Extendable/Foldable Arm Prosthetic Prop

I decided to make a ¨ third arm ¨ prosthetic for the 2025 Halloween build contest, I figured it would make a great prop for those planning on having a sci-fi/robot related costume. This is also my first time getting heavily involved in working with metal in my projects. Though it looks complex I found this project to be more simple and easy to assemble than my other projects and I am quite happy with how it turned out. If you couldn't tell already I like to go over the top for my class projects so I hope you enjoy looking at my building process. Also if you are planning on recreating my project I recommend you buy a lot of epoxy because JB-weld syringe epoxy will be your best friend for this project.

(Some tools mentioned are not provided on the image)

Tools:

Drill Press (With 8mm & Step Drill Bit)

Calipers

Drill

Compact Circular Saw

Dremel Saw

3D Printer

Sharpie

Safety Goggles

Gloves & Mask (for epoxy)

Vise

(4) Clamps

Alcohol & Steel Woll

Ruler

Cardboard

Materials:

(2) Metal Yard Sticks

8mm Thick Metal Rod (At Least 9.5in Long)

(4) Ball Bearings

4in Handle

(2) L-Trim Metal Beams (1 Inch Up & Across)

JB-Weld Syringe Epoxy

(2) Double Sided Velcro Strips

(2) T Squares

Sheet Metal

.3in Screw

Pen

String

Key Ring

1.) Measure and cut your yard stick into 6 pieces, (4) 12in long pieces (2) 8in long pieces (you may use the cirular saw or bend the yardstick at the length continuously until it snaps (saw is preferred))

2.) Grab two of your 12in pieces and marl/drill both ends with a 8mm drill bit (make sure the holes align)

3.) Grab your other two 12in pieces and mark/drill one end with your 8mm drill bit and drill a 22mm hole on the other end with your step drill bit (make sure the holes align)

4.) Take both of your 8in long pieces and mark/drill one ends with your 8mm drill bit then at the other end drill a 22mm hole (make sure the holes align)

Materials/Tools Used:

Drill Press

8mm & Step Drill Bit

Compact Circular Saw

(2) Yard Stick

Sharpie

Ruler/Caliper

In total you should have two 12in pieces with 8mm holes on both ends, two 12in pieces with 22mm holes on one and and an 8mm hole on the other end, and two 8in pieces with 22mm holes on one and and an 8mm hole on the other end. Refer to the image for help.

1.) Insert your ball bearings into the 22mm holes you made in your pieces

2.) Elevate your pieces with cardboard but leave the edge with the ball bearing hanging off so the ball bearing sits in between your piece

3.) Clean the ball bearings and the metal pieces with alcohol and steel woll

4.) Carefully mix and apply the JB-Weld Syringe Epoxy around the ball bearings making sure to cover the walls of the ball bearings and the surface of your metal piece and let it sit for 4-6 hours (wear mask and gloves when working with epoxy)

5.) Once dried, flip the metal piece and repeat on the other side

Materials/Tools Used:

(4) Metal Ball Bearings

Alcohol & Steel Woll

JB-Weld Syringe Epoxy

(6) Pieces Of Your Yard Stick

Mask & Gloves

Make sure you don´t get epoxy in the center of the ball bearings or in the interior wall or else the ball bearings wont be able to spin

1.) Measure your 8mm thick rod to the lengths provided below, then clamp the rod to your vise. Cut the rod with your compact circular saw to your measurements. Be sure you use eye protection while cutting and use hand protection after handling.

Piece I: 3.6 Inches

Piece II: 3 Inches

Piece III: 1.5 Inches

Piece IV: 1.5 Inches

Materials/Tools Used:

Compact Circular Saw

8mm Thick Metal Rod (<9.5in long)

Safety Goggles

Vise

Gloves

Ruler/Caliper

Sharpie

1.) Take both your T-Squares and unscrew the top parts from the rest of the T-Square until you are left with just the top part of the T-Square and keep the access metal for later

2.) Clamp your T-Square heads to your vise and use your compact circular saw to cut the ends of the T-Squares. Make sure you cut them so the hollow side is facing outwards for both sides (refer to the picture for help)

3.) Mark and drill a 8mm hole in the center of both T-Square heads with your drill press making sure they align

Materials/Tools Used:

Vise

Compact Circular Saw

(2) T-Squares

Drill Press (With 8mm Drill Bit)

Sharpie

1.) 3D Print the following file (I highly recommend you print twice in case you lose some pieces)

The pieces belong to certain parts on the prosthetic, to assemble the build properly follow the diagram so the spacers are at the correct spots. You may use the diagrams or written instructions below

--Bottom Joint--

Use the 3.6in long metal rod, insert the following parts into the rod in order:

