The Isabella Arm by FATHOM was designed for five-year-old Isabella, who was born with agenesis of the right arm. The FATHOM team partnered with the e-NABLE organization (EnablingtheFuture.org) and the Enable Community Foundation (EnableCommunityFoundation.org) to design an elbow-actuated prosthetic for Isabella, since existing prosthetic designs would not work for her. The Isabella Arm by FATHOM is targeted at users who lack wrist strength or who do not have a wrist but have a partial to full forearm. The principle designers are Dylan Millsaps (FATHOM summer intern), Bethany Casarez (FATHOM Applications Engineer), and Ava DeCapri (FATHOM Industrial Designer).
The Isabella Arm by FATHOM is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (http://creativecommons.org/licenses/by-nc-sa/4.0/). Please provide attribution to http://studiofathom.com/prosthetic/. If you remix this design, let us know! We encourage you to adapt it to your needs and share your versions with the e-NABLE community.
The Isabella Arm by FATHOM was designed and tested on the uPrint SE and Fortus 450 FDM-based 3D printing systems, which utilize soluble supports (http://studiofathom.com/blog/exploring-new-fortus-3d-printers-customer-support-engineer/). Modifications may be required to successfully build the prosthetic on other machines.
The following instructable explains how to assemble the Isabella Arm by FATHOM. The design has not yet been scaled or tested for multiple users. If you plan to build and test the Isabella Arm by FATHOM, please let us know! Send us a message or reach out to Bethany at firstname.lastname@example.org.
Step 1: Download and 3D Print the Components of the Isabella Arm by FATHOM.
The Isabella Arm by FATHOM was designed in Autodesk Fusion 360 software. The original CAD (a right-handed prosthetic) can be downloaded from A360 at the following link: http://a360.co/1W6q4Js. A STEP file of the full assembly is available at this link: http://a360.co/1km2MhW. The STLs for all 3D printed components can also be downloaded from A360 at the following links:
3D Printed Components:
Blank cover, QTY 1
Captive whippletree, QTY 1
Flower knob, QTY 1
Gauntlet, QTY 1
Hinge cover, QTY 1
Lower arm hinge, QTY 1
Palm, QTY 1
Palm plate, QTY 1
Finger, QTY 5
Ratchet cap, QTY 1
Ratchet cap retainer, QTY 1
Ratchet plate, QTY 1
Ratchet spool, QTY 1
Upper arm cuff, QTY 1
Upper arm hinge, QTY 1
Whippletree sled, QTY 1
Total Model Material Consumption: 15.5 cubic inches
Total Support Material Consumption: 3.89 cubic inches
Total time (uPrint SE, 0.010" layer slice): 26 hours
Total time (Fortus 450, 0.010" layer slice): 10.5 hours
Step 2: Obtain Hardware and Soft Goods.
Obtain all the hardware and soft goods denoted below. The FATHOM team used a laser cutter to cut the patterns for leather/foam and rubber. You can also print the pattern PDFs on cardstock and cut them out by hand. Patterns for the leather/foam and silicone rubber can be downloaded from A360 at the following links:
Universal laser files:
Metric Thread-Forming Screw for Plastics, aka "plastite" screws (18-8 SS, Torx, M1.91 Size, 8mm Length), QTY 30
Source: McMaster Carr, part no. 99397A072 (http://www.mcmaster.com/#99397a072/=zeaz72)
Cost: $11.58 per pack of 50
6-32 hex nut, QTY 2
Source: McMaster Carr, part no. 90480A007 (http://www.mcmaster.com/#90480a007/=zeazdf)
Cost: $1.24 per pack of 100
6-32, ½ inch length, torx pan head machine screw, QTY 1
Source: McMaster Carr, part no. 96710A261 (http://www.mcmaster.com/#96710a261/=zeazq)
cost: $8.30 per pack of 50
T6 Torx Screwdriver
No. 6 seed beads, QTY 5 (http://amzn.com/B004TCZBRC)
2 mm crimp tube beads, QTY 2 (http://amzn.com/B002CO0MFU)
0.32 mm craft wire, 12 inch length for cable threading (http://amzn.com/B00AUCH48Y)
Orthodontic elastics (3/8" diameter, light or medium force), QTY 5
Source: Amazon Prime, sold by Prairie Horse Supply (http://www.amazon.com/gp/product/B00OTA8MPM?ref_=s...)
