Group 20 - Black and Decker Jig Saw-Gate 3

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Contents

Management Review

There have been some problems with our group and they were discussed after we got our grade on our last gate. One small problem we have had is completing the work as a group instead of just finishing most of the work individually. I believe working as a group will help the quality of our work and make sure everyone is doing their equal part in this project. To solve this problem we scheduled an extra meeting this week on top of our regularly scheduled meeting. This extra meeting was very productive and we got a lot of work done, and it solved our issue of getting more work done as a group instead of working individually. At this meeting we completed the information that was going to be displayed in our presentation and we also worked as a group on what will be on our fourth gate. The issue of our grades has been discussed as a group and the only solution we could come up with is for everyone to work harder on their given duties. The members of the group that are not doing quality work have been told many times they need to do better work and put in more effort. We have given less responsibility to the members who haven’t been doing their part. Due to the amount of time between gates and everyone’s conflicting schedules there was not much time to correct some members efforts.

Components

Component Overview

The following table is a comprehensive overview of the parts that make up the Black and Decker Jigsaw.

Table 3-1: Component Overview
Component Image Function Number

of times used

Manufacturing

Process

Material Complexity
Housing/ Outershell
Figure 1: Housing Click to View
  • house almost all other components of the saw
  • protect the operator from fast moving parts
once Injection Molding Plastic Medium
  • Two manufacturing steps required
  • 3 dimensional
  • One main function, may aid another function


Counterweight
Figure 2: Counterweight Click to View
stabilize the armature assembly once Stamped Steel Low
  • One manufacturing step required
  • Simple geometry (2-dimensional)
  • Single function


Field Assembly
Figure 3: Field Assembly Click to View
  • create a magnetic field, which causes the armature assembly to spin
used continuously Machined
  • Lathe
*Aluminum
  • Copper Wire
High
  • Three or more manufacturing steps required
  • 3 dimensional with multiple pieces
  • 2+ functions


Thrust plate
Figure 4: Thrust Plate Click to View
is the divider between the thrust plate and armature assembly once Stamping Aluminum Low
  • One manufacturing step required
  • Simple geometry (2-dimensional)
  • Single function


Gear
Figure 5: Gear Click to View
turn the rotational energy from the main shaft to translational energy and transfer it to the shaft assembly used continuously Machined Steel Medium
  • Two manufacturing steps required
  • 3 dimensional
  • One main function, may aid another function
Locking screw and shoe assembly
Figure 6: Locking Srew and Shoe assembly Click to View
  • supports the weight of the saw
  • rests evenly on the material that is to be cut
used continuously

as a guide

Machining Aluminum Medium
  • Two manufacturing steps required
  • 3 dimensional
  • One main function, may aid another function


Gear assembly
Figure 7: Gear Assembly Click to View
  • Actuator
used continuously Injection Molding Plastic Low
  • One manufacturing step required
  • Simple geometry (2-dimensional)
  • Single function


Blade Guard
Figure 8: Blade Guard Click to View
  • protects the user from exposed saw blade
used continuously

while blade is in motion

Injection Molding Plastic Low
  • One manufacturing step required
  • Simple geometry (2-dimensional)
  • Single function


Shaft assembly
Figure 9: Shaft Assembly Click to View
  • connects to the gear
  • houses the blade
used continuously Machining Steel Medium
  • Two manufacturing steps required
  • 3 dimensional
  • One main function, may aid another function


Brush boxes
Figure 10: Brush Boxes Click to View
  • attatched to the motor
used continuously
  • Injection Molding
  • Machining
  • Copper Metal
  • Plastic
Medium
  • Two manufacturing steps required
  • 3 dimensional
  • One main function, may aid another function


Switch
Figure 11: Switch Click to View
  • recieves input from the trigger
  • sends a signal to activate motor
Once
  • Injection Molding
  • Machining
  • Copper Wire
  • Copper Metal
  • Plastic
High
  • Three or more manufacturing steps required
  • 3 dimensional with multiple pieces
  • 2+ functions


Housing/ Outer Shell

Figure 12: Housing Click to View


The Housing does not have an average geometric shape; it has some trapezoidal and circular features. It is 3 dimensional due to its depth and many cavities on the inside of the housing. The outer shell is two pieces and is split right down the middle. Both pieces have an almost identical outside shape making it close to symmetric. The inside cavities are not the same since it is designed to complement the interior components.

The function of this component is to house almost all other components of the saw and protect the operator from fast moving parts.

