Group 30 - Ryobi Contractor's Saw
Skil 2.3 HP Circular Saw
Our group was assigned to analyze the physical and operational characteristics oh a 2.3 hp Skill circular saw. This report will include a description of the overall design concept and manufacturing processes that were used to construct this saw. Additionally, we will examine the materials used and the overall technical concepts that were implemented to make the operation of this component possible and make assumptions based on knowledge we gained this semester to explain why many of these concepts were used.
We began simply by brainstorming assumptions of the basic concepts of how the saw worked and what components and materials we could expect to come across upon disassembly. During the disassembly we identified each component and considered its purpose. This process took approximately 45 minutes.
The saw can be broken down into two different sub-assemblies that are coupled together via a reduction gear and a few 1 7/8 in. 30 Torx screws. The first sub-assembly is the 12 Amp electric motor and its casing. The motor drives a shaft that is coupled to the saw assembly which is comprised primarily of mechanical components.
-30 Torx driver
-20 Torx driver
-Ball peen hammer
-3/4 Combination wrench
-Needle nose pliers
1) Using a 30 Torx driver we removed the 1-7/8 in. screws that attached the saw handle to the motor assembly as it interfered with the rest of the dissection.
2) Once the handle was removed we detached the motor housing from the saw assembly by removing the remaining 30 Torx 1-7/8 in. screws.
3) At that point we attempted to remove the motor housing, but found that the guide plate interfered.
4) Using a ball peen hammer and a die we tapped the pin that secured the guide plate in its place free.
5) From that point we were able to remove the motor housing and expose the rotor assembly.
6) Additionally, you could see the back side of the shrouded plate, the cog gear and the main support bearings for the saw blade and rotor.
7) The stator could be seen inside the motor housing.
8) Using a 20 Torx driver we removed the vented cap from the end of the motor housing.
9) That step exposed the two 2-1/2 in. 20 Torx screws that held the stator in place as well as the carbon brushes that rode on the commutator.
10) Once the Torx screws that held the stator in place were free we carefully removed the stator and brushes from the motor assembly by using the plyers.
Parts & Functions
The Motor Assembly
The purpose of the stator in an A/C motor is to induce a magnetic field. In this case 120V AC current flows through three different copper conductors, 120 degrees out of phase. This creates the instantaneous peak value of the current to spike a different times, yet continuously. The outcome is a rotating magnetic field. We assume that all of the plastic parts on the stator, the brushes and body, were created using the method of injection molding. An example of the three phase rotating magnetic field principle can been seen at the link below.
To manufacture the stator assembly first the central plastic cylinder would have to be made. We concluded through observations that we made that this was done through injection molding. Once the central cylinder was constructed, three individual copper coils were set in place on the cylinder with 14 gauge copper wire. The purpose of this is to create three separate phases for the magnetic field. Then two steel rings were adhered to the inner side and the outer side of the plastic cylinder.
The primary function of the rotor is to create rotational force to drive the saw blade. This is accomplished by inducing a magnetic field in the armature that opposes the rotating magnetic field in the stator. Polarity in the armature is generated by inducing a DC current though generator action in the commutator and carbon brushes (further discussion below).
Torque is created when the magnetic poles of the armature try to line up with the varying poles of the stator. That torque is transferred directly to the rotor for the saw blade via a pinion and cog gear arrangement. The main shaft or axel of the rotor was made by machining and polishing cold rolled steel in order to insure strength and a uniform composition. A few of the parts on the rotor are plastic which most likely means that the parts were made by the method of injection molding. We believe that the remaining small intricate metal parts are made from die casting.
A fan is attached to the axel of the rotor to remove any heat that is generated in the motor. As an auxiliary function the fan also helps to prevent carbon dust from building up on the commutator. We suspect that the fan is created by a combination of both machining and forming due to its shape and slots that must be cut out.
The Commutator and Brushes
The commutator is simply a series of copper plates adhered to a cylinder at the end of the rotor (shown above). As mentioned above the purpose of the commutator is to induce polarity in the armature by generating a DC current. Generator action is achieved through static friction due to relative motion between the brushes and the copper plates. In the image above you can see carbon residue from where the brushes previously rode on the commutator.
