Group 33 - Skil Circular Saw

Team Members

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.

Preliminary Appraisal

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.

Product Dissection

Dissection Notes

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.

Dissection Procedure

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.

Difficulty 1

Angle lock wing-nut.

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.

Difficulty 1

Stop to prevent the rotating guard from going around completely.

3. Remove the bolt, washers, and rings that hold the blade in place, using a ½ in socket wrench.

Difficulty 1

Bolt and spacers for the rotating blade.

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.

Difficulty 3

Spring in the fixed location.

The spring that holds the guard in the closed posistion.

Showing the coil of the spring used as a seat to hold the spring in position.

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.

Difficulty 3

Location of the screws above the shaft.

Location of the screws below the shaft.

Length of the Screws.

6. Remove the plastic handle from the rotating guard; it is attached by 1 small screw.

Difficulty 1

Lever that locks the depth of the blade.

7. Remove the two medium screws holding the back of the housing over the motor and take off the backplate.

Difficulty 1

Cover over the outside bearing and electrical brushes.

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.

Difficulty 2

Location of the screws holding the handle together.

Screws holding the handle together.

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. Difficulty 5

10. Lift the housing off the motor. This will separate the housing, stator, and baseplate from the rotor, shaft, and guards. Difficulty 4

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. Difficulty 2

12. Remove the two long screws holding the stator in place, and remove the carbon brushes. Difficulty 3

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. Difficulty 2

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

Product Components

Part numbers are from the manufacturer's parts list at http://mdm.boschwebservices.com/MDMCache/English%20%5BUS%5D/t10/0000000/r00753v-1.pdf

Part name: Housing

Part number: 2 610 912 983

Material: Plastic

Manufacturing process: Injection molding

Function: Encloses the motor, covering all of the moving and electrically active parts of the saw's internal workings

Complexity: Simple; this is a single molded part

Comments: Plastic is used here because it is a lightweight, easily molded, and cheap material. It is also non-conductive, so the housing protects the user from a short-circuit or other electrical malfunction in the motor. The part is injection-molded; this manufacturing process is suited to plastics, and is very good for producing large numbers of identical parts for mass-produced items like this one. This part is not subject to any forces, except at the bracket that attaches to the foot, which has a small force of less than 10lb applied to it from the weight of the saw.

Part name: Field

Part number: 2 610 341 367

Materials: Copper and iron wire, ceramic

Manufacturing processes: Wire: drawing; ceramic: casting

Function: Field magnet; produces the magnetic field required for the electric motor

Complexity: Moderate; part contains several different components

Comments: This part is made with a ceramic form, wrapped in iron wire on the outside and copper wire on the inside. When connected to a source of electricity, the coils of wire are used to create a magnetic field. The ceramic acts as an insulator between the two sets of conductive wires. The wire is made by drawing, which is the most common method of manufacturing wire. The ceramic piece is molded; as with the plastic parts, molding is a preferred method of making very large numbers of identical parts for mass-produced products such as this one. In this case, the motor is also used in other power tools on the same platform, so efficiently producing the parts in large volumes is desirable. This part is not subject to any forces.

Part name: Armature

Part number: 2 610 341 306

Materials: Steel, copper wire, plastic

Manufacturing processes: Wire: drawing; plastic: injection molding; steel: turning and casting*

Function: Interacts with the stator to create the torque required to turn the shaft

Complexity: Complex; part contains several different components that must be very carefully fitted

Comments: This is the rotor of the electric motor. It uses a coil of copper wire to convert the electrical energy from a wall outlet into mechanical energy to turn the saw blade. Copper is used because it has very good conductivity, and in both parts of the motor solid wire is used rather than stranded, since flexibility is not a concern. The coil is wrapped around a plastic core; plastic is cheap, easily molded, and lightweight, all of which are desirable characteristics here. It is also nonconductive, so no current is carried between the coil and the steel shaft. The plastic part is injection molded; this manufacturing process is suited to plastics, and is very good for producing large numbers of identical parts for mass-produced items like this one. The shaft, which transmits torque, is made out of steel for its strength. The shaft is turned, which provides a smooth shaft and reduces friction, and the gear teeth on the end are machined. The fins are cast as a separate part and force-fit onto the shaft. The shaft is subjected to all the torsion generated by the motor, which can be on the order of 5,000lb. Under normal use, the shaft should not experience any axial force, but this may occur if the saw blade is not lifted straight out after making a cut, or if the end of the saw accidentally strikes against something. In any case, this axial force is not likely to exceed 50lb.

