Introduction: Unlimbited Arm (Remix) - Edmund Edition
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The Unlimbited Arm (Remix) - Edmund Edition arm is an open source, 3D printed prosthetic arm that uses a pull-to-activate gripping system. By removing wrist or elbow grip activation, this hand doesn't require constant strain or bending to keep a solid grip. Instead, the user pulls a Velcro tab and locks it in place. Once locked, the grip of the hand is both strong enough to ride a bike and dexterous enough to use a pencil.
This design is based on the UnLimbited Arm v2.1 - Alfie Edition.
Inspired to help one young boy ride his bike.
- Added groove to the palm to fit handlebar
- Fused the first knuckle on fingers for improved grip and rigidity
- Added small grooves and inserts for elastic bands to support pen/pencil use
- Replaced wrist activation with pull and lock Velcro tab
- Added forearm support
This prosthetic arm is a useful tool for simple tasks, but there are some important safety considerations.
Check out the Safety Guidelines provided by Enabling the Future for a complete listing, as well as some useful cleaning and maintenance tips.
Step 1: Parts and Tools
3D Printed Parts
All of the necessary 3D part files can be found on Thingiverse.
If you have a 3D printer and want to print these files yourself, see Tips for successful prints for tips on print orientation, materials, and calibration.
If you don't have a 3D printer, you can contact an e-NABLE community chapter near you for help with getting parts printed.
Another option is 3DHubs.com, a 3D printing service that will contact a local maker to print and deliver the parts to you. The 3D parts for this project cost about $40 (with delivery) from 3DHubs.
If you're really interested in this project, but the 3D printing services don't work for you, reach out to: BionicArmH17@microsoft.com. We have limited production capabilities, so we cannot make any guarantees to deliver parts.
- 100 lb braided fishing line or strong string
- 3/16" orthodontic elastic bands
- 2" Velcro Straps
- Chicago screws
Optional Hardware Parts
- 1"x12" Velcro cinch straps for securing the arm in place
- Fingertip grips or Plasti dip to rubberize fingers for improved grip
- Super glue to solidify knots
- Tweezers and/or Dental Tool
- X-Acto knife or similar cutting tool
- Needle-nosed pliers
Step 2: String the Fingers
The tension strings run through the length of the finger, starting at the tip. If you jumped ahead and already attached the fingers to the palm, that's fine, but it's easier to string the fingers before they're attached to the palm.
First, cut a length of fishing line (or string, if that's what you're using) that's about three times the length of the hand. You'll need five pieces.
It's helpful to have a small, thin tool with a hook on it for stringing. A modified paper clip works well. Check out image 1 above to see a paper clip that was bent for stringing, and image 2 for an example of the paper clip with string hooked to it.
Insert the paper clip into the hole closest to the tip of the finger. Push the paper clip through the finger until it exits through the hole shown in figure 4. When your paper clip is in place, hook a piece of string on to it and gently pull it through the finger until it exits the finger tip (figure 5).
After the string has been pulled through the finger tip, use the paper clip tool to pull some of the string through the second hole to form a loop, as shown in image 6.
Use the free end of the string (from the finger tip) to tie a knot to the loop of string. For a more secure knot, use the improved clinch knot shown in the video above. If you want to take it one step further, add some super glue to the knot.
When your knot is tied (and dry, if you used glue), tuck it into the second hole so it's not exposed on the finger (figure 7).
Note: The holes in the thumb are not in the finger tip, but use the same procedure outlined above for securing the knot. See image 8 for an example of the thumb knot.
Step 3: String the Palm
The loose string from the fingers needs to be run through the palm, and eventually attached to the Velcro tensioner.
Use the loose end of one of the finger strings and run it through its respective knuckle hole of the palm as shown in image 1. Use the labeled image of the fingers to identify the fingers.
The fingers will be secured with pins and elastic bands in the next steps, but by stringing the palm, the fingers should be roughly in place.
Note: The pinky finger is slightly smaller than the middle and ring fingers. This is a subtle difference, but it's more noticeable when attached to the palm.
Continue threading the string through the palm until it reaches the end (image 2). Complete these steps for all 5 fingers.
Step 4: Attach Fingers
Start by identifying the four pins needed for attaching the fingers to the palm. You'll need:
- Long index-middle finger pin
- Ring finger pin
- Pinky finger pin
- Thumb finger pin
Check out the labeled image above if you're not sure which pins to use.
The finger pieces should be roughly in place after stringing the palm from the previous step.
Insert the index finger and middle finger into the palm as shown in image 1. After the two fingers are in place, insert the long index-middle finger pin from the outside of the palm. Make sure the square head of the pin is flush with the square hole in the side of the palm, as shown in image 2.
Attach the ring finger with the ring finger pin, then attach the pinky finger with the pinky finger pin. The pins are inserted from the other side of the palm. Be sure to line up the square holes on the palm with the square head of the pin, as shown in images 3 and 4.
Finally, attach the thumb to the palm using the thumb pin (this pin is slightly larger than the ring finger and pinky finger pins). After all the pins are added, the hand should look similar to images 5 and 6.
Note: No string is shown in these images, but your fingers and palm should be strung from the previous steps.
