The document is a report on the design of a prosthetic arm by a team of students. It details their research process, design choices, and preliminary tests of the prosthetic. The team decided on a direct tension design where extending the arm pulls a string to open spring-loaded fingers, allowing the arm to pick up and hold objects. Their low-cost design uses basic materials and direct human power rather than complex mechanical or electronic systems. The report evaluates the design and suggests areas for further improvement, such as enhancing the hand mechanism or tension system. It concludes the current design could be effective despite room for advancement.

1. MESA - Charles County, Maryland
Henry E. Lackey High School
Prosthetic Arm
6/3/2014
Team Members:
George Jenkins, Cody Rogers, Shade Jenifer, &
Christian Warren

2. Abstract
The Prosthetic Arm Challenge detailed design criteria and testing specifications for a
trans-radial prosthetic device. Devices are scored based on the distance accuracy relay, the object
relocation task, the dexterity task, and design efficiency. This report details the efforts of the
second Henry E. Lackey Prosthetic Arm Team to meet the goals of the challenge up to June
3rd, 2014. It includes a discussion of research, material choice, and other components of the
design evolution, along with STEM influence and principles, and an analysis of preliminary
tests. This design is a cost effective, efficient, and effective trans-radial prosthetic device. The
tension created by straightening the user's arm and the force created by the spring of the spring
clamp is enough to hold and relocate relatively heavy objects. The device can be used to thread
nuts onto bolts and throw ping pong balls, albeit with some difficulty. Additionally, the design is
composed of highly cost-efficient materials. The design can be improved by focusing on the
hand mechanism and the direct tension mechanism. If the direct tension method of design was
maintained, possible re-design concepts would include using simple machines or a pulley system
to enable an increase of the spring tension of the clamp but allow the force produced by
straightening the arm to remain the same. Even without these advancements the current design
remains an effective trans-radial prosthetic device and could be effective in a real life
application.

3. Table of Contents
TitlePage1
Abstract...2
TableofContents3
Introduction.4
Discussion...5-12
Conclusion.12
Recommendations..........13
References13-14
Acknowledgements15
AppendixA...16-18
Appendix B...19-20
Appendix C...21-22
Appendix D...22-23
AppendixE24
AppendixF.25
AppendixG...26-31

4. Introduction
Prosthetic devices have been mentioned throughout history. Early models were created
out of wood and metal and were often uncomfortable and had limited ranges of movement. At
the end of world war two, the National Academy of Sciences began to advocate better research
and development of prosthetics. The use of stronger, lighter materials like plastic, aluminum, and
composite materials, alongside the advent of microprocessors, computer chips, and robotics,
allows amputees to function with increasing efficiency.
The Prosthetic Arm Challenge detailed design criteria and testing specifications for a
trans-radial prosthetic device. The device could not utilize the elbow, forearm, wrist, hand, or
fingers of each arm to control the prosthetic device, must have had "fingers" that could open at
least ten centimeters, could not be modified mid-challenge, and could not exceed a monetary cost
of forty dollars and needed to be as light as possible. Devices were scored based on the distance
accuracy relay, the object relocation task, the dexterity task, and design efficiency. The distance
accuracy relay measured the distance and accuracy achieved by tossing balls of three different
sizes into target containers located at three different distances. The object relocation task
measured the mass-to-time ratio achieved by placing objects of varying weight into the specific
container. The dexterity task measured the number of bolts and nuts correctly placed and secured
onto the testing device. Design efficiency measured the greatest ratio of device performance to
device mass. Additionally, the team was scored based on its technical paper, academic display,
and oral presentation. This report details the efforts of the second Henry E. Lackey Prosthetic
Arm Team to meet the goals of the challenge up to June 3rd, 2014. It includes a discussion of
research, material choice, and other components of the design evolution, along with STEM
influence and principles, and an analysis of preliminary tests.

