Group 8 Gate 2

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Contents

Executive Summary

The purpose of this project gate is to document dissection process for the Craftsman 38cc 16" Chainsaw. A specific plan was laid out in gate 1 in how to adequately dissect the product and analyze each system and subsystem. Ideally, the plan laid out in gate 1 was to be followed entirely by the project team for gate 2. However, due to unforeseen circumstances, the plan must be altered in order to adequately complete the task in a timely fashion. These alterations to the original plan are documented within this gate as well.

Cause for Corrective Action

After significant discussion, the project team has determined that overall the plan for dissection initially set forth has been executed without any significant deviation from the plan. The team however, has decided to dedicate an extra half hour of meeting time on Mondays, Wednesdays, and Fridays at 5 pm for discussion in regards to the execution of the project plan as well as planning optimal documentation methods for Gate 2 and future Gates. This in return has allowed the group to focus on the actual analysis of the chainsaw more, where more time is reserved for proper documentation of the chainsaw parts and the dissection process. Although no unresolved challenges have arose yet, the group still plans to have all decisions “needing to be made within the group be made on a majority-rules basis.” Any significant issues that may arise that cannot be resolved by the group will continue to result in consultation with Instructors Phil Cormier and Andrew Olewnik, as well as the supporting Teaching Assistants.

It was originally planned that the chainsaw would be dissected “ with the focus being around accessing and analyzing the centripetal clutch first,” and then other components around the body of the chainsaw were removed accordingly. This was executed relatively well as planned. The most significant issues that arose during disassembly occurred when removing key components such as the engine block. These types of components are vastly complex as a result of many other components being connected to them. As a result, significant planning before execution was required in order to prevent damage to the chainsaw. The group was unable to properly anticipate the amount of time needed for dissection of these components during the planning stages. This was due to the fact that the group was only allowed to make observations on a macroscopic level without any parts being removed during the planning stages. Going forward however, the group has the ability to anticipate the amount of time needed for each component properly as a result of the dissection.


Product Dissection

Introduction

The chainsaw was disassembled with the initial focus being on accessing the Centripetal Clutch. From there the other components of the chainsaw were removed accordingly. Figure 1 below shows the exploded assembly of the chainsaw. This figure will be referenced multiple times when describing the dissection process.


Figure 1: Exploded Assembly of the Chainsaw. Courtesy of the owner’s manual provided with the chainsaw.

Note: Due to having two diagrams, when referencing parts from figure 1, it will be designated by a part number followed by a letter referencing which diagram it is on. The left diagram is “A” and the right diagram is “B.” So for example, the chain bar will be designated as part # 48A.

Degree of Difficulty

During the dissection process, several components required a different difficulty level for removal. The difficulty is the direct result of several factors such as planning before removal, physical effort required for removal (i.e. physical strength), tools need for removal, and research required . For reporting purposes a scale of 1-3 has been developed to describe the degree of difficulty in which a step was executed. Table 1 below describes the difficulty scale.

Table 1: Degree of Difficulty Scale
\'\'\'Scale\'\'\' \'\'\'Description\'\'\'
1 Little to no pre-planning required, physical effort to remove part(s) was minimal, only one or no tools were required for removal, and/or no research was required
2 Some pre-planning was required, moderate physical effort was required to remove part(s), 1-2 different tools were required to remove component(s), and/or little to moderate research was required to for disassembly.
3 Significant pre-planning was required, substantial strength required for part removal, 3 or more tools were required to remove component(s), and/or significant research was required for disassembly.



Dissection Process

Step 1: Chain Bar Removal

Figure 2: Chain Tightener and Bar Knob.

These parts are intended for easy removal in order to replace the chain quickly with minimal effort.

Degree of Difficulty: 1

Description:

For removal of the chain and chain bar, the bar knob (42A) and clutch cover (43A) are removed by hand. This leaves the chain bar (48A) and the chain (47A) exposed where they are easily removed by hand as well. No specific knowledge of the product is required to perform this step and minimal hand strength was required for this as well. It has been determined that for maintenance purposes, these components are removed in order to replace an old, dull chain with a new one as well as adjust the tension on the chain. This conclusion was drawn from the fact that no tools required these components. It is made easy for operators to remove components. It was also observed that Craftsman developed a chain-tension system that allows the user to fine-tune how taut the chain is. By loosening the bar knob and turning a knob located on the clutch housing, the tension on the chain can be increased.