EP-1 -> T1 -> P1 -> Arm 1 -> P2 -> Arm 2 -> T2 -> EP-2

--Lower Med Joint--

Use the 2.5in long metal rod, insert the following parts into the rod in order:

EP-3 -> Arm-3 (insert trough ball bearing) -> P3 -> Arm 4 (insert through ball bearing) -> MP-1 -> Arm 1 -> P4 -> Arm 2 -> EP-4

--Med Joint--

Use the 1.5in long metal rod, insert the following parts into the rod in order:

EP-5 -> Arm 3 -> Arm 5 (insert through ball bearing) -> P5 -> Arm 6 (insert through ball bearing) -> Arm 4 -> EP-6

--Top Joint--

Use the other 1.5in long metal rod, insert the following parts into the rod in order:

EP-7 -> Arm 5 -> P6 -> Arm 6 -> EP-8

Materials/Tools Used:

3D Printer

(4) Cut Pieces Of 8mm Thick Metal Rod

(2) T-Square Pieces

(15) 3D Printed Parts

1.) Once all of the pieces are assembled start removing the end pieces one by one and gluing them on with the JB-Weld Syringe Epoxy from the bottom up

2.) Clamp the end pieces once they are glued on to the rod before continuing with the next set of end pieces (Be mindful that the tighter the clamp, the harder it is to move the arm (try not to use too much force when clamping))

Make sure you don ´ t use too much epoxy, you want to try and glue the end pieces to the rod, not the part adjacent to it, this is so the arm wont get stuck and you are still able to move it. Be sure to use respiratory and hand protection when using epoxy.

Materials/Tools Used:

JB-Weld Syringe Epoxy

(4) Cut Pieces Of 8mm Thick Metal Rod

(2) T-Square Pieces

(15) 3D Printed Parts

(4) Clamps

1.) Find any flat sheet metal you are able to cut a 3in x 12in rectangle out of

2.) Mark & cut your sheet metal into a 3in x 12in rectangle with your compact circular saw

3.) Mark & cut your L-trim metal beams with your compact circular saw to the length of your T-square heads (Approx 7 inches)

4.) Clean the L-trim metal beams, T-square heads, and sheet metal with alcohol and steel woll

5.) Coat corners of the L-trim metal beams with JB-Weld Syringe Epoxy and glue the T-square heads to the corners of the L-trim metal beams

6.) With the remaining space left in the L-trim metal beams, slide the 3in x 12in sheet metal in between the T-square heads and stick them to the L-trim metal beams

7.) Weigh down the sheet metal and let the glue sit for 4-6 hours

Make sure you use hand and eye protection with the compact circular saw. When gluing make sure the arm is elevated so it doesn't stick to the base.

Materials/Tools Used:

JB-Weld Syringe Epoxy

Sheet Metal

Ruler/Caliper

Sharpie

Compact Circular Saw

1.) Find your .3in screw and clamp to your vise

2.) Use your compact circular saw and cut off the end of the screw (use hand/eye protection)

3.) Screw the thread through the holes in the centerpiece/P6 at the very top part of the arm

You are welcome to add/replace any object to your liking by unscrewing the thread and putting an object in between the thread and the part. As long as your object has a .30in/8mm hole in it and is at most .25in/6.35mm thick you are able to hold anything you like with this prosthetic, as long as its not too heavy.

Materials/Tools Used:

.3in Screw

Compact Circular Saw

1.) Get the spare metal you got from your T-squares and mark/measure them to the diagram above

2.) Put the metal on the vise and make your 4 pieces with your compact circular saw (use hand and eye protection)

3.) Cut the middle slits with your dremel (use eye protection , the blades tend to fly off) and find your handle (the prongs should be about 4 inches apart)

4.) Clean all the pieces and the handle with alcohol and steel woll

5.) Elevate the arm contraption and glue your pieces to the end of your metal L-trim beams with epoxy so that the side with the slip is sticking out then clamp

6.) With your elevated arm contraption lay your handle at the front of the arm contraption and move the other arms out of the way, clamp something underneath the handle so that it sticks out of the contraption then glue the corners with epoxy

7.) Measure and cut 2 strips of Velcro (add 5 inches to your lower and upper forearm length) then slide the Velcro through the slits in the metal

Tools/Materials Used:

JB-Weld Syringe Epoxy

Dremel

Compact Circular Saw

Handle (4in wide)

(4) Pieces of cut metal

(3) Clamps

Double-Sided Roll Of Velcro

Now that you are done with you arm, you are welcome to make as many attachments as you want. This could range from utensils to tools, blades, etc. You can also add some accessories like stickers or plier holders so you can always have pliers handy to change the object your holding. For me, I made a ring with a string attached to the end of the arm so I could pull it towards me when I need it. You can pretty much do as much as you'd like with this project and I had quite a lot of fun working with it and I hope you do as well.