The shape couples with the function by housing its components and making the overall size of the saw compact. This housing is made out of plastic and has some manufacturing advantages. There are many gaps, holes and cavities on the inside of these two pieces that make up the outer shell. It would be very time consuming and difficult to machine all of these areas out of most other materials. This component could be made out of another material such as aluminum or steel, so it is not required to be made of plastic but due to economic and social factors it is the optimal material for the component. Making the housing out of a metal like aluminum would be a lot more expensive to produce due to the cost of the material and the machining that would be involved in producing a final product. Increasing the production cost and purchasing cost would not make this saw as marketable for the companies target market. If these parts were made out of steel or a different metal, the saw would not only cost more but it would dramatically increase the overall weight of the saw. This saw is hand held and making it as light as possible for the consumer is a main objective of the company.

This component of the jigsaw has some unique aesthetic properties, mainly its bright orange and black color. The color has no functional purpose but is very important. Black and Decker is the company who manufacture the jigsaw and their company colors are bright orange and black. When people see this saw they automatically associate it with Black and Decker. Therefore, the color choice is positive advertising for the company. The smooth shiny surface finish on this is for aesthetic reasons.

The housing/outer shell of the saw was manufactured using the process of injection molding. The geometry of the housing suggests that this piece was not milled, as it would be far too expensive and time consuming. Also, there are marks on the inside of the shell from where the cooled piece was pushed from the mold. This is an indication that the piece was injection molded. The housing is made from plastic, a material that can be easily melted and formed. For this reason, injection molding was chosen as it can be used to produce a large volume of parts at a much lower cost than other manufacturing methods. Rather than the shape of the part determining the manufacturing method, it is more likely that it was designed to facilitate injection molding for reasons mentioned above. The only two factors that are really applicable in the selection of manufacturing method are the economic and environmental factors. While the machines and molds are expensive, in the long run, injection molding is a much cheaper alternative to milling for example. The operation of injection molding machines does not require skilled labor, cutting down on labor costs. As for environmental concerns, injection molding creates much less waste than a subtractive process would.

Specs:

Dimensions

  • Lenght: 7.5 inches
  • Width: 3 inches
  • Height: 7 inches

Weight

  • 2.8 lbs

Counterweight

Figure 13: Counterweight Click to View

The shape of this component is rectangular with trapezoidal features on the top. This part is 3 dimensional due to its depth but is two dimensional in nature. The holes that are drilled in the counterweight go all the way through the part and the front and back are the same. This component is axial symmetric.

The function of this part is to help stabilize the armature assembly, which spins rapidly causing the translational motion. The width of this component is the only part of the shape that contributes to its function. The counterweight is made out of steel and manufacturing did come into effect when it came to designing this component due to how easily this part could be machined. It is made of steel, which is not the easiest metal to machine, but it is easier than other metals. This material was chosen for the inherent weight of steel. The counterweight must be able to counter the forces created by the speed of the armature assembly and withstand the heat produced inside of the saw. The heat is produced by the gear adjacent to this component, which is attached to the motor.

When deciding what material to use economic and environmental factors were considered. The specific component needed to be heavy and there are heavier denser metals out there which could have save space by making the component smaller yet the same weight but steel is a cheap metal that is very common and still performs its function. Throughout the life of the saw this component shouldn’t have any problem doing its job but when this saw can no longer be used the pieces of this saw can be recycled. Steel is a metal that is recycled regularly and can be melted down and reused for something else making this particular component more environmentally friendly than if other metals were used.

The only aesthetic property of this component is its smooth shiny finish and it is grey the color steel. The aesthetic factor is of lesser importance since the component is never seen unless the saw is taken apart.

The counterweight was manufactured using the process of machining. The corners are sharp and the faces are smooth. It also does not show signs of being cast or stamped, leaving machining as the probably method. Before machining, it is likely that the steel was rolled to achieve the proper thickness. The counterweight is made from steel, a metal that responds well to machining. The geometry of the part is essentially two dimensional. This means that it can be machined fairly easily, without having to accommodate complex curves. The main factor that impacted the selection of this manufacturing process was economic. Machining the part is cheaper than casting it and for this reason, it was chosen.

Specs:

Dimensions:

  • Length: 3.375 inches
  • Width: 1.75 inches
  • Thickness: .25 inches

Weight:

  • 1.4 lbs

Field Assembly

Figure 14: Field Assembly Click to View

The shape of the field assembly is circular with two flat sides. This part is three dimensional and axial symmetric.

The shape of this component plays a major role in its function. Its function is to create a magnetic field, which causes the armature assembly to spin when put inside of this field assembly. The circular shape helps create a consistent magnetic field all the way around the inside of the round component.

This component is made out of aluminum on the inside and outside and has coils of copper wire on the inside of the aluminum. Aluminum is not a difficult metal to machine, which may have been a manufacturing consideration, but the aluminum was used because it is not magnetic. In order to function properly, the metal used cannot be magnetic. The copper wire used for the coils is also a specific material required for the saw to function. Both metal choices, aluminum and copper, are essential to the functionality of the saw.