5. Discussion
The team started the project back in November and like in any successful engineering
project, followed the design process. The team started by defining the problem of having to
design and construct a low cost prosthetic that could be used in application of everyday life to
perform various tasks. The team also evaluated the given constraints of the challenge and knew
exactly what criteria the arm had to meet and the expectations in terms of the performance. Once
the team had the problem clearly defined, the team moved into the research stages of the process.
In order to begin the preliminary research to find the particular solution desired, the team began
concept development by defining the purposes of four subsystems of the device: tension
mechanism, hand configuration, harness, and fastenings (Appendix A, Figure 1). The team
brainstormed possible implications of the enumerated criteria especially relevant to design: the
forty dollar budget, the inability to use the wrist, hand, or fingers in any way, and the necessity of
having at least two moving fingers. The team then met once a week to move thoroughly through
the design process.
The tension mechanism needed to open and close a device wide enough to pick up a two
liter bottle, which is the largest object in the competition being approximately 10cm, without
using the wrist, hand, fingers, or forearm of the arm equipped with the trans-radial prosthetic
device or any part of the other arm. Once the concept development was complete the team was
able to conduct preliminary research to find the types of prosthetics on market and options for
the project. Four options were explored, and extensive research was completed on each: direct
tension (Appendix B, Figure 1), pneumatics (Appendix B, Figure 2), hydraulics (Appendix B,
Figure 3), and mechanical devices (Appendix B, Figure 4). Direct tension involved stretching the
entire arm out to pull a string taut and to close the hand mechanism. For this to work as efficiently

6. as possible, research had to be conducted into how to gain the highest possible mechanical
advantage of the system in order for the arm to produce the most torque via the less human force
applied. Pneumatics and hydraulics both involved using a series of tubes with air and fluid,
respectively, to achieve the same result. Using the Pascal's Principle and the ideal gas law, the
team could have used either system which would have great efficiency since a system could have
been built which had a great change in area, resulting in a great output force to the arm.
Mechanical devices involved creating a switch mechanism that would turn a motor on and off,
again achieving the same result. This last idea involved potentially costly and heavy mechanical
motors and may have hindered the user's ability to time the opening and closing of the hand
mechanism to grasp objects effectively. Hydraulics involved potentially heavy quantities of
fluids, promised to be complicated to design and build, and raised concerns about leakage.
Pneumatics did not involve potentially heavy fluids, but did promise to be complicated to design
and build and also raised concerns about air leakage. Direct tension used only human muscle
power and would not be as efficient as the other options, but was the cheapest, lightest, and
simplest option. For these reasons, the team decided to pursue a direct tension trans-radial
prosthetic system design.
Once the preliminary research was conducted and the team had the main concept of a
direct tension prosthetic, further research was conducted in order for the team to have complete
understanding on the concepts behind direct tension. Online research indicated that a common
direct tension prosthetic system design involves running a string from the center of the user's
back (Appendix A, Figure 2), through a hook near the user's elbow, and to a gripping device.
The next topic of research was the natural state of the prosthetic. In a naturally closed direct
tension device, the user's naturally relaxed state would ensure the lack of tension in the string

7. and a spring or spring-like device would force the device's fingers together. The user could
extend his or her shoulder and elbow to create tension in the string, thereby opening the device's
fingers and grabbing the intended object before relaxing, letting the force of the spring pull the
fingers back together with enough force to continue holding onto said object. There are several
potential issues with this design. If the force created by the spring is not large enough, the fingers
may slide open and let the object fall out, but if the force created by the spring is too large, it
may be difficult to provide enough force using only your arm and shoulder to open the fingers
wide enough to grasp the object in question. These problems are somewhat alleviated by running
the string from the center of the back to the hand mechanism, thereby increasing the string length
and allowing the user to utilize the combined force of back, shoulder, and arm muscles to create
tension. This increased tension potential would allow for stronger spring devices and stronger
grips.
The other researched natural state is known as naturally open direct tension devices.
Naturally open direct tension devices are much like the naturally open devices, except the spring
is not in compression, yet tension. The spring actually presses out on the fingers and by the user's
motion of the extension of the user's arm, would compress the spring and the fingers would
come to. The team found this option very viable considering the user has complete control
over the forces compared to the spring having the control in the naturally closed device. The
major problem with this came when analyzing how a device such as this would perform in the
challenges given and if someone in real world would have to use the arm. The problem is user
fatigue while using the device, since in order to pick up an object the user would have the arm
fully extended for long periods of time since the extension of the arm is what holds the fingers
shut. Because of the fatigue issue and knowing the device is being made for real life application,
the team chose a naturally closed arm since the device would function and only a proper spring size
would be needed.

