14.3 Implementation
Once a final design is achieved, with all the link lengths and proper angles that will ensure the desired motion, it is time to get into aesthetic decisions and “the optimal way to build it,” which include manufacturing and assembly decisions.
We decided that our best option was to 3D print as many parts as we can, which will allow rapid prototyping and testing so we could redesign if needed. However, there were a couple of parts that were laser-cut due to their flat geometry, and other parts were ordered.
Here is a list of all the parts that have been either 3D printed, laser-cut, or bought:
3D Printed (custom design):
Fingers
Palm hand
Motorholder
Clevis Pins (6mm, 4mm diameter)
Gears (different sizes and teeths number)
Separators (cylinders between baseplates)
Laser-Cut:
Base plates
Outside clear shell
Bought:
Rods
Regular Washers
Silicone taps
Bearings
Set screwed shaft holders
Rollers
Toggle Switch
*More details could be found in the BOM
Explaining more on a finger design, our constraints were lightweight, optimal cross-section for contact friction and smooth motion, and avoiding any interference between rods or linkages. The result was a looks-like finger shell with holes for the pins that connect the linkages, allowing freedom to rotate.
Clevis pins were designed using a measure tool that we laser-cutted, that allowed us to have the tolerances that align the best with our requirements.
The motor holder was designed modular as a preventive action; the idea was to ensure an easy-to-replace system in case of motor failure or material fracture, since this is the part that will handle the highest torque. It is attached to the motion system using in-body rings, press fitted with bolts against the separators.
We cut a 3mm Acrylic for the right side of the hand that will allow to visualize how the system is actually working. An acrylic “thumb” was made from 4 3mm acrylics.
At this point we had all 3D printed, laser-cut, and ordered parts, so the last step before assembly was to cut the rods. For this task we used a table saw and sanding tool to smooth the edges.
Assembly
This section aims to show the struggles during the assembly process and how we got over them. They will be presented in the same order they showed up during the workflow. There were some issues about tolerances during this process, but for that kind of problem, we just reprinted the part.
The first problem we faced was a lot of vibration on the motor holder. This is a typical problem in dynamic systems, and usually they require extra parts like rubber that absorb enough vibration so you achieve your quasi-static constraint. However, in this case our motor holder was designed with an extra hole that allows the use of a larger screw connecting the holder rings with the upper section of the part, resulting in a stiffer structure.
After we solved this, we found out that the system was requiring so much more torque than expected; this was an unexpected behavior since we tested the motor before with the system, and the result was a smooth motion without an excessive torque requirement. This previous test helped us to be clear that the problem was neither the motor nor the system itself, which led us to the alignment issue. It only took a couple of manually tilting movements to find out the correct alignment for the shaft. We iterate a couple of times, tilting the whole system manually until we found out the position where the motor noise (which means less torque requirement) was the least and the motion was the smoothest. Since the alignment is determined by the position of the case (palm hand), we placed our shaft holders in a way that keeps the position press fitting the whole case. Other spares that help with this problem were the acrylic separators that act against the separator force, making a tighter system.
The entire assembly, without any problem should take 30-40 min.
After every part has been attached properly, the electronic components were connected using their pins, wires, and welding.
Finally, we paint it as scary as possible.
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