Step 2: Exposing Centripetal Chart

Figure 3: Clutch Drum and Washer Kit
: Figure 4: Chainbrake

Degree of Difficulty: 2

Description: In order to access the Centripetal Clutch for analysis, the clutch drum (11A) and the clutch washers (10A) had to be removed. A little a little discussion before removing the washer took place in order to ensure that this was removed without breaking. If this piece is broken, then the clutch drum will not be secured to the shaft properly therefore attention to detail is required. This removal was executed by two people each with a slotted screwdriver carefully prying the washer free from the shaft it was attached on. The clutch drum and other washer was easily removed by hand. Next the chain brake (35A) and hand guard (32A) are removed. In order to do this, a slotted screwdriver was used to remove two screws. Overall, it was determined that these components are not intended to be removed. This is due to the fact that it took multiple people and careful planning to remove the clutch washer. The chain brake is vital to the chainsaw because it serves as a safety feature by stopping the clutch from spinning which in return stops the chain. Removing these parts would not be part of general maintenance performed by the operator.





Step 3: Exposing The Engine

Figure 5: Top of engine exposed

Degree of Difficulty: 1

Description:

Upon making the Centripetal Clutch visible, exterior components were removed to expose the motor. First with the use of a slotted screwdriver, the front handle is removed (51A). Then the plastic shield (1B) is removed. This leaves the top-end of the motor exposed where it is easily accessible. It has been determined that these components are intended to be removed for maintenance purposes. By removing these pieces with little effort, the spark plug (16B) be replaced.





Step 4: Removing the Carburetor

Figure 6: The wiring harness is being disconnected from the air filter housing where the kill switch is located

Degree of Difficulty: 3

Description:

The removal of the carburetor proved to require significant pre-planning due to the fact there are several components connected to the carburetor. Initially the air filter (8B), Air filter cover (7B) Air filter housing (10B) were removed relatively easy with the use of an 1/8 inch socket wrench and star-key. Removing the actual carburetor (11B) proved to be complex. Several other components such as the fuel line (12B) and the wire (37B) that connects the throttle to the carburetor had to be removed first carefully using pliers. The wiring harness (6B) had to be disconnected after these components are removed, the carburetor is removed by hand. The intake tube (36B) and gasket (35B) come off by hand as well. Using a slotted screwdriver, the carburetor Adapter (14B) is then removed from the engine block (17B). The only part in this system intended to be removed is the air filter cover in order to replace the air filter. However, due to the overall complexity of this disassembly, it was easily determined that these components are not intended for disassembly. There are many delicate components that make up the carburetor system and if one is not careful, the carburetor will be taken out of tune and the engine will not run right. The removal of these components is not part of basic maintenance by the operator as well.




Step 5: Exposing the flywheel

Figure 7: The flywheel is exposed

Degree of Difficulty: 1

Description:

In order to expose the flywheel, the entire ripcord unit is removed (Parts 18-29 on diagram A). This was easily removed using a slotted screwdriver two remove two screws. The unit then slips off easily. It was determined that this component was intended to be removed as result of the ripcord needing to have the ability to be easily changed. The rip cord delivers mechanical energy to the flywheel which starts the combustion process in the motor. As a result, the ripcord is subjected to significant tensile forces and has the ability to rip. Therefore, easy access is required.






Step 6: Removing the Muffler

Degree of Difficulty: 3

Description:

Removing the muffler proved to require the most physical effort during disassembly. One nut had to be removed using an 1/8 inch socket wrench and two bolts removed using a star key. The two bolts connecting the muffler to the engine block were extremely tight. Significant physical force was required to loosen these bolts. At the same time however, cautionary measures had to be taken in order to ensure that the bolts did not shear when torque is applied. Once the bolts were removed, the entire muffler assembly (19B) came apart with ease. Overall the team determined that this system is not intended for removal. The amount of torque required to remove these components was substantial, indicating that these components are not intended for removal. The removal of these parts does serve maintenance purposes as well.






