Background Information
William Liang’s inspiration – William simply wanted to make a prosthetic for someone who is “lazy.” Losing a hand can definitely cause complications, but William saw an opportunity to utilize the space within a prosthetic. He wanted to design something that allows one to reach for something far away just from sitting in their couch. He enjoys things with a bionic and futuristic aesthetic and came up with a design that would attract someone with similar interests.
InitiaL Design
The initial idea for the project involved an expanding/retracting hand, with multiple pieces for movable fingers and wrist. The actual mechanism to carry out the function was still up for discussion. After discussing with William, the main focus of the design shifted to just the expand/collapse mechanism. Our updated design involved:
- a mechanism that is similar to a baton or toy lightsaber.
- One non-moving hand piece due to time constraints
- Holes for straps
- Twist to lock mechanism for the hand
- Button controlled LED light in hand for bionic aesthetic. Possible ping-pong ball light diffuser.
Final Design
The decision was made to eliminate the lighting up hand piece and to solely focus on making the arm design strong enough to bear the weight of William’s sculpted hand. Due to this decision, the twist to lock mechanism was also eliminated from the final design.
The final model involved a main arm piece and three expand/collapse pieces, with the smallest piece attaching to the hand through a threaded extrusion. Each smaller piece would slide in through the bottom of the next biggest piece, but would not fall out due to it’s cone-like shape. The user’s arm stub would go in approximately 4 inches, indicated by the exterior ring, and would block each piece from falling out of the bottom. Four slits are placed at the bottom opening to allow for two straps to hold the arm in place as well. Additionally, holes and grooves are designed all around the arm pieces for a bionic aesthetic, as requested by William. Along with the pin, the side holes are utilized as a locking mechanism to prevent the pieces from extending without the user wanting them to.
Test Print and Final Product
The test print for the arm was scaled down to 25% and the default MakerBot print settings were used. It took about 2 hours to print with diamond fill and only raft support. The main focus was to see if the design worked as intended, which it did.
The final print was scaled at 100%, used 35% infill density, 3 shells, diamond fill, raft support, and took about 24 hours to print. The increase for shells and infill density allowed the print to be stronger and bear more weight, allowing William to attach his plastered hand without the piece breaking. Additionally, the largest arm piece needed to be split into two pieces to allow the model to fit on the printer where the smaller piece was able to slide into the larger piece. After the print, William was able to drill a hole into the hand, matching the size of the threaded extruded piece, and attach it to the arm.
During the process of making the arm, we faced various challenges:
- Dimensioning
- For this project, we designed our prosthetic for someone who has an amputated forearm, however everyone’s body is different.
- Our prosthetic was designed for four inches of the amputated forearm to fit, but the design can be adjusted before printing for different arm lengths for different people
- Hand function
- Hand function is very complicated to recreate and involves a lot of time and research
- Due to the time restraints for this project, we kept our focus on the expand/collapse mechanism and the Ardunio electronics, since that was the main intention of the artist and project.
- In the future, we can create another hand piece(s) that would allow for finger movement and that could also be screwed onto the mechanism. We could also add a ball socket mechanism that allows for wrist movement.
- Locking mechanism
- Our initial idea for a twist to lock mechanism did not consider that it would completely block the expanding pieces.
- We later added a pin locking system, but this adds more complexity to the design and adds a small part that can be easily lost by the user.
- In the future, our design can change to allow for an optimal locking system.