Group 33 - Skil Circular Saw
Sehwan Jun : Group Manager Responsibilities include a central contact for the group as well as scheduling and work assignments.
Brian Mitrowitz : Publication Manager Responsibilities include taking of pictures and video and editing information to be viewed on the Wiki page.
Jane Pattison :Technical Writer Responsibilities include the writing of technical papers related to the product.
Yukie Furukawa : Organization Expert Responsibilities include the organization of parts and paperwork.
Tomoaki Furukawa : Dissection Expert Responsibilities include the disassemble and measuring of parts of the Skilsaw.
We are dissecting a Skil saw model 5400 circular saw. Its primary use it cutting relatively thin wood such as sheets of plywood or boards up to its maximum cut depth of 2-1/2 in. With a suitable blade it can also be used to cut other materials such as plastic, veneer, and metal. This model is primarily targeted at non-professional users.
The saw uses a small motor to convert electricity from a standard wall outlet into mechanical energy to drive a shaft on which the saw blade is mounted. It is not currently working; we think either the switch is broken or the carbon brushes in the motor are missing. Most of the visible outer parts of the saw are plastic, including the handles, housing, and the covering of the power cord. The guards are aluminum. The base plate and blade mount are steel. It also has a rubber stop that keeps the rotating guard from turning too far when it's released. Internally, the motor and power cord contain copper, and the motor uses carbon brushes.
This is not a very complex product; it uses a common mechanism (the electric motor) to perform a single task (running a saw blade). It doesn’t have a lot of settings or alternative functions. According to the manufacturer’s parts list, it has 57 components, which includes all of the fasteners and the individual parts of the motor. None of these components are complicated.
Under normal use, this product does not require regular maintenance; under unusually heavy use it might be necessary to replace the brushes in the motor occasionally. It is solidly constructed, and unlikely to need frequent repairs.
There are a large number of very similar hand-held saws on the market. This model runs from about $25 to $60, which is at the bottom end of the price range. It has a definite advantage in cost; some of the alternatives cost over $200. All of them have similar safety features. The engine used in this product has a good weight-to-power ratio, but is noisy and inefficient. The more expensive alternatives are frequently packaged with extra blades (this one is sold with a single blade suitable for wood), or have extra capabilities for cutting other materials. There are several cordless alternatives, which have the advantages that they can be used without easy access to an outlet, and the user can't trip over the cord, but they are also considerably more expensive, and are heavier due to the added weight of the battery.
The following are the steps we went through to disassemble the saw. This device is not intended to be taken apart as thoroughly as we did; under normal use only the ½ in bolt and the rings that hold the saw blade in place would be taken off. That being said, it was not exceptionally difficult to disassemble; there are only a couple tricky parts, which are noted below. In the course of taking it apart, we discovered that one of the wires leading from the power switch to the motor has been disconnected from the switch; this is probably why the saw doesn't currently work, although there may also be other problems that we didn't find. We used T20 and T27 screwdrivers, a ½ in socket wrench, and a flat head screwdriver in the process. There are only three types of screw used; 2 in (long), 1 in (medium) and ½ in (short). Torx screws are commonly used in automated assembly plants because they resist cam-out better than Phillips screws. The limited number of fasteners simplifies the manufacturing process. We were forced to deviate from our original plan in several places. First, the base plate is attached by a pin, which we were not able to remove. In addition, we discovered that the guards can't be separated from the handles until the small handle connecting the base plate and the curved slot that controls the depth of cut is removed. Each step is rated with a difficulty of 1 to 5, 1 indicating a very simple step, no more complicated than removing some screws, and 5 indicating a very complex step that required several tries or more than one pair of hands. Most of the steps are at the low end of the scale.
1. Remove wing nut and carriage bolt holding the base plate in place; this lets the base plate move out of the way of accessing the blade mount assembly.
2. Remove rubber stop; this allows the rotating guard to move freely and gives access to several screws behind the guard. The stop can easily be removed by hand.
3. Remove the bolt, washers, and rings that hold the blade in place, using a ½ in socket wrench.
4. Remove spring attached to the rotating guard by unhooking it from the fixed guard and pulling it back through the hole in the rotating guard. This allows the guard to move in a full circle, and stops it from springing back into place when released, which makes it easier to access the screws holding the guard assembly onto the housing covering the motor.
