Introduction: "Spock" Prosthetic Basketball Hand
The Spock is a 3D printable prosthetic hand that was designed by 3D4E at UCLA with input from the UCLA Women’s Basketball Team. We were inspired by Raptor Reloaded from e-NABLING the Future and decided to contribute to the growing open source prosthetic hand movement.
This prosthetic hand was designed to assist those without fingers but with wrist motion. For those missing both wrist and hand, refer to Step 16 for possible modifications.
Step 1: Gathering Materials
Bill of Materials:
There are several options when it comes to strings, bolts, and paddings. Below are a selection of materials that we used.
- Seattle Sports ½” Bulk Foam with Adhesive, Grey
- Jaybird and Mais 30/31 Adhesive Foam: ¼” x 5” x 2 yards
- Lee Tippi Micro Gel Fingertip Grips - Size 3 Extra Small - 10 Pack
- MagicShield 500M 100LB Super Dyneema Strong Braided Fishing Line
- Beadalon Fabric Elastic 1.0mm Black, 100-meter
- M3 x 20 Bolts
- PLA Filament
- Tools: 3D Printer, Scissors, Files, Sandpaper, Screwdriver, Hot Glue Gun
Step 2: Measurement and Design Modifications
To start, measure the width of the individual’s forearm, wrist, and residual limb. Using the .f3d files from the Autodesk Fusion 360 design software (or .step files if you use a different software) provided by 3D4E at UCLA, scale the parts to their desired measurements and leave 3 mm of space for the velcro straps and the foam padding, particularly at the wrist. Additionally, .stl files are provided in case you want to simply print a hand. IMPORTANT NOTE: When scaling the palm, makes sure the diameter of the small holes through which the elastic strings are inserted are kept at 2 mm. This may require editing the sketches of the holes, shrinking the diameter to a calculated value, and then scaling the part to size. Additionally, for the screw holes in the tensioner, you can either keep the screw holes the same size by using the aforementioned procedure or consult a screw chart and adjust the hole size accordingly. Make sure to alter the diameters of the circular holes on the tensioner block mounted on the gauntlet so the screws do not slip past.
Step 3: Printing the Parts
Print on the 3D printer available to you. It is essential to mention that every printer has different tolerances. For instance, an AIO Robotics Zeus will deposit a line filament with a width dissimilar to that extruded from a Makerbot Replicator. Hence, multiple prints and modifications to the design may be needed to ensure that the fingers do not wiggle around and the pins fit snugly while allowing for rotational movement. The design files given in this Instructable are specifically tuned for the Zeus 3D printer from AIO Robotics. 30% infill works for all parts except the pins and first joint, which are 100% and 60%, respectively. NOTES: Holes are generally smaller than they appear on the design interface software. When sizing screw holes, compensate for this. I have personally found that increasing the screw hole tap size by 0.027 inches (about 0.7 mm) higher than the value given by a chart. You may have to print test blocks to see which holes work.
Step 4: Filing, Sanding, and Smoothing the PLA Plastic
Using the files (especially the thin round file--those help tremendously!) and sandpaper, sand the rough edges on plastic parts. For the finger tips and joints, file the holes until the pins are loose enough to permit rotational movement while snug enough so that they do not fall out. When the supports are removed, the undersides of the palm and the gauntlet may be very rough, depending on the quality of your printer. Though this will eventually not matter because foam padding will be placed over these imperfections, it is suggested that acetone should be applied to these areas to melt and smooth the plastic. Be careful, though; acetone tends to remove the pigment of the PLA, so try to avoid getting acetone all over the uncovered areas. Acetone vapor baths may produce better results. For more information, refer to other online resources, i.e. Youtube.
Step 5: Assembling the Fingers
Insert the small finger pins through the holes of the first joints and the finger tips as shown in the picture. The finger tip should be able to rotate smoothly, and the pin should fit nicely. Repeat this process for the other four fingers.
Step 6: Attaching the Fingers to the Palm
Insert the longer knuckle pins through the side of the palm and the holes in the first joint. The first joints should be able to rotate freely, and the pin should not be able to fall out. Repeat for the other fingers and the thumb. NOTE: the diameter of the thumb pin may be larger than that of other knuckle pins. Hence, additional filing of the hole in the first joint may be necessary.
