  • Step 11: Attach Plug of Cord to Circuit Board and attach strain relief of Cord to Mouse Base
  • Step 12: Attach Horizontal Gear to Mouse Base
  • Step 13: Attach Vertical Gear to Mouse Base
  • Step 14: Attach Mouse Cover to Mouse Base
  • Step 15: Transfer tray out of Station 2
  • Step 16: Transfer kit out of Station 2
  • Station 3:
  • Step 17: Transfer tray into Station 3
  • Step 18: Transfer kit into of Station 3
  • Step 19: Place Ball into Mouse Base
  • Step 20: Place Ball Holder into Mouse Base
  • Step 21: Transfer kit from rack to takeaway area
  • Step 22: Secure Mouse Base and Mouse Cover by inserting and tightening Screw
  • Step 23a: Transfer tray out of Station 3 (go to Step 28) OR
  • Step 23b: Transfer tray to test area
  • Step 24: Plug in Cord’s connector into Test Fixture Connector Block
  • Step 25: Functional Test (no operator required)
  • Step 26: Transfer tray out of Station 3
  • Station 4:
  • Step 27: Remove Cord’s connector from Test Fixture Connector Block
  • Step 28: Transfer tray into Station 4
  • Step 29: Attach top Sticker Pad
  • Step 30: Attach bottom Sticker Pad
  • Step 31: Attach Hologram Sticker
  • Step 32: Bundle Mouse Assembly with Product Documentation and Software
  • Step 33a: Pack into OEM packaging OR
  • Step 33b: Pack into aftermarket packaging
  • Step 34: Transfer finished product out of Cell
  • Step 35: Transfer empty tray to empty tray storage area
A cellular design allows a single-piece flow operation that is very flexible to product changes. Much more skilled and active operators are required since it is the operators that not only physically assemble the product but also control the material flow throughout. Because the cell requires that all the components be present before assembly, additional employees are needed to create each kit. However, a missing operator does not necessarily shut down production, as operators will likely be cross-trained and shift between stations as batches are completed. No sophisticated machinery is needed with this sequence other than the Test Fixture.

Analysis of the Chosen Assembly Sequence

An analysis of the complexity of the Mouse assembly process and the estimated volume led us to decide upon a hybrid assembly process that uses both manual stations and automatic stations. A fully automated assembly sequence is clearly not a viable option even with the proposed redesigns mentioned earlier. The section below describes the general advantages and disadvantages with pursuing the proposed hybrid design instead of the cellular design.

Advantages of Selected Assembly Sequence

The selected hybrid design provides a number of advantages over a manual assembly. The automated Workstations provide a uniformity and repeatability of task completion. We assume that the cycle time for a product is much longer in the manual assembly sequence and therefore a larger product volume could be achieved with an automated assembly. Because most of the costs of a manual assembly are variable, they increase proportionately to an increase in product volume. An automated assembly, however, is comprised largely of fixed costs causing unit costs to drop as the volume is increased. The heavy machinery is typically customized to respond to the product’s design. However, since computer mice are relatively similar, the cost of machinery can be distributed over product families. Minimal manual labor is required in the hybrid design, thereby reducing the cost of fringe benefits and salary associated with manual labor. Similarly, much of the employee training can be eliminated.

In general, the capacity of automated machines can be modified to respond quickly to changes in product volume. While it is a viable option to use a full manual assembly process, we feel it was important to complete our analysis of the hybrid assembly selected from previous reports to explore the capabilities of robotic assembly. A subsequent report will compare the two assembly processes on cost of capital, cost of labor, and product cycle time among other things.

Drawbacks of Selected Assembly Sequence

There are a number of disadvantages to a hybrid assembly sequence that make the full manual assembly more attractive.

To put an automated assembly in place, invariably higher up-front costs of assembly equipment and material handling equipment are needed. However, not just the cost of the equipment must be considered. The cost of the integration software, which is substantial for each new product, must be included. This raises the initial costs of building an assembly line that has some automated equipment.

In addition, once the assembly line is operational, there are still additional drawbacks to the selected assembly sequence. First, to accommodate the inherent variability in manual operation output, it is necessary to keep relatively large buffers of work in process between stations. Also, because product tests are not performed until the end of the assembly process and visual inspection is difficult to integrate into the sequence, it is more likely that defective products are created that must be either scrapped or reworked. Finally, the high cost of training operators to troubleshoot and maintain the automation equipment could be considerable.

