Group 36 - Ryobi Contractor's Saw

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Contents

Executive Summary

For this project, our group was instructed to examine the Skilsaw contractor's saw. We first disassembled the saw, and analyzed its components. We then made recommendations for design improvements, before reassembling it.

Introduction

The Skilsaw circular saw is primarily designed for cutting wood, but with a change of blades can also cut through other materials, such as metal piping. This hand-held saw is powered by an AC current, which is converted to mechanical energy in the rotation of the blade. The electrical engine has a power output of 2.3 HP, and spins at 4,600 RPM. During this project, the group disassembled, analyzed, and reassembled the saw.

Group Members

Rita Groetz acted as the group leader. She was responsible for scheduling meeting times, keeping the entire group up to date on the project with emails and phone calls. She also was the photographer for the group, and the primary contributor and editor for the CIBER-U web page.

Kevin Bush was responsible for assembling and disassembling the saw. He had prior experience with both the materials and mechanics of the saw; knowledge which was invaluable to the group. He also contributed to the CAD design, and presented the project.

David Pohl was the primary CAD designer. He created the majority of the CAD drawings, and also created an animated movie clip, using those drawings to illustrate how the motor looked on the inside, as it was running. He also took notes during most meetings, and presented the project.

Kevin Ho contributed to the power point presentation and contributed to the CIBER-U web page.

Tai Nguyen took notes for the disassembly process, contributed to the power point presentation, and contributed to the CIBER-U web page.

All group members collaborated on analyzing the components of the saw.

Before Disassembly Section

Since the saw was brand new when given to the group, we assumed that its initial appearance and operation was correct. Apart from basic outer components, like housing and a handle, the saw contained a safety switch, a movable foot, and a detachable wrench to tighten the center bolt.

The saw is operated by engaging a safety switch, and then a power switch. It was very loud, and even without the torque of the blade, kicked back when it was initially operated. This demonstrated the physical power of the motor.

The group estimated that the saw would contain about 40 components, and predicted that those components would be made of plastic, steel, aluminum and copper.

Disassembly Procedure

1. Removed Wrench with hands.

2. Removed rubber stopper with T-30 torx bit.

3. Removed lower guard lift with T-20 torx bit.

4. Removed spring from lower guard with needle nose pliers.

5. Removed blade-holding bolt and “washers” with 13mm socket.

6. Removed spring ring with swiss army knife.

7. Removed C-clip and depth adjustment lever with pliers and a 12mm wrench.

8. Removed lower guard with hands.

9. Removed 3 short screws with T-20 screw driver and bearing flange.

10.Removed pinion shaft.

11.Removed 4 long screws from upper guard with a T-20 torx bit.

12.Removed upper guard with hands.

13.Removed rotor with hands.

14.Removed plain washer and washer from the rotor.

15.Removed 6 medium screws to take handle apart with T-20 torx bit.

16.Pulled the handle off with hands.

17.Removed 2 medium screws and pulled off housing cover with hands.

18.Slid out motor brushes with needle nose pliers.

19.Took apart brushes with hands.

20.Removed 2 long screws from stator.


The disassembly was relatively easy except for pulling off the C-Clip, which needed to be slowly worked out of its former position, so as not to damage it.

After Disassembly

Component List

Part # Component Name Number of Parts of This Type Material(s) Manufacturing Process Image
1 Motor Housing 1 Plastic, Brass, Galvanized Steel Injection Molded, Die Cast and Machined, Stamped
Housing and stator.jpg
2 Stator 1 Plastic, Copper, Magnetic Alloy Injection Molded, Extruded, Stamped and Soldered
Stator.jpg
3 Trigger and Safety Switch 1 Plastic, Aluminum Injection Molded, Extruded and Machined
Switch trigger.jpg
4 Power Cord 1 Copper, Plastic Both Extruded
Power cord.jpg
5 Upper Guard 1 Aluminum, Brass Die Cast and Machined, Machined
Upper guard.jpg
6 Pinion Shaft 1 Steel Extruded and Machined. Also dipped in an anti-corrosion coating.
Gear.jpg
7 Central Washer 2 Steel Die Cast
Center Washers.jpg
8 Depth Adjustment Lever 1 Plastic Injection Molded
Depth Adjustment Handle.jpg
9 Rubber Stopper 1 Rubber Injection Molded and Vulcanized
Rubber Stopper.jpg
10 Handle 2 Plastic Injection Molded
Handle 1.jpg
11 Rotor 1 Steel, Copper, Plastic Stamped, Extruded and Machined, Extruded, and Injection Molded
Rotor.jpg
12 Vent Cover 1 Plastic Injection Molded
Vent cover.jpg
13 Lower Guard 1 Aluminum Die Cast
Lower Guard.jpg
14 Carbon Brush 2 Plastic, Copper, Carbon, Brass Injection Molded, Extruded, Machined, Stamped
Carbon brushes disassembled.jpg
15 Lower Guard Lift Lever 1 Plastic Injection Molded
Lower Guard Handle Parts.jpg
16 Long Screw 6 Steel Extruded and Machined
Long screw.jpg
17 Medium Screws 8 Steel Extruded and Machined
Medium screw.jpg
18 Short Screws 4 Steel Extruded and Machined
Small Screw.jpg
19 Central Bolt 1 Steel Extruded and Machined
Center Bolt.jpg
20 Long Spring 1 Steel Extruded
Spring.jpg
21 O-Spring 1 Steel Extruded
O-Spring.jpg
21 C-Clip 1 Steel Stamped
C-Clip.jpg
22 Large Gauge Screw 1 Steel Extruded and Machined
Rubber Stopper.jpg
23 Wing Nut Bolt 1 Steel Extruded and Machined
Bolt from butterfly.jpg
24 Wing Nut 1 Steel Stamped
Butterfly.jpg
25 Wrench 1 Galvanized Steel Stamped
Wrench34.jpg
26 Plain Washer 1 Aluminum Stamped
Plain washer.jpg
27 Washer 1 Aluminum Stamped
Washer.jpg
28 Bearing Flange 1 Steel Machined
Flange.jpg
29 Depth Adjustment Nut 1 Steel Machined
Depth Adjustment Nut.jpg


CAD Drawings

Bearing Flange

CAD flange.jpg


Pinion Shaft

CAD gear.jpg


Rotor

CAD rotor.jpg


Upper Guard

CAD upper guard.jpg


Assembled Parts

CAD assembly.jpg


We also produced a video clip, using the CAD drawings. It can be viewed here.



Design Changes

While the long, medium, and short screws all took the same size bit and had the same thread, the large screw took a different bit. This group suggests that the large screw be changed to accommodate the same size bit as the others.

Also, the carbon brushes are simply set into the saw and held in place by the vent. The group suggests that the brushes are fastened in place independently.


Assembly

1. Placed stator and two long screws into housing, using a T-20 torx bit.

2. Reassembled carbon brushes with hands, and placed into housing with hands.

3. Replaced vent cover and two medium screws, using a T-20 torx bit.

4. Slid handle parts back onto housing, and realigned the switch and wires inside of it with hands.

5. Replaced 6 medium screws in handle with T-20 torx bit.

6. Replaced plain washer and washer on rotor with hands.

7. Placed rotor into housing with hands.

8. Reattached upper guard to housing with 4 long screws and a T-20 torx bit.

9. Replaced pinion shaft with hands.

10. Replaced bearing flange with hands, and secured with 3 short screws and the T-20 torx bit.

11. Eased lower guard back into place with hands.

12. Replaced depth adjustment lever and c-clip with needle nose pliers.

13. Replaced spring ring with hands.

14. Replaced center bolt and washers with hands.

15. Replaced spring with pliers.

16. Replaced lower guard lift with T-20 torx bit.

17. Replaced rubber stopper and large screw with a T-30 torx bit.

18. Replaced wrench with hands.

Reassembly was much easier than assembly, because by that time, we were quite familiar with the saw's components.


After Assembly

How the Saw Works

Depressing the safety switch allows for the trigger to be pulled, which completes the electrical circuit of the saw. The electricity runs through wires to the stator, and the AC current causes its electromagnet to become charged, and to flip its poles very quickly. At the same time, the rotor becomes charged as well, but its poles remain constant. The rapid flipping of the poles of the stator causes the rotor to turn, and the carbon brushes allow for electricity to reach the rotor, even while it is spinning. The spinning rotor causes the gears to turn, which then spins the blade.

When the trigger is released, the circuit is broken, and the rotor begins to slow, and then finally ceases spinning.

Reassembly

Upon reassembling the saw, the group successfully tested it. It ran almost exactly as it had before, but once the switch was disengaged, the motor continued to spin from its momentum. The group surmised that the broken copper wiring in the carbon brushes must have had something to do with the braking of the motor. This problem could be quite dangerous if the lower guard malfunctioned.


Design Problems/ Changes

The group discovered that there was a lot of plastic used in the saw's construction. We felt that there was not enough metal at pivotal load-bearing locations on the saw. Instead, we found plastic at these locations. The high power of the drill and the brittle nature of the plastic could lead to a dangerous malfunction, were some of the plastic supports to break. We suggest that high-load areas be reinforced with some sort of steel.

We also noticed that there were no ball bearings on the bushings. Instead, the bushings were heavily lubricated with grease. If the lubricant was burned away by regular use, there could be a large amount of friction created, efficiently "burning out" the motor. We suggest that to increase the longevity of the saw, ball bearings are added to the bushings.

A nonessential change that the group also recommends is a gel grip for the handle. This would allow for the operator to use the saw for long periods of time without discomfort.


References

Brain, Marshall, "Howstuffworks 'How Electric Motors Work'". (n.d.). Retrieved December 8th, 2007, from http://electronics.howstuffworks.com/motor.htm .

Product Detail - 7 1/4" Skilsaw - Model #5400-01. (n.d.). Retrieved December 3, 2007, from http://www.skiltools.com/en/AllTools/Category/Product/default.htm?pid=5400-01&cid=192160 .