For manufacturing our mechanism, we used rapid prototyping methods such as laser cutting and 3D printing for their perfect balance between precision and lead times among all the other options available. Precision was necessary for our press-fits and to reduce play in joints. Shorter lead times helped us come up with iterations faster.

3D Printing

We used FDM 3D printers – CraftUnique CraftBot+, Raise 3D and Creality Ender 3V2. Most of our 3D-printed parts were made from PLA. Selective parts which were exposed to fatigue were printed with PETG. The Geneva mechanism, gears, pulleys, and end effector were 3D printed.

Laser Cutting

We used the Full Spectrum 48 x 36” 140W CO2 Laser Cutter available in Texas Inventionworks. We used ¼” thick Baltic Birch Plywood. This thickness gave us the perfect balance between the required strength and weight for the base, vertical support, and links. We chose plywood over Acrylic as it was less brittle, allowing us to maintain tighter press-fits.

Precise press fits were crucial for the smooth operation of the mechanism. All the bearing housing holes were slightly undersized to press the bearing into place using a vice. We 3D-printed hubs to transmit motion from shafts to links, without slipping. The hubs were press-fit onto the shaft and held in place with a set screw.

The most difficult part to tune was our inverted spherical Geneva mechanism. The roller (mounted on the green Geneva Crank) had to be perfectly aligned with the slots in the white Geneva sphere, especially during the entry, for smooth motion transfer. We wanted the green Geneva crank to be stationary and the white Geneva sphere to rotate. One of our biggest challenges was to increase friction between the green Geneva crank, shaft, and base so that it does not rotate due to the large forces on contact with the Geneva Sphere. We used an inverted hub that was screwed onto the base and held the shaft in place with a set screw. The hub is attached below the wooden base and houses the shaft.

When two links make a pin joint, we use a collar in between the surfaces to avoid sliding friction. In this case, we use bearings on both links as we want them to rotate freely on the shaft.

We added counterweights to balance the weight of our pick and mechanism mounted on the large white gear. Balancing the large gear ensured it meshed with the driving gear smoothly.

Design for Manufacturing

The Geneva sphere was designed to be 3D printed without supports. This saved us 2 hours in print time and gave us a better surface finish. The overhang angle was designed to be less than 45 degrees. The motor mount and collars were among other components which went through DFM best practices for faster and better 3D prints.

Off-the-Shelf Components Used

Thrust bearings and 686 rotary ball bearings were used for smooth rotary motion between surfaces. Off-the-shelf timing belts were used with 3D-printed gears to transmit motion. We used un-hardened stainless steel shafts which could be cut with a saw.

Assembly

Our design was modular which helped us with as easy and accessible assembly. We had to go through multiple iterations for the Geneva mechanism, which required us to assemble and disassemble it. The pick and place mechanism could be disassembled by loosening two screws. The vertical support could be taken out by unscrewing 4 screws.

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