Step 7: Connecting Palm and Gauntlet
Connect the palm to the gauntlet using the wrist pins. Prevent the pins from sliding using the pin caps. Some sanding may be needed to make the wrist rotation smooth.
Step 8: Tensioner Pins and Mount
Slide the tensioner pins through the tensioner mount to check if they slide smoothly. It’s okay if the pins are slightly loose. If there is a large amount of friction between the pin and the mount, sand the pin until it slides through nicely. Insert the screws through the back end of the tensioner pins. If scaled properly, the screws should be unable to slide beyond the back end of the tensioner mount.
Step 9: Slide Mount Onto Gauntlet
Slide tensioner mount onto slot on the gauntlet. Slide retaining clip into the same slot until the click is heard. It should be firmly clicked into place, but not impossible to remove.
Step 10: Fitting the Elastic Strings
Cut a section of elastic string and tie a knot at one end. Rotate prosthetic hand such that the inside of the palm and gauntlet are facing upwards. There is a pair of small holes on the inside of the palm associated with each finger; slide the elastic string through the hole. Bending the fingers in a fist-like position, push the elastic string while using a screwdriver to ease the string through the rounded trench at the end of the first joint. In the end, you should have the elastic string beyond the palm as much as the knot at the end of the elastic string will allow. Refer to pictures. Slide the elastic string through designated holes in the first joint and the finger tip. Depending on the size of the fingertip, fasten the elastic string by tying a knot or tying the string to the beam. Make sure to pull the elastic string tight before fastening.
Step 11: Inserting the Inelastic Strings (Tendons)
Flip the prosthetic hand such that the palm is facing upwards. With the inelastic fishing string, tie a clinch knot around the tensioner pins. (Refer to this video to learn how to tie a clinch knot.) Direct the inelastic string through the bump on the gauntlet, the angled holes in the palm, the channels along the inside of the palm, and out the angled holes in the palm near the fingers. Guide the string through the holes on the underside of the first joint and tie a clinch knot on the beam on the underside of the fingertip.
Step 12: Adjusting the Tendon Tension
Use the screwdriver to adjust the tensioner pins until the ball is grasped when the palm is bent back.
Step 13: Palm Padding and Velcro Straps
With the hot glue gun, attach the padding and the velcro to the palm as shown. First, apply the hot glue to the palm (NOT the velcro) in such a way that the hot glue does not touch the inelastic strings in the channels. Before the glue cools, press the soft loop side of a velcro strip onto the hot glue. In a similar fashion, apply hot glue to the palm and press the side of the velcro strip WITHOUT the hook onto the glue. Cutting appropriately sized pieces of foam, glue the padding to all surfaces around the inside of the palm.
Step 14: Gauntlet Padding and Velcro Straps
Flip the prosthetic hand such that the inside of the palm and gauntlet are facing upwards. Apply enough hot glue to attach a section of the soft loop side of the velcro strip. Guide the strip through the lower slot to the outside, back through the upper slot, to the lower slot on the other side, and finally back up to the upper slot on the other side (refer to picture). Cut and glue appropriately sized foam along inside of gauntlet. Depending on how comfortable you would like the gauntlet to be, you may add foam to the inside walls of the gauntlet (not shown).
Step 15: Gauntlet Velcro Strap Fastener
Cut two reasonably-sized adhesive-backed velcro hook squares of equal area and press the adhesive surfaces against each other. Alternatively, you can use two squares of non-adhesive velcro hook strips and hot glue them together.
Step 16: Fastening Down the Gauntlet Velcro Strap
Attach the now double-sided velcro hook strip to the soft loop surface of the strap on the gauntlet as shown in the pictures. The strap on the gauntlet should be able to be held fixed now.
Step 17: Completed Hand and Potential Modifications
The hand should be ready to go! Use the elastic straps to tighten the gauntlet to the arm. Feel free to make any adjustments in accordance to your needs. For those without a hand and wrist, try the following modifications. First, design and print a block that fits snug in the inside of the palm in order to fill the space where the user normally puts their palm. Then, use strips of elastic bands to fasten the gauntlet to the arm better. Finally, create an upper arm band using the elastic straps, inelastic or elastic strings, and possibly the velcro hook and loop strips (in a way, this upper arm band looks like a one of those blood pressure cuffs). This will prevent the entire palm and gauntlet from slipping off during the basketball shot process.
Step 18: References and Contact