8. The performance tasks provided several distinct challenges. The prosthetic device had to
open at least two fingers at least ten centimeters apart, pick up objects of various shapes, sizes,
and weights, throw three sizes of balls different distances, and thread nuts onto bolts. Now that
the research had been conducted the team moved into the brainstorming and design portions of
the design process. The team chose to go through each aspect of the design; claw, mounting
apparatus, harness, and cable respectively, separately, and for each brainstorm possible solutions
to chose the best component rather than focus operations on one main device. The hand was a
spring clamp that was naturally closed and opened with the application of tension from the
string. One spring clamp design (Appendix B, Figure 5) had a wide, circular opening directly
between each finger that can hold larger balls and two pieces of rubber attached laterally to the
tips of the fingers. The opening allowed it to pick up, and subsequently release and throw, balls,
and the laterally-attached rubber squares moved with the shape of the targeted object, increasing
the likelihood of successfully picking that object up. A flat piece of plywood provided a stable
base for the clamp, but several challenges remained: mounting the device on a stable surface and
holding everything in place. Particular attention was paid to the kinetic friction of the constantly-
moving string and the force required opening the clamp.
Two different team members had to attempt each task. The system, in order to fit more
than one person, could not rely on any singular set of measurements. A t-shirt seemed a likely
solution. Velcro straps could be used to secure the plywood platform and the elbow hook for the
string in place in addition to fastening the t-shirt securely onto the user's arm. Additionally, long
sleeves could prevent any harmful abrasion on the user's skin during testing. Another hook could
be added to the back of the t-shirt to extend the length of the string and allow the user to fully
utilize his or her back, shoulder, and arm muscles to create tension. Unfortunately, a t-shirt
would not provide enough stability for the overall system, as the Velcro straps would not prevent
rotational sliding on the arm or slippage on the back hook. The team used a free body diagram

9. which showed that the shirt did not provide sufficient horizontal support to prevent such motion.
A leather "8" harness looped around both shoulders could secure the back hook in place and
multiple hooks around the upper arm, elbow joint, and wrist could prevent rotational slippage on
the arm. In order to limit the amount of materials required, however, a looped dog leash
(Appendix B, Figure 6) was incorporated into the design instead of a leather harness, eliminating
the need for attaching an additional hook on the back, and PVC pipe was used in place of several
Velcro straps and a long-sleeve t-shirt to hold the hand mechanism.
Originally the team was going to use a cardboard shipping tube, but overall, less wear
could be done with the PVC, and the team needed the device to be as durable as possible. The
string began at the loop already on the secured dog leash loop in the back, ran through the PVC
pipe and a loop located above the wrist, and through each of the handles of the spring clamp. The
string, however, chafed against both the user's forearm and the edges of the PVC pipe. Original
designs called for latex tubing to act as the string, but its relative weakness made it difficult to
create enough tension to force the spring clamp open, and so it was replaced with the stronger,
more durable butcher's twine. During testing, however, friction from the constant fraying on the
edges of the PVC frayed the twine, causing it to snap. The butcher's twine was then replaced with
the much-more-durable trimmer line and a hole was drilled into the side of the PVC pipe, allowing
the trimmer line to be fed in around the area where the user's fist rested, preventing any harmful
chafing.
The spring clamp was connected to a plywood rectangle of wood by the use of two bolts
(Appendix A, Figure 3). It was further held in place by a doubled rubber band attached to
the center of gravity of the clamp and the two screws. The top half of approximately five inches
was cut off of the PVC pipe so the plywood could be placed inside. The plywood was secured
via wood screws. Foam padding was placed behind the end of the plywood in order to minimize
user discomfort. The trimmer line was tied to the looped dog leash, run through a hole in the side