Part name: Switch

Part number: 2 610 917 325

Materials: Plastic, copper

Manufacturing processes: Plastic: injection molding; copper: casting

Function: Controls whether the tool is on or off; contains safety features so that the switch must be held closed for the product to function

Complexity: Moderate; contains several components

Comments: The outer casing of the switch is made out of molded plastic; this material is used because it is suited to molding into small parts, and is nonconductive, so it insulates the user from the conductive circuit inside. The conductive parts of the switch are copper, which is used for its good conductivity. The connection inside the switch is subject to a small force (about 1lb) from the pressure of the user's finger. This force needs to be maintained to keep the circuit closed.

Part name: Mains connection cable

Part number: 2 610 915 148

Material: Copper wire, plastic insulation

Manufacturing process: Wire: drawing; insulation: extrusion

Function: Conducts electricity from a wall outlet to the switch

Complexity: Simple; this is simple copper cable wrapped in plastic insulation Comments: Copper wire is used because of its conductivity. Since the cable needs to be flexible, it uses stranded rather than solid wire. The insulation is flexible plastic; this is a cheap, nonconductive, and easily molded material. This part is not designed to take any force; the wire and insulation may break if too much tension is applied to the cable.

Part name: Bearing sleeve

Part number: 2 610 356 680

Material: Bronze

Manufacturing process: Sintering

Function: holds the shaft of the armature where it passes through the protective cover; protects the cover and shaft from each other

Complexity: Simple; this is a single cast part

Comments: The bearings in this product are made from sintered bronze. Bronze has very little metal-on-metal friction, and the porosity of sintered bronze allows a lubricant to be trapped in the part. These properties let the shaft spin freely in the bearing with very little friction. It is a very tough metal, and protects the aluminum guard from wear from the rotating steel shaft. This part has a friction force due to the rotation of the shaft and the weight of the armature and blade applied to it; this force is less than 5lb.

Part name: 8 1in T20 screws

Part number: 2 610 341 364

Material: Steel

Manufacturing process: Thread rolling

Function: Hold the exterior parts (housing, handles, and housing cover) together

Complexity: Simple; these are a standard type of screw

Comments: These screws hold the two halves of the handle set together, and are also used to attach the housing cover. These are self-tapping screws, so the holes that they fit into do not need to be threaded, which makes manufacturing the plastic housing and handle components easier. The length of the screws is determined by the parts they connect; these 1in screws are used in the thickest parts of the handles. There may be small shear forces, less than 5lb, applied to these screws as the components they are fastening shift, and there are very small forces applied to the threads, on the order of 1lb.

Part name: 2 washers

Part number: 2 610 313 808

Material: Steel

Manufacturing process: Casting

Function: Cushions the

Complexity: Simple; these are a standard fastener

Comments: These two washers are slightly curved, and are used on the shaft of the armature between the fins and bearing sleeve at one end, and the flat washer and bearing at the other. The curvature and stiffness provide a spring effect, which protects the components from any unwanted lateral forces, and also serves to dampen any vibrations from the motor. Steel is used here because it has a high stiffness and will absorb considerable amounts of force. Under normal use, the shaft should not experience any axial force, but this may occur if the saw blade is not lifted straight out after making a cut, or if the end of the saw accidentally strikes against something. In any case, this axial force is not likely to exceed 50lb.

Part name: 1 flat washer

Part number: 2 610 341 465

Material: Steel

Manufacturing process: Casting

Function: Distributes force

Complexity: Simple; this is a single common fastener

Comments: This washer is placed between the curved washer above and the commutator on the armature. It distributes any forces that the armature is subjected to over a larger area to avoid damage to the armature.

