Group 33 - Skil Circular Saw
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.
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.
Part numbers are from the manufacturer's parts list at http://mdm.boschwebservices.com/MDMCache/English%20%5BUS%5D/t10/0000000/r00753v-1.pdf
Solid works was chosen as the modeling program due to the availability and familiarity of it to Brian Mitrowitz.
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.