10. of the PVC pipe, the wrist hook, and a hole pre-drilled into one of the spring clamp handles
before being tied to the pre-drilled hole in the other handle. Now the build of the prosthetic
device was complete.
Further modifications were made to the prosthetic build after competing in the state
competition on May 1st
. The changes made were to increase design efficiency since the main
weakness of the device was the weight. The PVC pipe of the arm was cut down two inches, now
being the ideal length for arms of members (Rogers and Jenifer) who will be competing in the
National event. Additionally, the piece of plywood used initially was longer than needed, so the
wood was cut to proper length. Lastly, to reduce the weight of the device even further, a series of
holes were drilled in rows around the arm to reduce the weight as much as possible. These
changes increase the design efficiency and overall quality of the device since at a lighter weight
the prosthetic is easier to operate.
Three preliminary tests were designed. First, various objects were placed on a table to be
picked up and moved to a designated zone via use of the trans-radial prosthetic device (Appendix
C, Figure 1). These objects were chosen from the list of given objects (e.g. 2 liters, soup cans,
and notebooks) and based on their particular weights or ergonomic shapes (e.g. power drill,
coffee mug) in order to determine whether or not the prosthetic device could perform the
challenge adequately and the relative ease with which it was possible to pick up heavier or oddly
shaped objects. During testing, it was noticed that the laterally-attached rubber rectangles did not
have significant gripping properties. Electrical tape was added to the rubber to provide extra
friction between the device and objects. This increased the grip tremendously. The device was
able to move every single object the user attempted to move. During a timed test, one team
member moved fifteen items from a table to a designated location two meters away in forty six
seconds (Appendix D, Graph 1). These results clearly suggest that the prosthetic device can pick

11. up, move, and drop items of various shapes, sizes, and weights.
Secondly, balls of various diameters as defined in the testing challenge were collected
and thrown into buckets (Appendix C, figure 2) of the specified meters away fifty times. Fifty
ping pong balls were thrown in a bucket two meters away, three-and-a-half meters away, and five
meters away. Trials were conducted for each type of ball. The results (Appendix D, Graph 2)
show a significant downward trend in accuracy as the distance of the buckets increases. A ping
pong ball successfully thrown into the closest target earns twenty-five points.
Landing in the next target earns thirty points. A successful landing in the farthest bucket earns
forty points. If all fifteen ping pong balls are thrown into the nearest bucket, approximately six of
them will get in, earning a total of one hundred and fifty points. If they are all thrown into the
bucket after that, approximately four of them will get in, earning a total of one hundred and
twenty points. If they are all thrown into the farthest bucket, approximately one of them will get
in, earning a total of forty points. These results show that all balls should be thrown into the
nearest bucket unless accuracy can be significantly improved at three-and-a-half meters.
Lastly, the prosthetic was used to thread a nut through a bolt (Appendix C, figure 3). The
team constructed a replica of the actual board and was able to get the same nuts and bolts as the
competition would provide. The team member would go exactly as if in the competition and put
the bolt in the hole then fasten the nut onto the bolt. It took approximately twenty seconds to
thread a nut completely on the bolt (Appendix D, Graph 3). The indentations on the bolt guided
the nut up the shaft. Although the non-equipped hand could hold the head of the bolt in place, the
bolt had to be repositioned several times during some trials. Overall the device was very
successful with the challenge proving the device could be implemented in terms of use for fine
motor control tasks.
As stated previously, the Lackey Prosthetic arm team has been meeting every Monday

12. since November. An estimated time worked on the project would be approximately 114 hours,
including work done in school and then the various outside practices and work done on the
project (Appendix E). The team is committed to the success of the arm and has gone above all
expectations now having acquired much practice time for the challenges and looks very
promising. Team members worked together through every aspect of the design process. The
members did everything together, no one person in control, everyone working together to do
everything to the highest potential. Everyone had the same role, all important. During testing the
two performers (Rogers & Jennifer) performed the tasks, while the other two members (Jenkins
& Warren) helped to set-up and record data. Teamwork helped the success and in the end, made
the team what it is.
Conclusion
This design is a cheap (Appendices F & G), efficient, and effective trans-radial prosthetic
device. The PVC pipe and dog leash adequately constrain the trimmer line and the single wrist
hook adequately aligns it with the spring clamp. The tension created by straightening the user's
arm and the force created by the spring of the spring clamp is enough to hold and relocate
relatively heavy objects. The device can be used to thread nuts onto bolts and throw ping pong
balls, albeit with some difficulty. The data collected through the ball tossing challenge indicates
the device has accuracy up to the two meter line, which is where the teams focus will be and the
device has relatively acceptable fine motor control. Additionally, the design is composed of
highly cost-efficient materials. The design has much room for improvement, but is an effective
trans-radial prosthetic device.