Part name: Protective cover

Part number: 2 610 312 398

Material: Aluminum

Manufacturing process: Casting

Function: Covers the moving blade to protect the user

Complexity: Simple; this is a single cast part

Comments: Aluminum is used here because it is tougher than plastic, and less likely to be abraded or weakened by use, but it is lighter than steel. It is also both rigid and not brittle; the cover should not bend or break under any normal use. Casting is the easiest way to get the necessary shape; the part needs to be curved to fit over the blade, and has rounded corners as a safety feature. Casting also allows the part to be easily mass-produced. The cover has a protective coating on it to protect the metal from being scratched or abraded. The socket that the gear on the pinion shaft fits into is heavily lubricated to reduce friction.

Part name: Bearing flange

Part number: 2 610 341 347

Material: Steel, with a force-fit bronze bearing

Manufacturing process: Steel: casting; bronze: sintered

Function: holds the shaft where it passes through the protective cover; protects the cover and shaft from each other

Complexity: Simple; this is two simple shapes force-fitted together

Comments: This part fits into the guard plate where the pinion shaft passes through it, and provides a bearing for the shaft. The bearings in this product are made from sintered bronze. Bronze has very little metal-on-metal friction, and the porosity of sintered bronze allows a lubricant to be trapped in the part. These properties let the shaft spin freely in the bearing with very little friction. There is a force of less than 1lb applied by the weight of the blade pressing the shaft against the bearing, and an equally small friction force between the bearing and the shaft.

Part name: 4 ½in T20 screws

Part number: 2 610 341 365

Material: Steel

Manufacturing process: Thread rolling

Function: Hold the protective guard and housing together

Complexity: Simple

Comments: These screws are used to fasten the protective cover to the housing, and the lever to the guard plate. These are self-tapping screws, so the holes that they fit into do not need to be threaded, which makes manufacturing the housing and guard components easier. These are the shortest screws used in the product, and are used where a longer screw would interfere in the mechanism. The screw fastening the lever to the guard plate, in particular, must be shorter than the combined thickness of the lever and guard to avoid scratching the blade. There may be small shear forces, less than 5lb, applied to these screws as the components they are fastening shift, and there are equally small forces applied to the threads to keep them in place.

Part name: 2 Supporting discs

Part number: 2 610 341 361

Material: Steel

Manufacturing process: Casting

Function: Holds the blade in place

Complexity: Simple

Comments: These discs are magnetized so that they stick to the steel saw blade to hold it stable while the hex screw that holds the blade in place is being tightened. As a result, they need to be made out of a ferrous material, and steel is used because it is more resistant to corrosion than iron. Casting is the easiest way to get the necessary shape, and allows the parts to be made in large numbers for mass production. There is a compressive force applied to the discs by the hex screw and the guard plate, but it is less than 5lb.

Part name: Housing cover

Part number: 2 610 916 204

Material: Plastic

Manufacturing process: Injection molding

Function: Covers the back of the motor

Complexity: Simple

Comments: Plastic is used here because it is a lightweight, easily molded, and cheap material. The part is injection-molded; this manufacturing process is suited to plastics, and is very good for producing large numbers of identical parts for mass-produced items like this one. The only forces applied to this part are the small (less than 1lb) compressive forces applied by the screws that attach it to the rest of the saw.

Part name: 2 Connecting cables

Part number: 2 610 341 377 and 2 610 916 885

Material: Copper wire, plastic insulation, brass contacts

Manufacturing process: Wire: drawing; plastic: extrusion; contacts: forging

Function: Connects the switch to the stator

Complexity: Simple; these are both simple insulated wires

Comments: These are two small wires that conduct electricity from the switch (when the circuit is closed) to the field generator in the motor. Copper is used for its conductive properties, and in this case stranded wire is used for flexibility. The insulation is flexible plastic; this is a cheap, nonconductive, and easily molded material. The contacts are brass rather than copper because it is stiffer, and therefore less likely to bend and lose contact. No force is applied to these connecting wires.