5. Access and remove the screws holding the fixed guard onto the housing. There will be 4 long and 3 short screws. At this point, it is possible to pull the housing an inch or so away from the guard assembly, but it can't be removed completely yet.
6. Remove the plastic handle from the rotating guard; it is attached by 1 small screw.
7. Remove the two medium screws holding the back of the housing over the motor and take off the backplate.
8. Remove the 6 medium screws holding the handle pieces together, and separate the two parts of the handle. The step requires a screwdriver with a long, straight shank since two of the screws are set in deep holes; a screwdriver with interchangeable bits will not work because the collar is too wide to fit.
9. Remove the red handle in the curved slot that controls the depth of cut. This is the last fasteners holding the guard assembly and the housing together; once this is removed, the guard assembly and the shaft that the blade mounts on can be separated from the motor. The handle is held on by a snap ring around the nut, which can be removed with a flat head screwdriver. This is a more complicated step than most of the others because the guards and shaft need to be held in place while the handle is removed.
10. Lift the housing off the motor. This will separate the housing, stator, and base plate from the rotor, shaft, and guards.
11. Once the guards, shaft, and rotor have been separated from the housing and stator, the rotating and fixed guards can be taken apart, and the gear and shaft that the blade mounts on can be separated from the rotor.
12. Remove the two long screws holding the stator in place, and remove the carbon brushes.
13. Pull the stator out of the housing. It will not separate completely from the housing due to the power cord, which runs through a hole in the housing and connects to the stator.
14. Remove the ends of the power cord from the contacts on the stator, completely separating the power cord and housing from the stator. Remove the bronze bearing from the end of the motor housing.
15. Remove snap ring; pull spindle out of rotating guard
Part numbers are from the manufacturer's parts list at http://mdm.boschwebservices.com/MDMCache/English%20%5BUS%5D/t10/0000000/r00753v-1.pdf
1) The saw could be assembled using Philips rather than Torx screws; this would make maintenance easier since Philips screwdrivers are more common than Torx. It would not affect the manufacturing process; automated assembly lines handle Philips screws very well, and should not make the product more expensive. If anything, the Philips screws may be slightly cheaper.
2) Given that the moving parts of the motor in this product are not likely to generate excessive heat, the bronze bearings could be replaced with polymer bearings; these are lighter than bronze, but should fill the same role in this case.
3) The pin that connects the housing and casing foot and controls the angle of cut is very stiff; since the saw is held at the desired angle by a screw and wingnut, this joint could be made with more lubrication or a slightly looser fit. This would make the product slightly easier to use, and would not raise the price or weight.
The pinion shaft transmits torque from the motor to the blade; one end is attached to the armature, and the blade is fastened by a screw to the other end. There is also force applied from the other end of the system, as any force created by friction between the blade and the material being cut. These two forces can be represented by moments applied to either end of the shaft. The moment due to friction on the blade can never exceed the moment generated by the motor: at most, friction can stop the blade from moving; it will never push the blade backward. These forces need to be taken into account when designing the shaft so that it is made of a strong enough material to withstand them without breaking or deforming.
The moments applied to each end of the shaft are at their maximum when the blade has completely stalled while the motor is running. The torque generated by the motor can be calculated from the power output and the angular velocity using this relationship:
where P is the power, T is the torque, and ω is the angular velocity. Note that the power must be expressed in in•lb/s and the angular velocity must be in rad/s; since these are usually given in horsepower and rpm, they will need to be converted.
The shaft should be designed so that it does not twist under the applied torque, so that the power from the motor is transmitted as efficiently as possible to the blade. As a result, the angle of twist should be as close to zero as possible.
The angle of twist is found using this equation:
where Φ is the angle of twist in radians, T is the torque, L is the length of the shaft from one end to the point where the angle of twist is being measured, J is the polar moment of inertia of the shaft, and G is the shear modulus of the material.
If the torques applied to the shaft are equal and opposite, as described above, each end is fixed and the angle of twist will be greatest where the ratio of the distance from the end and the moment of inertia is greatest. Since the shaft is not a constant diameter, this will probably need to be calculated at small intervals all along the length of the shaft to find its maximum value.