Cost: $5.49 per pack of 100
Monofilament line (Fins Spectra 150-Yards Windtamer Fishing Line, 6 Lb test, 1.5 mono diameter)
Source: Amazon Prime, sold by Fins Fishing (http://www.amazon.com/dp/B009B7JJD2/ref=twister_B0...)
Cost: $16.95 per 150 yard spool
Closed cell foam padding
Source: Patterson Medical, part no. 7173 (http://www.pattersonmedical.com/app.aspx?cmd=getPr...)
Cost: $38.35 per 1/8" x 6" x 72" (3.2mm x 15.2cm x 1.83m) roll
Source: Hide House: Fuchsia Lambskin 2-2.5oz, available in 33 colors (pictured color: 510FU-Fuchsia)(https://hidehouse.com/products/index.html?Category...)
Cost: $45.00 per 6-8 square foot hide
Adhesive backed silicone rubber (FDA-Compliant, Adhesive Back, 1/32" Thick, 12" x 12", Transparent)
Source: McMaster Carr, part no. 86915K24 (http://www.mcmaster.com/#86915k24/=zeb5hd)
Cost: $26.81 per 12" x 12" sheet
Step 3: Install Elastic Anchors on Fingers and Palm.
The Isabella Arm by FATHOM uses dental elastics to maintain an open grasp. Plastite screws are used to anchor the elastics to the finger and palm.
- Each finger has a 1.5 mm diameter pilot hole on the top of the distal digit. Use a T6 torx screwdriver to install a M1.91 plastite screw onto each finger.
- Make sure that all threads are completely buried in the plastic to avoid the risk of breaking the elastics.
- The palm has a 1.5 mm diameter pilot hole at the base of each finger. Use a T6 torx screwdriver to install a M1.91 plastite screw onto the palm at the base of each finger.
Step 4: Assemble Fingers to Palm.
The fingers for the Isabella Arm by FATHOM are printed in place and snap into the palm.
- Snap each finger onto the palm.
- Install a dental elastic onto each finger, first anchoring the elastic to the palm, then to the distal finger.
Step 5: Assemble Palm to Gauntlet.
The wrist can be rotated on the fly to better accommodate the task at hand. A set screw with a knob is used to lock the wrist orientation.
- Insert a 6-32 hex nut into the hexagonal cavity on the inner surface of the gauntlet. This is for the set screw that locks the wrist in place.
- Install the palm onto the gauntlet. This will "capture" the hex nut.
- Install the palm plate on to the palm using three M1.91 plastite screws. This will keep keep the palm from falling off the gauntlet. The face with the chamfered edge should be in contact with the palm.
Step 6: Assemble Captive Whippletree to Whippletree Sled.
The captive whippletree is printed in place and press fit into the sled. To better secure the sled, use superglue to bond the whippletree to the sled.
- Apply a small amount of superglue to the cavity in the sled.
- Quickly press the whippletree into the sled and follow the hold instructions on the super glue.
- Make sure not to add too much superglue. The whippletree should still be able to rotate back and forth once glued onto the sled.
Step 7: Assemble Elbow Hinge.
- Place the upper arm hinge on top of the lower arm hinge.
- Install the cap using 3 M1.91 plastite screws.
- When you tighten down on the hinge, there should be a small clearance (25-50 microns). If there is not enough clearance, the hinge will have too much friction. In that case, simply sand the bottom of the cap until the desired clearance is achieved.
Step 8: Wire Fingers to Whippletree.