As this is a key component to the saw, functionality became the most important factor. Without the field assembly, the saw would not operate.

This component is shiny and silver and bronze the color of the aluminum and copper which is aesthetically pleasing but is more for the function of the material than a designed aesthetic property. There is a smooth shiny finish with the two flat sides having machine marks on them but the finish only for its function. When the armature assembly is placed inside of the field assembly there cannot be friction due to the speed of the spinning assembly. Also the two flat sides with machine marks assist in holding this component in place not allowing it to move.

The field assembly was manufactured using the process of machining. Evidence of this includes machining marks on the piece. It is made from aluminum, a soft metal that is also easily machined. The field assembly contains grooves and cavities which indicate it may have been first turned on a lathe to achieve the round shape. The manufacturing process in this case was dictated by the material above all else. It is also possible that it was machined rather than cast to avoid any structural inconsistencies. This would fall under the societal category because it is important that customers see the product as solidly built and not prone to manufacturing defects.

Specs:

Dimensions:

  • Length: 2 inches
  • Circumference: 8.75 inches

Weight:

  • 3.2 lbs

Thrust Plate

Figure 15: Thrust Plate Click to View

This component is rectangular with rounded corners. It is very thin making it mainly 2 dimensional and this part is also axial symmetric. The shape of this part couples with the function of this component.

This component attaches directly to the armature assembly and it is the divider between the thrust plate and armature assembly. The shape is very similar to the thrust plate and identical to the shape of the assembly it attaches to making the shape important.

The material of the thrust plate is aluminum and the manufacturing decision made when choosing this material was the strength of the metal and how easy it would be to machine and make. This component could have been made out of a different material so there is no specific property of this material that is needed for it to function. The only property needed for this to function is to be able to withstand some heat and a little bit of pressure from movement.

A societal factor was considered when choosing this material due to trying to save weight and keep it light for the consumer. Also economic factors were considered, because it wouldn’t cost as much to produce many of these components as opposed to using a different material.

The aesthetic properties of this component are the shiny smooth finish that doesn’t aid in the function of it but makes it look sharp. It is the color of the aluminum but is not seen by the consumer during everyday use so doesn’t serve any specific purpose.

The thrust plate was manufactured using the process of stamping. Before stamping, the material is first rolled into sheets. Stamping is a method that can produce large volumes quickly and cost effectively. For this reason it is most likely the process used. The piece is very thin, allowing it to be stamped. The geometry of the piece did not dictate the method of production, however it allowed the preferred method to be used. The decision to use stamping was above all else, an economic decision as it allowed for lowered production costs.

Specs:

Dimensions:

  • Length: 2.5 inches
  • Height: 1.25 inches
  • Thickness: .1 inches

Weight:

  • .15 lbs

Gear

Figure 16: Gears Click to View

The shape of this gear is circular with many teeth around the outside and one round pieces of metal attached on each side. It is a 3 dimensional part that is axial symmetric other than the one small metal piece on the backside. The shape is the most important aspect to the function of this component.

The function of this gear is to turn the rotational energy from the main shaft to translational energy and transfer it to the shaft assembly. The teeth on around the outside of the gear fit with the teeth on the end of the main shaft attached to the motor, which rotates the gear. The metal piece on the side fits into the shaft assembly that holds the blade and moves the shaft assembly up and down rather than the same direction the gear moves.

This component is made out of steel, which can be manufactured fairly cheap and effectively. This specific component must be made of a fairly hard durable metal due to the friction from coming into contact with the main shaft and shaft assembly. It must withstand the heat caused but such friction. The main factor considered with this component was economic due to how much this part would cost to produce.

The aesthetic properties of this gear are of a secondary importance to its functionality. It is the normal grey color of steel with a smooth finish that serves a functional purpose to help reduce friction among all of the moving parts associated with this component.

The gear was manufactured using the process of machining. Again, steel is a durable metal with the ability to be machined. All faces and corners have been cleanly cut, indicating it was in fact machined. Because of the geometry of the gear, it is unlikely that any other process was used. For large volume production, this method is more cost effective than die casting for example. This would be considered an economic factor.

Specs:

Dimensions:

  • Diameter: 1.5 inches
  • Thickness: .375 inches

Weight:

  • .12 lbs

CAD Parts and Assembly

Autodesk Inventor Professional was chosen to CAD the components in Gate 3. This program contains the ability to replicate individual components as well as create an assembly of multiple parts. An exploded view was also made that clearly displays how the components fit together.