13. Recommendations
As of writing this report, the device would perform better if the user did not have to apply
as much force to open the clamp because great strength is needed to open the device all the way,
but the same amount of spring force is necessary to grasp the provided objects. The team needs
to look into alternate direct force applications, such as simple machines. The design needs to
hold more objects, no matter their shapes, and be more easily utilized. This can be achieved by
focusing on the hand mechanism and the direct tension mechanism, respectively. Additional
testing should be done to see how the device can handle more delicate objects, such as a fish
bowl, to see the true real life application of the device.
References
Norton, K. M. (n.d.). inMotion: A Brief History of Prosthetics. Amputee Coalition - Resources
for amputees, amputation, limb loss, caregivers and healthcare providers. Retrieved March 15,
2014, from http://www.amputee-coalition.org/inmotion/nov_dec_07/history_prost
Burck, J., Zeher, M. J., Armiger, R., Beaty, J. D., & Laboratory, J. H. (n.d.). Developing the
World's Most Advanced Prosthetic Arm Using Model-Based Design - MathWorks News &
Notes - 2009. MathWorks - MATLAB and Simulink for Technical Computing. Retrieved March
15, 2014, from http://www.mathworks.com/company/newsletters/news_notes/2009/jhu-model-
based-design.html
Scott, D. (n.d.). University of Illinois Students Design $300 Prosthetic Arm | Complex. Complex
| Style, Music, Sneakers, Entertainment, Girls, Technology. Retrieved March 15, 2014, from
http://www.complex.com/tech/2012/11/university-of-illinois-students-design-300-prosthetic-arm

14. Chorost, M. (n.d.). A True Bionic Limb Remains Far Out of Reach | Wired Science | Wired.com.
wired.com . Retrieved March 15, 2014, from http://www.wired.com/wiredscience/2012/03
AMO Arm pneumatic prosthetic does mind-control on the cheap. (n.d.). Engadget. Retrieved
March 15, 2014, from http://www.engadget.com/2011/04/05/amo-arm-pneumatic-prosthetic-
does-mind-control-on-the-cheap/
Nylon Dog Traffic Leash 1-inch x 2 foot Blue - Dog Leashes Nylon. (n.d.). Arcata Pet Supplies -
Online pet shop for all your supply needs. Retrieved March 15, 2014, from
http://www.arcatapet.com/item.cfm?cat=794
Magellan self-actuated prosthetic device can be operated with your smartphone | The geek's
guide to awesomeness | DamnGeeky. (n.d.). The geek's guide to awesomeness. Retrieved March
15, 2014, from http://www.damngeeky.com/2012/08/15/4071/magellan-self-actuated-prosthetic-
device-can-be-operated-with-your-smartphone.html
Alfred Mann Foundation » Limb Loss. (n.d.). Alfred Mann Foundation. Retrieved March 15,
2014, from http://aemf.org/our-research/current-focus/limb-loss/
spring clamp. (n.d.). plastic spring clamp for hundreds of uses. Retrieved March 15, 2014, from
http://www.alibaba.com/product-gs/341861413/plastic_spring_clamp_for_hundred

15. Acknowledgements
The Henry E. Lackey Prosthetic Arm team would like to thank our sponsors;
Mr. Crawford and Mr. Liston, for helping us flush out our design ideas and their continuous
support through our entire design process and as we have advanced through numerous
competitions to the National level. Also special thanks go out to Dr. Warren for his constant
support of the team and allowing the team to use his tools for construction and also to be there for
the team for assistance when needed in the construction portion of the design. The team would
like to recognize Michael Laury who aided in the training of the competition portion of the
challenge. Mike served as an outside observer, helping us refine our techniques for the
performance. Thomas DeLoache is very fluent in Inventor, our 3-D modeling program, and
answered any questions the team had while utilizing the CAD program. The team would also like
to recognize Mai-Lin Quinto and Melissa Nelson who helped in the layout of the backboard.
Thank you to Henry E. Lackey High School, Charles County Public Schools, and Maryland
MESA for supporting the team as the team represents Charles Countyandthe StateofMarylandat the
National level.

