Welcome to my prosthetic hand page!
As an intern at AIO Robotics, I designed this prosthetic hand to assist those with limb differences so that they may grasp objects by simply curling their wrist. This hand is completely mechanical, requires no electronics, and is tendon-driven. While its functionality is not revolutionary, this project exploits the use of flexible filament to remove the need for pins at the joints, making it easier to scale the prosthetic based on screw sizes without having to worry too much about the tolerances of 3D printer extrusions. Additionally, I built the prosthetic to uphold a streamlined look, retaining the tendons in channels and underneath a forearm shell. The resulting product looks pretty awesome to me. While this prosthetic has not undergone live testing with a recipient, I have posted the details of the printing and assembly to Instructables for anyone interested in expanding the outreach of this project. I am completely expecting a flaw(s) in some form or fashion, so feel free to make modifications as you please! I am also very open to suggestions.
Before I get started, I would like to give a quick shout-out to AIO Robotics for giving me the incredible opportunity to intern at the company and providing that exciting start-up vibe. Check out AIO's printer, the Zeus--I have yet to come across a printer that is as high quality and dependable as it is (seriously). Their emphasis on software really pays off. While the Makerbots, XYZ, and Airwolf printers I've used in my club keep breaking down, the AIO keeps churning away excellent prints, and there are more features to come. Furthermore, I would also like to thank the Tampa-based Handling the Future Organization for giving me the ideas, advice, and vision for improvement. Its members played a huge role in the direction of this project. Without them, I would have never created this prosthetic.
With that, continue on and enjoy!
Step 1: Materials List
These are the materials that I used for the prosthetic that I printed. Feel free to find alternatives to these materials if they are more convenient and/or less expensive.
- Seattle Sports 1/2" Bulk Foam with Adhesive, Grey
- Jaybird and Mais 30/31 Adhesive Foam: 1/4" x 5" x 2 yards
- https://shop3duniverse.com/collections/3d-printabl...(I chose these over the ones on Amazon because they are all clear colored, great for maintaining the clean aesthetic I wanted. Same price, too. Check them out!)
- MagicShield 500M 100LB Super Dyneema Strong Braided Fishing Line
- Screws (refer to "Scaling the Prosthetic" step)
- All screws are the same thread as what is dictated on the scaling chart (i.e. the default prosthetic only uses 6-32 screws)
- Screws for fastening the lower palm to the upper hand (length on scaling chart)
- Screws for fastening buckle clips to the inner forearm (0.25" long for default prosthetic scaled by a factor of 1.00)
- Screws for tensioner pins (1-1.25" long for default prosthetic scaled by a factor of 1.00)
- To find approximate lengths of tensioner pin and buckle screws for other scaling factors, multiply the length by the scaling factor.
Step 2: Download the STL Files
First things first, download the STL files. They are STL files of the default scaled prosthetic, which is based off of my arm (specifically, a wrist of around 2.6 inches).
To design these files, I used Autodesk Fusion 360 and have attached the .f3d and .step files if you would like to modify the files at all. The prosthetic was designed using the tolerances on the AIO Robotics Zeus printer, and I'm currently working with Handling the Future to print the prosthetic on the Ultimaker 2+. Every printer has different tolerances (i.e. a hole might be printed smaller on an Ultimaker than on the Zeus), so feel free to alter the design dimensions. Beware, however; the CAD is somewhat complicated and has caused Fusion 360 to crash multiple times.
Step 3: Scaling the Prosthetic
It is important to scale the prosthetic to fit your recipient.
I have attached a scaling chart here based on the screw sizes and dowel pins you should use for certain wrist sizes. If you have access to a drill press or hand drill, you can round out and clear the 3D printed holes to easily thread your desired screws into the prosthetic. If you do not have access to such tools, the chart provides the screw sizes that will fit the holes for the listed scaling factors, based on the tolerances of the extrusion from AIO Robotics's Zeus.
There is a minimum size to the prosthetic--if the channels become too small, the tendons cannot be guided through easily. While I haven't tested this out, the minimum size should work out.
Step 4: Print the Rigid Parts of the Prosthetic
Most important note: don't use supports!
I used the default settings on the Zeus for these parts, applying 15% infill and a ton of glue and blue tape to reduce the warping. Feel free to experiment with different infill patterns; I've heard Archimedean chords do pretty well when it comes to preventing warping. Alternatively, you can just use a printer with a heated bed.
Step 5: Screwing on the Clips
Once everything is printed, time for assembly!
Start by fastening the "buckle clips" to the inner forearm with screws, the size and thread of which depend on the scaling factor. It is important to note that the clips must go in certain locations. For instance, "BuckleBackRight" must be fastened to the back right notch of the inner forearm if the hand (for a right handed user) is pointing away from you and the top surface of the forearm is facing up. Gauge which end of the buckle points up by comparing the top forearm shell side-by-side with the inner forearm; if it looks like the top of the buckle will hook over the edge of the slot in the shell, you have the buckle oriented correctly in the vertical direction.
Step 6: Glue on the Wrist Joints
Glue on the wrist joints using hot glue, super glue, a soldering iron, or all three. Refer to the CAD design to see which wrist joint goes where. The larger circular section attaches to the forearm, but there is a difference between the joints (look at the slight chamfer on the arm of the joint--it matches the slight chamfer on the slot in the hand/palm).
Step 7: Tie the Tendon Knot
With the inelastic fishing string, tie a clinch knot around a relatively round, ~1/4 inch diameter cylindrical object (I used a dowel pin, but a #2 pencil should work fine) in order to create a small loop. Upon tying the clinch knot, use some super glue to prevent the knot from unraveling. Refer to this link to learn how to tie a clinch knot.