Selected Assembly Sequence

The choice to design a hybrid assembly process instead of a full manual assembly process was driven largely by our own intellectual curiosity about robotic assembly. Both fully manual and hybrid assembly are viable options for this product, and they should be thoroughly analyzed and evaluated before either one is implemented. A curious discover is that, as the product was redesigned to make robotic assembly possible, it became much easier to assemble the product manually. Thus, many of the justifications for the need for robotic assembly, have been rendered moot, after design simplification.

Production Rate, Production Volume, and Cycle Times Calculations

The production volume was based on Microsoft’s claim that 2 million products were in the market. We have made the assumption that a production run would last roughly 2 ½ years. Using these estimates, this requires that roughly 800,000 assemblies would be required each year. We know from the product label that the assembly was made in China. Assuming that there are roughly 250 workdays in a Chinese production year, a production rate of 3200 products per workday is required. Assuming there are 8 hours per Chinese shift, and 2 shifts per Chinese workday, a cycle time of 18 seconds per Mouse Assembly is required.

In the previous report, we stated that it would be efficient for multiple assemblies to be placed on the same pallet to minimize the total number of tool and gripper changes required. Based on recommendations, we selected a pallet design that would hold 4 assemblies at once. Thus, using a batch size of 4, with the cycle time (per Mouse Assembly) of 18 seconds, a pallet of four assemblies must be produced every 72 seconds.

Note that this cycle time calculation assumes a production yield of 100% fault-free assemblies. Since this is an unrealistic assumption, we must increase the production requirement for the process. Since we can expect that the circuit board (the major source of product failure) to be tested at the supplier’s facility, it is likely that there will be few unusable assemblies. Thus, if we assume a yield percentage of 95%, the required production rate becomes roughly 3400 Mouse assemblies per day, or a cycle time of 17 seconds per Mouse assembly, and a resulting batch cycle time of 68 seconds.

Note also that this batch cycle time of 68 seconds is the end-of-line rate. Upstream stations in the assembly process will have to produce at a marginally faster rate, to account for minor production losses due to repositioning/rework at the manual stations, which could result in unscheduled downtime losses due to blocked or starved conditions. This source of yield loss could compound over time to be a major source of lost production volume, so it is vital that it be accounted for when planning the cycle time of the process. Assuming an in-line yield loss of 90%, the first stations in the assembly process should be capable of producing 3800 Mouse assemblies per day (a cycle time of 15 seconds per Mouse assembly, or a batch cycle time of 60 seconds). However, these stations should just be capable of this production rate, and must not produce at this rate unless it is required to account for in-line losses of yield. Producing at this higher rate would cause unneeded in-process inventory, which would negatively affect the balancing of product between stations in the process.

Finally, planned downtime must be accounted for in any cycle time determination. However, since we assume that the assemblies are produced with a two- shift operation, it is expected that any required preventative maintenance could be completed during the third shift.

In the previous report, a batch cycle time of 55 seconds was calculated. This calculation was based on empirical numbers suggested by the Boothroyd & Dewhurst design analysis method. Revisiting these calculations, we believe that this is an overly optimistic estimation. Four assembly steps, involving four parts, are required in the stage that was studied. In addition, the pallet must be located and later released from this Workstation. Assuming it would take 1.5 seconds to raise the pallet off the conveyor and then locate the pallet properly, and another 1.5 seconds to lower the pallet back onto the conveyor, 52 seconds remain in which to accomplish the required assembly stages. This would require rapid accelerations and steady-state speeds, which could result in less accurate positioning of parts in the assembly. Recalculating the assembly time using an estimated time of 5 seconds per assembly stage (rather than 3.3 seconds, suggested by Boothroyd & Dewhurst) yields a total time of 20 seconds per pallet per assembly stage. In addition, a gripper change (estimated to take 5 seconds per change) is required for each assembly stage. This adds another 20 seconds onto the cycle time. Therefore, the total cycle time to produce a pallet of four Mouse assemblies would be 103 seconds.