The Motor Housing
The stator and rotor are housed by an injection molded lightweight plastic casing. The casing is consists of two different pieces that are screwed together by two 1 ½ inch Torx machine screws. The casing assembly is then screwed to the motor housing and maintains proper alignment of the rotor shaft and the cog gear of the saw assembly.
We can assume that this design was used to promote durability, while maintaining a lower cost of assembly. Torx heads on the screws were probably used so the final assembly process could be automated.
The Saw Assembly
Coupled to the motor assembly is a shrouded plate in which the saw blade rotates parallel to. The shrouded plate appears to be cast molded from aluminum. For additional safety a second shroud, this one being spring loaded, is pressed into the stationary portion of the saw assembly’s main journal bearing. The saw blade (not provided) rides within this shroud.
While the saw is not in use an extension helical spring holds the protection shroud in place. When using the saw, the operator will push the saw over a horizontal surface. To maintain stability the saw rides on a steel guide. As the user applies force to move the saw forward, the surface that is being cut will offer opposition to both the user and the spring loaded shroud, overcoming the spring force holding the shroud in place causing it to rotate approximately 180 degrees about the saw blade. When the cutting operation is complete the helical spring will snap the shroud back into place.
The arbor size is based on the internal width of these shrouds for where a safe clearance exists for free rotation of the cutting tool.
Bearings and Gears
Previously it was stated that the motor assembly was coupled to the saw assembly via a pinion and cog gear arrangement. As you can see in the image of the rotor gear teeth are machined into the end of the rotor shaft forming a pinion. The gear arrangement produces a single reduction stage stepping down the rotation speed of the motor to a safe and effective rotational speed (4600 rpm) for the saw.The gear ratio is approximately 3.3:1*
The cog gear appears to be machined from either brass or a brass alloy.
The guide plate is used to maintain the stability of the tool. The operator can set the saw to cut at a certain angle by locking the guide plate into position. By locking it into position, the guide now ensures a clean uniform cut into the surface. The guide plate is equipped with a pointer that the users uses to guide the saw from above. It is set at the front of the guide plate and is used to make sure that the saw is moved in the desired direction.
Technical Drawings & Animations
In this section we will focus on how all the moving internal parts of the saw are integrated to work together. Below is a technical drawing of the gear and journal assembly(1), the bushing housing(2), the blade(3), the bushing(4), the fan spline and rotor assembly(5), the nut(6) and spacer disc(7).
Shown below is an animated drawing that shows the rotation of the rotor assembly and fan spline, cog gear and saw blade while resting in the main journal bearing and bushing assembly for the saw assembly.
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Shown below is an animated drawing that displays the assembly of the mechanical portions of our project. The process below begins with the Cog gear engaging with the fan spline at the end of the rotor assembly. Then the bushing housing slides over the end of the cog gear. The cog gear and bushing are stabilized and rotate freely within the bearing. Next one half of the spacer slides over the square milled tip of the cog gear. The saw blade then slides in place and is retained by the other half of the blade spacer and a central nut.
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Recommended Design Changes
In the image below you can see the top view of the motor assembly after its removal from the frame plate of the circular saw. From above you can see the shape of the housing and a clear view of the fan.
Upon reassembly of our product, we noticed some major mechanical concerns. It seemed as thought the saw did not fit perfectly in slide as imagined. It seemed to rub against the outer shell causing scratches. We suggest that the case be a little bigger to insure that wear and tear doesn’t happen to these products in the future. If this wear and tear were to continue, our saws lifetime expectancy would be diminished to a few uses.
For this reason we think that it would be a better design to maintain symmetry in the motor housing by making it completely round, allowing reasonable clearances for the fan to rotate.
In the image below you can see a side view of the top of the motor assembly with the vent cover removed.
We noted that once the vented cover was removed from the motor housing that there were two clips on the cover that lined up with the brushes to hold them in place. Once the cover was removed the brushes had a considerable amount of play in them (shown in the image above). Even with the cover securely installed there was no lateral support to keep the brushes in place while the commutator rotated. This could cause uneven wear on the brushes. Additionally, any slop between the brushes and commutator could effect the magnetization of the armature. For these two reasons we would recommend a change in design to include metal clips that would wrap around each brush individually and snap into place directly onto the motor housing for further support.
- Gear ratios were calculated based on gear and pinion diameters