Part name: Guard plate

Part number: 2 610 916 428

Material: Aluminum

Manufacturing process: Casting

Function: Covers the moving blade to protect the user

Complexity: Simple; this is a single cast component

Comments: Aluminum is used here because it is tougher than plastic, and less likely to be abraded or weakened by use, but is lighter than steel. Casting is the easiest way to get the necessary shape; the part needs to be curved to fit over the blade and has rounded corners as a safety feature, and allows the part to be made in large numbers for mass production. The guard also has a protective coating on it to protect the metal from being scratched or abraded.

Part name: Spring

Part number: 2 610 914 057

Material: Steel

Manufacturing process: Cold winding

Function: Holds the guard plate in place

Complexity: Simple; this is a single simple component

Comments: The spring pulls the guard plate back into place after it has been moved. When the guard has been rotated either to allow access to change blades, or to expose the blade for use, the spring pulls it back until it either contacts the stop or the surface being cut. It is shaped with a splayed end, which stops it from sliding through the guard, and a hook at the other end, which attaches to the protective cover. This part is constantly under a tensile force of less than 2lb. Since all it needs to do is move the guard, which is a lightweight piece, it has a very low spring constant, not more than 1lb•in. This is desirable since a stronger spring would cause the guard to snap back into place with more force, potentially causing injury by catching the user's hand between the guard and the blade.

Part name: Lever

Part number: 2 610 924 015

Material: Plastic

Manufacturing process: Injection molding

Function: Lets the user rotate the guard plate

Complexity: Simple; this is a single molded piece

Comments: This is the handle used to move the guard plate. The lever itself is plastic; weight is not a concern with this part, since it's quite small, but plastic is cheap and easily molded. Also, no great strength is required; the spring has a very low spring constant and does not stretch far, so it does not exert more than about 2lb of force on the guard.

Part name: Retaining ring

Part number: 2 610 341 359

Material: Steel

Manufacturing process: Cold winding

Function: Holds the bearing flange in the cover

Complexity: Simple; this is a single simple part

Comments: This part fits into a groove on the bearing flange and prevents it from slipping through the cover. It is formed from a thin strip of steel wound into a circle; this shape lets it stretch enough to slide over the end of the flange into its groove. There is a compressive force of less than 10lb applied to the part by the flange and the cover.

Part name: 6 2in T20 screws

Part number: 2 610 347 103

Material: Steel

Manufacturing process: Thread rolled

Function: Fasten the stator and protective cover to the housing

Complexity: Simple

Comments: These screws are used to fasten the protective cover and field generator to the housing. They are self-tapping screws, so the holes that they fit into do not need to be threaded, which makes manufacturing the housing easier. These longer screws are used to provide a more secure attachment to the housing than the shorter screws that are also used for the same parts. There may be small shear forces, less than 5lb, applied to these screws as the components they are fastening shift, and there are equally small forces applied to the threads to keep them in place.

Part name: Casing foot

Part number: 2 610 913 231

Material: Aluminum

Manufacturing process: Casting

Function: Rests on the surface of the material being cut and supports the saw during use

Complexity: Simple; this is a single cast piece

Comments: This part supports the weight of the saw (12lb) during use. Aluminum is tougher than plastic, which is desirable here since the foot may be abraded by rough materials, but is relatively lightweight; this keeps the overall weight of the saw at an acceptable level. Casting allows the part to be made in large numbers for mass production. The weight of the saw and any forces due to kickback, caused when the blade stalls during use, are concentrated in the two slots that connect the handles and housing to the foot, and should not exceed 30lb..

Part name: Round-head bolt and wing nut

Part number: 2 610 017 262 and 2 610 029 562

Material: Steel

Manufacturing process: Bolt: cold-forging; Nut:  ?