Table 3-2: CAD Modeling File:Total parts.zip


Component CAD Image
Counterweight
Figure 17: CAD counterweight, click to view


Thrust Plate
Figure 18: CAD Thrust plate, click to view


Gear
Figure 19: CAD Gear, click to view


Spring
Figure 20: CAD Spring, click to view
Guide
Figure 21: CAD Guide, click to view
Support
Figure 22: CAD Support, click to view
Connection
Figure 23: CAD Connection, click to view






The following is a image of the assembly where all the individual components previously displayed are fit together.

Figure 24: CAD Assembly, click to view
The exploded view gives an accurate representation of the order in which the parts fit together.
Figure 26: Exploded view, click to view

Engineering Analysis

While designing the jig saw, a key function that would be subjected to the design process is the motor and its angular velocity. The engineers that design the saw have to find a motor with the proper angular velocity, revolutions per second, to give the jig saw the necessary oscillations for a given voltage. Each full rotation is equivalent to a single oscillation.

After stating that they need to find the angular velocity given a voltage, a simple schematic diagram would be drawn. A simple circuit consisting of an input voltage, V, that would cross a resistor, R, an inductor, L, and the motor, Kmech. The voltage input would create the motor to rotate giving the angular velocity, omega. Summing the voltages using Kirchhoff’s Voltage Law, the first order linear differential equation of two variables, the current, i, and angular velocity, omega is stated as:

R * i + L * di/dt +Kmech * ω = V

If the motor is spun without a voltage input, a current is created. Summing the moments in the system creates the second equation. The moment of inertia, J, is multiplied by the angular acceleration, time derivative of omega. There exists some friction so we say that it is linearly proportional to omega, h. The current created is linearly proportional as well denoted by Kcurrent. After summing the moments a second linear differential equation appears as a function of the same two variables.

J * dω/dt +h * ω = Kmech * i

These two equations can be set together in matrix notation, and solved using matrix exponentiation.

[dx/dt] = A * [x] + [b]

Where A is the square coefficient matrix, x is the vector of the unknown values, current and angular velocity, and b is the input voltage vector. Initial conditions will be known and used in solving for the two constants of integration.

Using this equation the engineers doing the analysis can numerically calculate the angular velocity delivered by the motor given an input voltage. This is assuming that Kcurrent and Kmech are found experimentally and with precision.

Design Revision

There exists a small hole in the forward top section of the jigsaw. After dissection and inspection of this area it appears to be a void. This is assuming all pieces are present. The small hole could be contributed to a button that would hold the trigger down during prolonged use. But, without the actual pieces, we must assume the area to be wasted. If, in fact, the area should be empty, the jigsaw should be redesigned. A new mold should be struck, perhaps of minor to considerable initial cost. The new design should move the trigger back and round the front. This has an economical advantage in that it would reduce the amount of plastic needed for this particular saw. Rounding the front has an ergonomically benefit as well; it would make guiding the saw simpler from behind the cutting tool instead of directly above it. This modification can also be directly related to the safety of the user, working from behind the cutting tool takes the user out of the direct line of debris if kicked upwards.


Another safety consideration that could be considered is the guard. The guard is located directly in front of the blade and above the shoe assembly. Its main purpose is to prevent unwanted material from entering the blade area. The design as it stands now does just that; but it could be improved upon. The blade guard should also wrap around the sides of the blade to stop things from entering from the side. This type of guard might have been considered but deemed impractical for serviceability. If the guard was a solid piece that wrapped around the front of the blade, the saw would have to completely need to be disassembled, almost completely in order to change the blade. This being said, the saw should be concealed more fully. Ideally with a locking mechanism that is located by the side of the saw that would be easily locked and slightly more complexly unlocked, in order to stop it from inevitable opening while in use.


A disadvantage of this jigsaw is in part are the safety issues, but also the cord. Using the saw is assuming that there is a constant and seemingly unlimited production of electricity at a six foot radius from where the saw is needed, six feet is about the length of the cord. By virtues of ergonomics alone, the jigsaw should have wireless capabilities. It would be of great convince to the user to be able to be plugged in to a constant input and when needed be able to detach the cord and run solely on battery. Such a modification has the capability of being of great utility. The ability that it would gain was weighed against the ease of completing such a task, and presumably the weight that an internal battery would add. In theory, what needs to be changed is trivial, a power converter is needed and a place to store the power. In practice, the power storage could result in a thirty to fifty percent increase in weight making it less easy to wield. Making the jigsaw cordless is a possibility, but such a convenience of free movement for a period of time is not necessarily balanced by the excess cost and weight. For the cost of making the jigsaw have a cordless ability could be equally applied to making the cord considerably longer.