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Most of our parts were 3D-printed using PLA. We chose to 3D print many of our parts because it enabled them to include the multi-layered extrusions and cuts used to interface the pin-jointed parts together, rather than needing to machine or cut many pieces and then laminate or otherwise attach them together. PLA was chosen for the material because many of the parts we needed to print exceeded the printing time limit imposed by Texas InventionWorks, so we found an alternative way to print them but only had access to PLA filament. One advantage of the PLA filament was that its pliability allowed very secure press fits between our mechanical components to be achieved. This same compliance did have a negative effect on the final prototype, though, as even though the mechanism was not back-drivable, the compliance of the PLA parts meant that they deformed under load while grasping, and prevented as firm a grasp as desired from being achieved. Most of our other parts were either taken directly off-the-shelf or only required slight modifications easily performed with basic tools.
An early prototype built revealed several limitations related to the way certain parts were toleranced and other issues with the design of individual components and the larger mechanism, including:
- The large offset between the sliding and pinned joints of the link that interfaced the actuation arm and distal finger meant that link was under higher than anticipated stress, and because of the mild compliance of the PLA, this led to inconsistent actuation.
- The original prototype, which involved the actuation slider moving away from the gripper jaws, had a much lower resistance to force applied to the gripper jaws than expected. Altering the mechanism such that the actuation slider moved toward the gripper jaws while closing resolved this issue.
- Some parts featured longer extruded sections of PLA that were subject to high stress, meaning they failed eventually. Redesigning these sections to allow steel shafts to pass through them eliminated this problem.
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These discovered issues informed a revised design of the larger system and of many subcomponents. After manufacturing these parts, the assembly process went very smoothly and the assembled mechanism was operational as intended with smooth actuation and the desired high-holding capacity.
The assembled prototype is shown below.