Function: Used to connect the casing foot to the housing and adjust the angle of cut

Complexity: Simple; part includes two standard fasteners

Comments: The bolt fits through a curved, vertical slot in the foot and controls the angle of the blade. A wing nut is used here because it can easily be tightened or loosened by hand; this is intended to be user-adjustable, and the nut does not need to be more than hand-tightened. There are friction forces between the bolt head and nut and the slot, forces applied to the threads, and shear forces where the bolt rests against the slot. The first two are small, less than 5lb, but the last may be a large fraction of the weight of the saw, which is 12lb.

Part name: Roller pin

Part number: 2 610 914 058

Material: Steel

Manufacturing process: Casting

Function: Attaches the housing to the casing foot

Complexity: Simple; part consists of a single component

Comments: This pin connects the housing and the foot. The pin needs to support a small force due to the weight of the saw. The pin is not removable; it is fixed in the bracket so that it will not work loose due to vibration when the saw is in use.

Part name: Stop

Part number: 2 610 341 394

Material: Plastic

Manufacturing process: Injection molding

Function: Stops the guard from rotating too far

Complexity: Simple; this is a single molded piece

Comments: The spring pulls the guard back into place when it has been moved, and this part stops it at the correct point, leaving a gap between the edge of the guard and the edge of the protective cover. It is made of a rubber-like, slightly soft plastic so that the impact when the edge of the cover hits it is cushioned; this ensures that the guard is not chipped or cracked when it hits the stop. The force of the spring pulling the guard closed is applied to this part, but that force is less than 2lb.

Part name: Spacer bolt

Part number: 2 610 341 366

Material: Steel

Manufacturing process: Formed from a rectangle of metal shaped into a cylinder

Function: Stops the screw below from running all the way through the stop

Complexity: Simple

Comments: This part fits tightly into the stop and prevents the head of the screw that fastens the stop to the cover slipping all the way through. There is a friction force holding the spacer in place, and a compressive force exerted by the head of the screw and the cover. Both of these are less than 5lb.

Part name: 1¼ in T27 screw

Part number: 2 610 347 100

Material: Steel

Manufacturing process: Thread rolled

Function: Fastens the stop to the protective cover

Complexity: Simple

Comments: This screw fastens the stop to the protective cover. It is a self-tapping screw, so the hole that they fit into do not need to be threaded, which makes manufacturing the cover and housing easier. The length of the screw is determined by the thickness of the stop and cover; to fasten the stop securely it must go through the stop and cover and into the housing. There are small forces applied to the threads of the screw, and a shear force due to the pressure of the guard on the stop; both of these are less than 5lb.

Part name: ½ in hex screw

Part number: 2 610 346 554

Material: Aluminum

Manufacturing process: Thread rolled

Function: Holds the supporting discs and blade in place

Complexity: Simple

Comments: This screw runs through the supporting discs, blade, and guard, and is threaded into the end of the pinion shaft to hold the blade in place. It has a ½ in head, which makes it easy for the user to manipulate when mounting or removing the blade. Steel is used because of its toughness; the threads and head of the screw need to survive being repeatedly tightened and loosened. There are forces applied to the threads due to tightening the screw, and a small shear force due to the weight of the blade; both of these are less than 1lb.

Part name: Bushing

Part number: 2 610 997 773

Material: Bronze

Manufacturing process: Sintered

Function: Provides a bearing for the pinion shaft.

Complexity: Simple; this is a single cast part

Comments: This part is force-fitted into the protective cover, and provides a bearing for the pinion shaft. The bearings in this product are made from sintered bronze. Bronze has very little metal-on-metal friction, and the porosity of sintered bronze allows a lubricant to be trapped in the part. These properties let the shaft spin freely in the bearing with very little friction. Bronze is also very tough, and protects the cover from wear from the rotating shaft.

Part name: Box wrench

Part number: 2 610 968 670

Material: Aluminum

Manufacturing process: Casting

Function: Used by the user to tighten the hex screw that holds the blade in place

Complexity: Simple; this is a single cast part

Comments: A wrench is included with the saw to fit the hex screw that fastens the blade in place. Since it is only intended for use with that screw, it can be a simple tool cast as a single part; casting allows the part to be made in large numbers for mass production. This part is subjected to the torque required to tighten the hex screw, which may involve a force up to 10lb.