In this product, the pinion shaft is steel, which has a shear modulus around 11 x 106psi. This is several orders of magnitude greater than the torque, and should be high enough to minimize deformation of the shaft.
The saw did not work before we took it apart due to a broken wire between the switch and the motor. We did not have the necessary tools to repair the wire, so it is still nonfunctional. We were able to completely reassemble the product using the same T20, T27, and flat head screwdrivers and ½ in hex wrench that were used during disassembly. The flat head screwdriver was primarily useful for straightening some bent contacts so they could be reconnected, and replacing the retaining ring.
In the course of reassembling the product, we found that the switch and electric field need to be reconnected very early in the process, because the wires running from the switch run inside the housing. Since the switch is set into the handles, they are reattached to the housing at the beginning of the process. Unlike the disassembly process, which effectively works from the outside in, the reassembly has to being with these outside components before reassembling the motor.
Each step below is is rated with a difficulty of 1 to 5, 1 indicating a very simple step, no more complicated than placing and tightening some screws, and 5 indicating a very complex step that required several tries or more than one pair of hands.
1. Insert bushing into back of housing.
2.Fit switch and wiring into handle assembly and fit handles together; this is a bit tricky; the wires need to be carefully fitted into the spaces inside the handle, or the parts will not fit together correctly. Use 1” T20 screws to fasten handles together. At this stage, only fasten the 4 screws that hold the handle sections together; not the two that attach handle to the housing.
3.Feed the wires that run from the switch through the hole in the housing.
4.Connect the wires from the switch to the field, making sure not to connect them to the contacts used by the carbon brushes.
5.Insert the field into the housing. This is quite difficult; it is a tight fit, especially with the wires running through the housing next to the field.
6.Insert and tighten the two 2” T20 screws that fasten the field in place in the housing.
7.Put the flat washer and one curved washer on the end of the armature shaft that goes into the field; insert the armature into the field and seat the end of the shaft in the bushing.
8.Insert the carbon brushes. This must be done after the armature is in place, or they get in the way. This requires careful placement and some pressure to seat the contacts fully.
9. Attach the housing cover using two 1” T20 screws.
10.Place and tighten the two 1” T20 screws that hold the handles and housing together.
11.Put the second curved washer on the end of the armature shaft, then fit the protective cover onto the shaft; the armature goes through the top hole in the cover.
12.Fasten the cover to the housing using three 2” T20 screws. At this point, only use the 2” screws, not the three ½” screws around the bearing flange.
13.Insert the pinion shaft into the socket in the protective cover, then slide the bearing flange over the end of the shaft. Make sure that the flattened side of the flange lines up with the flat edge of the cover.
14.Fasten the bearing flange in place using three ½” T20 screws.
15.Feed the spring through the hole in the rotating guard, put the guard on the bearing flange, and hook the spring onto the protective cover.
16.Fit the retaining ring over the end of the bearing flange.
17.Put the spacer bolt into the stop, then attach the stop to the protective cover using the 1¼” T27 screw.
18.Slide the supporting discs onto the end of the pinion shaft.
19.Tighten down the ½” hex screw that fastens the supporting discs.
20.Put the lever on the rotating guard; it fits over a pin. Fasten it on using one ½” T20 screw.
21.Slide the round-head bolt through the angle control slot on the foot and the housing, and fasten it using the wingnut.
22.Thread the nut onto the screw that runs through the depth-control slot from the cover, put the lever handle on the nut, and attach the snap ring that holds the lever in place.
23.Place the wrench back into the slot in the foot.
Our team dissected a Skilsaw 5400 circular saw, which is an inexpensive handheld saw targeted at home users. The following report describes the procedures and tools used to dissemble and reassemble the saw, and contains a component list describing all of the parts and materials used and several design recommendations that we think would improve the product. We also include an analysis of the force applied to the pinion shaft.
This is a relatively simple product; it is intended to perform a single function in a straightforward, user-friendly manner. It is also functionally simple; an electric motor connected to a wall outlet turns a shaft, which in turn rotates the saw blade. As seen below, the dissection and reassembly procedures are straightforward and do not require any tools more specialized than a set of Torx screwdrivers.