The whippletree enables the individual fingers to bend at different rates. Use the finger wiring diagram as a reference to wire the fingers to the whippletree appropriately. The index and middle fingers are wired to one side of the whippletree and the ring and pinky fingers are wired to the other side of the whippletree. The thumb is wired directly to the whippletree sled. Seed beads are used to secure the filament at the tip of each finger.
- Cut a piece of monofilament roughly twice the length of the arm (~24 inches). This will be used to wire the index and ring fingers to the whippletree.
- Thread a bead to the end of the filament. Loop the filament back through the bead once. This allows you to adjust the bead as needed wile holding it onto the filament.
- Tie a clinch knot to anchor the bead to the end of the filament. Tie 2-4 single overhand knots. This will make it easier to tuck in the remaining filament when wiring the fingers.
- Thread the filament through the index finger, starting at the tip of the finger and continuing through the underside of the proximal digit and into the palm.
- Create a threading tool using a 20 inch piece of beading wire. Bend the wire in half and twist. Continue to twist until you have a small opening, like the end of a sewing needle. This will help you thread the filament through the palm and whippletree.
- Thread the filament through the palm making sure to pass the cable through the hole in the center of the palm plate, using the threading tool to assist you. This keeps the cables centered, which allows you to rotate the wrist on the fly without needing to adjust the finger tension.
- Thread the filament through the middle hole on the front of the gauntlet.
- Slide the whippletree sled into the gauntlet.
- Thread the filament down through the appropriate hole on the whippletree, and back up through the adjacent hole. (Use the threading tool to assist you).
- Thread the filament back through the middle hole on the front of the gauntlet.
- Thread the filament through the center hole in the palm plate.
- Thread the filament back through the middle finger, using the threading tool to assist you.
- Thread the filament through the proximal and distal segments of the middle finger.
- Thread a bead onto the end of the filament. Loop the filament back through the bead once so you can easily adjust the bead.
- Bend the finger slightly to further tighten the bead. Tie a clinch knot to secure the bead. Tie 2-4 single overhand knots and trim the remaining filament. With the finger bent, Tuck the loose end of the filament into the hole at the tip of the finger to hide it.
- Repeat steps 1-15 to wire the ring and pinky fingers to the other side of the whippletree. This time, thread them through the hole on the front of the gauntlet that is closer to the ring and pinky fingers. (For a right handed prosthetic, this will be to the right, and vice versa for a left handed prosthetic).
- To wire the thumb, cut a piece of monofilament roughly the length of the arm (~12 inches).
- Thread one end of the filament through the horizontal hole on the whippletree sled. This will be the hole closest to the thumb (furthest to the left on a right-handed prosthetic and vice versa for left handed).
- Loop the filament back through the hole in the sled once. To secure the filament to the thread, tie a clinch knot, followed by 2-4 overhand knots.
- Thread the filament through the remaining hole in the front of the gauntlet.
- Thread the filament through the center hole on the palm plate.
- Thread the filament through the palm and thumb, using the threading tool to assist you.
- Thread the filament through the proximal and distal segments of the thumb.
- Thread a bead on to the end of the filament, looping back through once. As you did with the middle and pinky fingers, bend the finger to tighten the bead and secure it using a clinch knot followed by 2-4 overhand knots. Trim the remaining filament and tuck it in as before.
Step 9: Assemble Hinge to Gauntlet and Cuff.
The counter-bored slots in the hinge allow you to adjust the length of the prosthetic.
- Using QTY 4 M1.91 plastite screws, install the lower arm hinge (the one with the straight thru slot) to the gauntlet. Do not screw them in all the way.
- Using QTY 4 M1.91 plastite screws, install the upper arm hinge (the one with the curved thru slot) to the upper arm cuff. Do not screw them in all the way.
- Adjust both the upper and lower hinges so that the center of the hinge lines up with the center of elbow rotation. Tighten the screws to secure them in place.