Part name: 2 Carbon brushes

Part number: 2 610 993 156

Material: Plastic casing, steel spring, carbon brushes, brass contacts

Manufacturing process: Molding (casing and brushes)

Function: Complete the circuit between the stator and armature in the motor

Complexity: Complex; part contains several components requiring a high degree of precision

Comments: The brushes complete the circuit between the stator and rotor in the electric motor. Carbon is used here because it provides just enough resistance to smooth out the the change in current as the brush moves from one section of the armature to the next. The springs provide enough pressure to keep the brushes against the rotor; these springs and the brushes are subject to very small (much less than 1lb) forces as a result. The brushes themselves are molded, fired, then ground to the exact shape required. The plastic casing is made by injection molding, which is a preferred method of making very large numbers of identical parts for mass-produced products such as this one. Since the same brushes are used by all power tools on the same platform, efficient mass production is desirable.

Part name: Pinion shaft

Part number: 2 610 997 776

Material: Hardened steel

Manufacturing process: Shaft: turning; gear: csating

Function: Transmits torque from the armature to the blade

Complexity: Moderate; this part has two components force-fitted together

Comments: Steel is used for this shaft for its strength; since this part transmits torque from the motor to the blade, it is subjected to more force than most other parts of the saw. It is turned to produce a smooth finish, which reduces friction with the bearing sleeve, and to create the groove at the end. The gear is cast separately and force-fitted onto the shaft. This part is subjected to all the force generated by the motor, which can be on the order of 5,000lb. Under normal use, the shaft should not experience any axial force, but this may occur if the saw blade is not lifted straight out after making a cut, or if the end of the saw accidentally strikes against something. In any case, this axial force is not likely to exceed 50lb.

Part name: Set of handles

Part number: 2 610 915 260

Material: Plastic

Manufacturing process: Injection molding

Function: Provides a grip for the user

Complexity: Moderate; this part has two molded pieces with a fairly complex pattern

Comments: This is the largest single component of the saw. Plastic is used here because it is a lightweight, easily molded, and cheap material; making the handles out of metal would make the product unacceptably heavy. The handle needs to be able to support the weight of the saw (just under 12lb), and also needs to be able to withstand any forces due to kickback, caused when the blade stalls during use. These forces my be greater than the weight, but should not exceed 30lb. The handles are shaped to provide an easy and comfortable grip on the saw during use. The force is concentrated at the lever which attaches the handles to the foot, and the screw that fastens the handles to the housing.

Part name: Lever

Part number: 2 610 921 181

Material: Plastic, steel

Manufacturing process: Plastic: injection molding; steel screw: thread rolling

Function: Lets the user adjust the depth of cut

Complexity: Simple; this is a single molded piece with a standard fastener

Comments: This part has a screw that fits into the vertical slot on the foot and holds the handles and foot together. The lever is used to raise the saw to control the depth of cut. The lever itself is plastic; weight is not a concern with this part, since it's quite small, but plastic is cheap and easily molded. Also, no great strength is required; at most, it needs to raise the 12lb weight of the saw. The screw that holds it in place is steel; plastic is not used because it's threaded and uses a split ring to keep enough pressure on the edges of the slot to hold the saw in place. Plastic would not hold the thread pattern effectively.

Part name: Bearing

Part number: none

Material: Bronze

Manufacturing process: Sintering

Function: Provides a bearing for the end of the armature shaft where it meets the housing cover

Complexity: Simple; this is a single molded part

Comments: This part is shown on the assembly drawing (at the URL given at the beginning of this list), but does not appear on the parts list. The bearings in this product are made from sintered bronze. Bronze has very little metal-on-metal friction, and the porosity of sintered bronze allows a lubricant to be trapped in the part. These properties let the shaft spin freely in the bearing with very little friction. It is a very tough metal, and protects the plastic housing cover from wear from the rotating shaft. This part has a friction force due to the rotation of the shaft and the weight of the armature applied to it; this force is less than 5lb.