Feel free to explore better options for tendons! Handling the Future suggested to use hand-made piano wire springs, and I definitely think this would be a viable option, maybe not through the channels but at the joints. Adding the springs would remove some rigidity in the string, allowing the fingers to grasp an object tighter upon encountering the wall of the object.
Step 8: Pin Through the Loop
Push the loop into the hole of the finger tip while peering through the hole along the side. When the loop has cleared the hole, slide the respective dowel pin (according to the scaling chart) through. Deposit a dab of hot glue into the hole to hold the pin in place.
Here, a drill press or hand drill would be very helpful to make sure that the dowel pin will fit snugly.
Step 9: Printing the Flexible Filament Joints
Print the flexible filament joints!
Find any printer that can print using flexible filament such as NinjaFlex. I personally used Verbatim Primalloy on the Zeus using a specially designed filament guide, but feel free to use whatever is most convenient to you.
If you are using the Zeus, refer to this page for more information regarding the filament guide you need to print and install in order to use the flexible filament. It should be on the local drive on the Zeus. If not, it is here for your convenience. Replace the current filament guide and push the flexible filament through. Then, change the settings of the printer to the following:
External perimeter speed: 28 mm/s
Infill speed: 33 mm/s
Top solid infill speed: 33 mm/s
Travel speed: 70 mm/s
Temperature: 220-230 degrees Celsius
If the prints are not turning out nice, try slowing down the speed or increasing the temperature. If the flexible filament is bunching up in the gap between the filament guide and the extrusion motors, you can try propping the guide up with a paper clip or binder clip (or any thin, U-shaped part).
Step 10: Glue in the Flexible Filament Joints
Using the hot glue gun or super glue, deposit glue on the flexible filament rectangular strip as you slide on the finger tip, middle finger section, and phalanx. The result should look like the photo shown above.
Guide the tendon through the holes on the underside of the fingers.
Step 11: Screwing in the Lower Palm
Using the screws dictated in the scaling chart, screw the lower palm into the upper palm. Once again, a hand drill or drill press will assist you greatly in this step by widening the 3D printed holes to the exact desired length for inserting the screws. Utilize a drill/tap chart to figure out which drill bit to use.
If you're feeling adventurous, try printing the final layers of the palm (and the fingers) using AIO's pause feature, which allows you to swap filament mid-print! Right before the printer deposits the final layers, switch to flexible filament to create a surface with more grip. Do this by using the pause editor on the Zeus's "Edit" screen (before you slice your STL on-board). Make sure the flexible filament isn't swapped in before the Zeus prints overhangs--flexible filament doesn't do that too well.
Step 12: Glue in the Fingers
First, push the tendon up and through the channels on the upper side of the hand.
Next, stick the exposed flexible filament joints of the fingers into the slots on the hand. Super glue or hot glue the joints into the slots. Be very careful at this step; some printers may not print the thin wall between the back of the slot and the channel behind it. You don't want glue to go into the channel.
Step 13: Glue in the Thumb
Assemble and glue the thumb as you did for the fingers. Note that the wider strip of flexible filament serves as the joint between the thumb and the hand itself. Additionally, the joints of the thumb are two pieces rather than one long strip, the least widest piece going between the thumb tip and the phalanx.
The channel for the thumb tendon is open in the thumb joint slot on the hand, so be especially wary while gluing the thumb joint.
Step 14: Glue the End of the Wrist Joint to the Hand
Once again using hot glue, super glue, and/or a soldering iron, fuse the wrist joints onto their respective slots in the hand. Wait for the glue or melted plastic to settle before continuing.
Step 15: Guide the Tendons Through the Channels
Guide the tendon through the channels to the end of the hand and into the circular slots of the inner forearm as shown. Repeat for the other four tendons.
Step 16: Tie to the Tensioner Pins
Once again using the clinch knots, fasten the end of the tendon to the tensioner pins. To make sure the fingers will curl upon bending the wrist, pull the tendons until the fingers are slightly curled and then tie the knot. Put a dab of hot glue or super glue to prevent the knot from unraveling.
Insert the longest screws through holes at the end of the mount on the inner forearm and into the back of the pins. Adjust them until the tension is correct. Feel free to carefully use a hand drill or drill press to clear out the 3D printed holes in the back of the tensioner pins.
Upon inserting the screws, the tensioner pins may split at a certain layer. To prevent this, you can clamp the top and bottom surfaces of the print (which keeps the horizontal layers pressed against each other) and then insert the screw.
Step 17: Clip on Top and Bottom Shells
Clip on the top and bottom shells to conceal the mess of tendons for that (beautiful, in my opinion) streamlined look. They should just snap onto the buckle clips. To remove them, press the clips inwards and pull up on the shell. The clips should be thin enough to bend but not break.
Step 18: You're Done! Enjoy
Hooray! Thank you for taking the time to complete this prosthetic.
Adhere foam pads in the desired places to increase comfort. Velcro straps can fasten onto the forearm of the user through the slots of the inner forearm (see the buckle section--the pictures there show these slots). For a better idea of how to place the foam and velcro straps, refer to the "Spock" basketball prosthetic hand my club, 3D4E at UCLA, created and displayed on this page. Specifically, the steps are shown in Steps 13-16. You will probably need to add finger grips to the tips for stronger grasping of objects, found here: https://shop3duniverse.com/collections/3d-printabl...
Please feel free to describe any problems you may have come across in the design, especially when it comes to scaling. I'm sure there are some creases that need to be ironed out.
Step 19: Thank You!
Thank you for taking the time to read this extensive Instructables! If you would like to contact me, please send me an email at firstname.lastname@example.org (no spam please). I am always open to suggestions and challenging design projects.