Step 10: Wire Whippletree Sled Through Elbow Hinge and Install Ratcheting Tensioner.
The prosthetic is tensioned using a single ratchet. By twisting the ratchet, you adjust the tension of the fingers by pulling on the whippletree sled. Reference the upper arm wiring diagram to wire the ratchet through the elbow hinge to the whippletree sled.
- Cut a piece of monofilament roughly 2 times the length of the prosthetic (~24 inches).
- Attach a crimp bead to the end of the filament, looping back through once and securing with a crimp knot (as before).
- Thread the filament through the one of the horizontal holes in the ratchet spool, so that the bead acts a stopper.
- Thread the filament through one of the side holes in the ratchet plate. The thread should pass from the inside of the ratchet plate outward.
- Thread the filament through the small vertical hole in the ratchet plate.
- Thread the filament through the upper arm hinge, then through the lower arm hinge, using the threading tool made in a previous step to assist you.
- Thread the filament through the one of the horizontal holes on the whippletree sled and back through the adjacent hole.
- Thread the filament back through the lower arm hinge, then the upper arm hinge, using the threading tool to assist you.
- Thread the filament through the vertical hole in the ratchet plate (in the reverse direction as the first pass).
- Thread the filament through the remaining side hole in the ratchet plate, this time passing the thread from outside inward.
- Thread the filament through the other horizontal hole in the ratchet spool and attache a bead to act as a stopper, as before.
- Flip the ratchet spool over, so the scalloped teeth face upward, and place the spool in the ratchet plate.
- Pull the cables taught from outside the ratchet plate so the spool sits flat in the ratchet plate.
- Press the ratchet cap onto the spool to engage the scalloped teeth.
- Twist the cap clockwise to wind the cables onto the spool.
- Continue to twist until the fingers reach the desired tension.
- To decrease the tension, gently lift the cap to disengage it from the spool.
- Optionally, install the ratchet cap retainer with a M1.91 plastite screw.
Step 11: Assemble Wrist Knob.
The wrist knob functions as a set screw, locking the wrist in a particular orientation.
- Pass a 6-32x 1/2" screw through the front of the knob.
- Install a 6-32 hex nut in the hexagonal cavity on the back side of the knob.
- Tighten the nut as much as possible to secure the screw.
- Screw the knob into the captive hex nut on the gauntlet.
- Adjust the wrist to the desired angle and tighten the knob by hand to lock the wrist in that orientation.
Step 12: Etch (optional), Cut, and Install Adhesive-backed Rubber Grips to Palm and Finger Tips.
To improve the grip of the hand, install adhesive backed silicone rubber onto the palm and finger tips. Use the templates provided or create your own. If you have scaled your prosthetic, remember to scale the pattern by the same factor. To further improve the grip and aesthetics of the rubber, use a laser cutter to etch a pattern into the grips. For a lower cost option, use a rubber stamp carving tool to carve ridges or patterns into the silicone.
- Peel off the paper backing on the silicone and install it on the palm and each finger tip.
- Press gently to ensure good adhesion between the silicone and plastic.
Step 13: Cut Leather and Foam for Gauntlet and Upper Arm Cuff.
Use the patterns provided for the gauntlet and upper arm cuff or create your own pattern for the leather and foam that line the prosthetic. To make your own pattern, felt can be used to test the shape and size before cutting out leather and foam.
- Trace your pattern onto the wrong side of the leather hide.
- Cut out the leather.
- Peal the paper off the foam and gently adhere the wrong side of the leather to the adhesive side of the foam.
- Cut out the foam, following the outline of the leather.
Step 14: Install Velcro Straps and Leather Backed Foam Padding.
Install the velcro straps and leather lined foam padding onto the gauntlet and upper arm cuff, with the soft side of the padding facing inward and the colorful leather facing outward. Optionally, use an exacto to cut slits in the leather-lined foam to pass the strap through the leather lined foam. This can help to secure the padding to the prosthetic.