Step 7: Removing the Engine Block

Figure 8: picture of piston assembly after engine block is removed

Degree of Difficulty: 3

Description:

Before the removal of the engine block (17B), the group took a significant amount of time to prepare a plan of action. Moderate research was done as well before performing this step. If careful planning did not take place, the internal combustion process would not function correctly upon reassembly. Four bolts are removed from the bottom side of the chainsaw using an allen wrench. From here the block is separated from the frame. The cylinder guides (24 & 32B) come out once the piston is freed. It is easily determined that this system is not intended for removal. By taking this apart, one is subjecting the integrity of the combustion process to failure. The seals in the piston could potentially fail to work properly leading to improper lubrication of the engine, which ultimately leads to engine seizure. The team has determined that there is no maintenance value to removing the engine block from the frame.







Subsystems

Figure 9: Energy Flow


Figure 9 above shows how the various sub-functions involved with the chainsaw’s overall function of removing material. All of the sub-function run together meaning they can’t run independently of each other. If you look at figure 9 you will see how the sub-function are connected with each other. All transfer of electrical energy is connected by wires. Air, gas, and exhaust are all connected using some type of tube or channel. The choke and throttle signals are connected to the carburetor by mechanical switches opening or closing the entrance of the air channel into the carburetor. The path of energy from the combustion chamber to the chain is connected physically as follows. The pneumatic energy caused by combustion pushes the piston which turns the crank shaft which the centripetal clutch is attached to. When enough speed is applied the clutch opens up and catches turning the gear which turns the chain around the chain bar.


They are all connected in that certain order to allow for proper transfer of signal and energy. If the pneumatic energy created by combustion was immediately transferred to the centripetal clutch nothing would happen. Similarly if gas and air were imported into the muffler instead of the combustion chamber, no ignition would occur and even if there was ignition the muffler would most likely explode with no positive outcome. The sub-functions and connected are designed and made in a certain way to ensure proper functionality.


This chainsaw compared to other models is relatively small and light duty. It is not meant to run for long periods of time and take on extensive projects. That being said, most of the connections in the chainsaw are light and inexpensive. For example, the channel that transfers air from the throttle input is connected to the engine only by a rubber tube pressed down by just the screws holding the carburetor in place. No other fastener is holding the tube in place. This chainsaw can be sold around the world because there is a demand for this class chainsaw everywhere so no global factors influenced the connections. It can be made the same and sold globally and the chainsaw will function properly. The fact that the chainsaw is meant for light use is a societal factor. This influences the durability and cost of implemented connections making them cheaper and simpler. The only environmental influence is making implementing an engine size appropriate to the size of the chain bar. There is no need to power a 16 inch chainsaw with a lawnmower engine. As seen from dissecting the product many of the parts were cheap and light. This may show that an economic influence played a part when Craftsmen was deciding what materials to use for each part. As stated earlier many times this chainsaw was meant for light duty work. The performance demand for this product is not great because of this meaning the connections were of the simplest of all connection options at the time like the example with the air channel tube. If a larger chainsaw were to be dissected better quality connections would be implemented into the making of the product.


The chainsaw contains only one system that can be broken down into the above function in the figure. The arrangement of sub-function’s is important and is properly placed because certain functions need to run in series and parallel with one another. The figure 9 shows how energy is transferred and what kind of energy is transferred from function to function in specific order. The carburetor, gas tank and starter all need to run in parallel to import energy into the combustion chamber. The path from the combustion chamber to the removal of material function all needs to be in series because the energy created in the combustion chamber needs to be converted and transferred through the function path in figure 9 to allow the chain to utilize the energy created. The functions in series between this are the piston which creates translational energy, and the crank shaft to chain functions all create rotational energy. The muffler function runs parallel to the path from the combustion chamber to the removal of material function because as long as energy is created and transferred there will be heat and noise coming from that component. The path from the combustion chamber to the removal of material function, can not be adjacent because the energy needs to be converted and transferred before it reaches the chain and chain bar.



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