Step 5: Add Elastic Bands
After the pins are added and the fingers are held in place, you'll need to add an elastic dental band to the top of each knuckle. Elastic bands are also used to secure a pen or pencil.
The tops of the knuckles where the fingers join the palm have indents for elastic bands to keep tension in the fingers. To attach the bands, hook the band onto the palm side of the knuckle. Hold the attached band in place and secure the other end of the band to the finger side of the knuckle. It may be helpful to use a paper clip or small tweezers to ensure the bands are securely fastened.
Make sure the bands are secured to each of the five fingers as shown in image 2.
Elastic bands are attached to the inside of the palm and the index finger to keep a pen or pencil in place for writing. The images for these steps show an orange palm and fingers instead of the usual green, but it doesn't make a difference.
To secure the band, use a small cylinder type object. For this assembly, 3D printer filament is used (see image 3). Toothpicks trimmed to size also work well for this.
Start by inserting the band into one of the small holes in the palm as shown in image 4. From the top of the palm, insert the 3D printer filament or toothpick into the palm and through the opening in the band to secure the elastic band in place. Stretch the band to the other hole and repeat this process. Your elastic band will look similar to images 5 and 6 when you're finished.
Repeat these steps for the index finger. When both bands are in place, your hand will resemble the hand in image 7.
To use a pen or pencil, lock the grip of the hand (locking will be added in a later step), pull the bands out a little and insert the pen from the top of the palm to the index finger. The final result will look like image 8.
Step 6: Assemble the Tensioner
Cut an 8 inch piece of Velcro (it doesn't matter which side of Velcro you choose, for now). Line up the Velcro with the inside of the bottom piece of the tensioner as well as the top of the forearm brace. You'll need to trim the top of the Velcro to fit inside of the tensioner and trim the sides so it fits inside the top unit of the forearm. See image 2 for an example of where to cut the Velcro and how it aligns with the pieces. The Velcro should fit inside the tensioner piece as shown in image 3.
Thoroughly coat the tensioner piece with epoxy (using a tooth pick) and attach the cut Velcro. Coat the inside of the top tensioner piece with epoxy (including the pegs) and snap it to the bottom piece of the tensioner, with the Velcro "sandwiched" in between. Smooth any overflow of epoxy with the tooth pick. Images 4-7 demonstrate these steps.
Depending on the type of epoxy used, you may have to wait several hours for it to completely dry.
Step 7: Add Arm Support
Before the arm support is attached, you need to glue the Velcro strap to the top of the forearm channel. Cut a piece of Velcro that fits the size of the top of the forearm channel (image 1). You should use the side of the Velcro that can attach to the side of Velcro you used for the tensioner in the last step.
Use a cutting tool to score the top of the channel. Scoring the channel allows for a better seal from the epoxy. Add epoxy to the top of the channel and attach the Velcro. This may take several hours to completely dry.
Fit the bottom forearm support to the palm and insert a Chicago screw on each side (image 3). Place the top of the forearm support over the palm and secure all pieces together with the other end of the Chicago screw. Do this for each side of the palm.
If you're using an elbow support piece (optional), use the Chicago screws to secure it to the end of the forearm support. An example is shown in image 6.
Step 8: String the Tensioner
After your tensioner is finished drying, you need to attach the loose ends of the finger strings to it.
Before you begin stringing the tensioner, make sure the finger strings are not going through the palm, but instead are going over it, as shown in image 1. If the strings exit from the bottom of the palm, they won't be able to move as easily to lock the hand (this will make sense after you attach the forearm braces).
Starting with the pinky finger, insert the pinky string into the top of the tensioner and pull it through the top of the hole, as shown in images 2 and 3. Repeat this for all 5 finger strings.
Insert the tensioner piece into the top of the forearm channel until it reaches the edge, holding the loose ends of the string out of the way (image 4).
Keeping the tensioner piece even with the forearm channel, tie a small overhand knot in the string as shown in image 5. Repeat this for all of the strings. These knots need to be as level as possible. If the knots are not level, the fingers will not have an equal amount of tension, which will cause them to close at different rates.
Once you have tied 5 level knots, tie another overhand knot on top of them for extra security. Your knots should look similar to the knots in image 6.
Step 9: Velcro Strap Support
For Velcro strap support, you can either use the 2 inch Velcro straps or simply purchase the optional Velcro cinch straps (simpler).
Add the 2 inch Velcro straps to the forearm brace and elbow brace to keep the arm secure. Slide the straps through the sides of the arm brace as shown in the image.
Slide the cinch strap through the sides of the arm brace and secure the cinch strap. The cinch strap is optional, but recommended due to ease of use and simplicity.
Add padding such as moleskin or medical foam to the inside of the arm brace for added comfort and a better fit. This step is unique to the individual, so it may take some trial and error to get this right.
Step 10: The Team
This team came together for Microsoft's Oneweek Hackathon 2017 with the goal of producing an inexpensive DIY 3d printed prosthetic hand for children to use to ride their bike and hold a pencil. We are now embarking on building a motorized version and we are also trying to distribute 1000 3d printed prosthetic hands like this outs and others to those that need them.