16. Appendix A: Design Notebook Photocopies
Figure 1: 4 subsystems of design

17. Figure 2: Team's Direct Tension Method

18. Figure 3: Plywood Layout

19. Appendix B: Design Ideas
Figure One: Direct Tension Prosthetic Device
Figure Two: Pneumatics Prosthetic Device
Figure Three: Hydraulics Prosthetic Device

20. Figure Four: Motorized Prosthetic Device
Figure Five: Spring Clamp
Figure Six: Looped Dog Leash

21. Appendix C: Testing
Figure 1: Object Relocation Test Set-up
Figure 2: Distance Accuracy Relay Test Set-up

22. Figure 3: Block used for dexterity challenge practice
Appendix D: Testing Data
Trial # Items Relocated Time (Sec)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Average
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
15
51
47
42
46
41
51
48
43
45
48
46
52
45
46
47
46.533
Graph 1: Chart of Time to relocate all objects.

23. 80
70
60
50
Successes40
30
20
10
0
ArmThrows
TennisBall
KickBall
PingPongBall
2meters 3.5meters 5meters
Distance
Graph two: Chart of the number of balls that landed in buckets of varying distances
Trial#
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Average
BoltsSecured
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Time(Sec)
63
57
69
59
72
64
67
59
62
54
63
57
59
61
63
61.933
Graph three: Chart of time to secure all nuts on bolts

24. Appendix E: Cost-Labor Summary
Cost-LaborSummary
Component TimeperSession Number of Sessions Total Time
Device-InSchool
Meetings/ Practice
Device-Out ofSchool
Meetings/ Practice
Paper
AcademicDisplay
Presentation
Additional Time -
Build with Volunteer
1Hour
2Hours
2Hours
1Hour2
Hours
5Hours
35
10
6
10
8
1
35Hours
20Hours
12Hours
10Hours
16Hours
5Hours
Support
Additional Time -
ResearchConducted 2Hours 8 16Hours
for Project
TotalTime 114Hours
*In school meeting consisted of the group going through the design process and coming up with
ideas. All research was split in between the group members to be done outside of school
individually. Out of school practice was when the group meet up most of the time after school on
our free time to do additional work on the arm and also to practice the challenges. Then on a
single day, the group meet together at a members house (Warren) and constructed the arm, using
the tools and time volunteered by the member's father (Dr. Warren). Above is the chart
reflecting the average time spent on each one of these sessions and also the exact values of
sessions had for each component. The total time is an estimate, but is very close to the actual
time spent.

25. Appendix F: Itemized Budget Sheet
Part Unit Dimensions Retail Price Price per Unit Quantity Used Total Cost Retail Source
PVC Pipe
Charlotte Pipe 6- in x 10 ft
Sch 40 PVC DWV Pipe
$42.48 $0.354/inch 15.5 Inches $5.49 Lowes.com
Hardwood Oak, Red 3/4" x 4" x 48" $15.75 $0.08/inch^2 18 Inches^2 $1.44 Woodcraft.com
Leash Large, 6ft $6.49 $6.49/Leash 1 Leash $6.49 Chewy.com
Trimmer
Line
Husqvarna 639 00 51-05
230 ft Titanium Force
Premium
$15.99 $0.08/foot 3 Feet $0.24 vminnovations.com
Wood
Screws
75 Pack, 1-3/4" $3.14 $0.042/screw 8 Screws $0.33 midlandhardware.com
Spring
Clamp
Jaw Opening Capacity
(Inch): 3 Overall Length
(Inch): 9
$8.36 $8.36/clamp 1 Clamp $5.85 use-enco.com
Sponge 9" x 5" $1.79 $1.79/sponge 1/2 Sponge $0.90 Amazon.com
Bolts
100 Pack, Width: 3/8",
Length: 1-1/2"
$165.50 $1.655/bolt 2 Bolts $3.31 Lowes.com
Nuts 25 Count, 3/8" $2.74 $0.1096/nut 2 Nuts $0.23 Lowes.com
Washers 50 Count, 3/8" $3.99 $0.0798/washer 2 Washers $0.16 Amazon.com
Butt
Conector
50 Count, 12-10 Gauge $7.15
$0.143/Butt
connector
1 Butt
connector
$0.14 protelecomsupply.com
Total $24.58

26. Documentation of Retail Prices – Print screens of Retail Prices
- Highlights Price of Material
PVC Pipe:
Hardwood:

27. Leash:
Trimmer Line:

28. Wood Screws:
Spring Clamp:

29. Sponge:
Bolts:

30. Nuts:
Flat Washers:

31. Butt connector: