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• Team Members
• Industry Contact
• Academic Advisor
• Dr. Faryar Etesami
Overview• Introduction / motivation• Scope• Mission statement• PDS review• Beginning design• Initial design and parts• Initial testing• Refining• Prototype / final testing• Conclusion
Introduction / Motivation
• According to the Consumer Product Safety Commission, injuries suffered from the use of hand held portable circular saws is second only to power drills. The reason for this is due to the inherent danger involved with the saw and its availability to a large cross section of the general population.
Scope
This project involves the following.
•Isolating the blade and electronics
•Stopping the blade
•Adaptation to current circular saws
Mission Statement“To design a blade-arresting device for a hand held portable circular saw that will reliably and effectively stop a moving blade upon contact with human tissue. The device must be readily adaptable for mass production by either existing circular saw manufacturers or by the client company, SawStop LLC.”
PDS ReviewStopping Time: < 0.005 Sec Target 0.003 Sec
Compatible With Other Safety Features
Reliability: > 98% Target > 99%
Does Not Change Ergonomics Or Obstruct View
Lightweight: < 2 Lbs Target < 1 Lb
Cost: < $8 Target < $5
Maintenance Free
Durability: (Withstand a Drop From 6 Ft)
Working Life: > 10 Years Target > 40 Years
Beginning Design• Forces
• Methods of Stopping Saw Blade
• Placement on the Saw
Force On Stopping Block I=moment of inertia=angular momentum=3500rpm=angular accelerationk=radius of gyrationm=mass=0.678 lbmd=diam=7.25inT=torqueF=force
<__Known Parameters
Design Time of .002sec
Force
Methods of Stopping BladePrevious Design Ideas
• Motor Braking– DC current injection. Not Fast Enough
• Friction Braking– Similar to car brakes. More bulky and complex, less
reliable than others and more weight.
• Gear Braking– Pawl in gears. Possible gear damage, blade will still
move
• Impact Braking– More to follow
Placement•Must Be Compatible With Current Safety Features
•Must not obstruct view
•Must not create a hazard
Due to the above restrictions the front of the housing was selected as the best placement.
Initial Design and Parts• Spring to actuate block. Most reliable• Block• Housing• Pivot• Wire
Concept Block
Initial Testing
Testing Materials and Shape
RefiningTesting refinement
Redesigned Test Apparatus –
Uses Spring Actuation to Force the Block into the Blade
Test blocks from second test. Polycarbonate blocks are on the left, and aluminum blocks are on the right.
Test Block Results
Block MaterialBlade Radius R (in)
L R+L D L/D
(deg)d Blade Type # of rev
time (ms @ 4600 rpm)
Comments
3 Polycarbonate 3.63 0.50 4.13 0.69 0.73 32 1.3Freud Diablo 24 tooth blade limiting shoulder (DO 724P)
0.917 23.913 Not Satisfactory
4 Polycarbonate 3.63 0.50 4.13 0.69 0.73 32 1.3Freud Diablo 24 tooth blade limiting shoulder (DO 724P)
0.917 23.913 Not Satisfactory
6 Polycarbonate 3.63 0.50 4.13 0.69 0.73 32 1.3Freud Diablo 24 tooth blade limiting shoulder (DO 724P)
0.958 25.000 Not Satisfactory
7 Aluminum 3.63 0.42 4.04 0.69 0.60 29 1.3Freud Diablo 24 tooth blade limiting shoulder (DO 724P)
0.125 3.261
Works well, sheared off one tip and fractured
another
10 Aluminum 3.63 0.50 4.13 0.69 0.73 33.5 1.18Freud Diablo 24 tooth blade limiting shoulder (DO 724P)
0.125 3.261 Worked Well
1 Polycarbonate 3.59 0.53 4.13 0.69 0.77 34 1.3Oldham Combination Industrial Carbide--24 tooth (725-4524)
0.125 3.261 Works well
2 Polycarbonate 3.59 0.53 4.13 0.69 0.77 34 1.3Oldham Combination Industrial Carbide--24 tooth (725-4524)
0.125 3.261 Works well
9 Aluminum 3.59 0.48 4.07 0.69 0.70 30 1.18Oldham Combination Industrial Carbide--24 tooth (725-4524)
0.083 2.174Broken Tooth,
stopped immediately
5 Polycarbonate 3.25 0.53 3.78 0.69 0.77 34 1.3 Oldham Max Life 140 tooth (D650P)
0.293 7.640A little over the time allotted
11 Aluminum 3.25 0.45 3.70 0.69 0.66 33.5 1.18 Oldham Max Life 140 tooth (D650P)
0.186 4.845Worked Well for
this type of blade
Block Material & Blade Type Performance
Depth lim
iting Shoulder1
Com
bination 2
Fine Tooth3
Springs
Spring Force
<---as design number at 30 degT 54lbf in
T G
30 deg
Let = 26 to 30 deg to account for assumptions and simplifications
<---as minimum at 20 degT 36lbf in
T G
G 1.784lbf indeg
G 2I
t2
20deg
I 0.0197332lb in2
t 0.001s
G I 2
t2
solve G 2I
t2
Sub (eqn 1) and (eqn 2) into (eqn 3)
(eqn 3)T I
Also
(eqn 2)T G
For a tortional spring
(eqn 1) t
2 2
solve 2
t2
Spring Design--Helical Tortional Spring
For the small travel we will assume the angular acceleration to be constant
Spring refinement
Restraining
Prototype
Seal / Insulator
Housing
Final Testing Setup
Testing apparatus
Electrical testing setup
Testing Circuit Schematic
Prototype Test Setup
Prototype Parts
Testing Results
Housing fracture
Testing Results
Blade Stopped in one tooth and simply manufactured block fractured
Final Design
Refinement and changes to be made
•Strengthened housing. Depends on manufacturer
•Better manufactured block
•Cover for housing. Will depend on manufacturer.
Main components including spring, insulator, wire attachment, block and housing performed as expected.
ConclusionThe final design is a stopping device
that is reliable, light (less then ½ pound), fast (one tooth of blade),
Compatible with other safety features, inexpensive (approx. 1.70$),
maintenance free and does not change the ergonomics of the saw.