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When designing and fabricating parts for future prototypes, two important things to consider will be the tolerancing of 3D printed parts and their print orientations. Designing parts for a press fit means that not only are the parts difficult to place on the assembly without significant force, which causes concern over breaking the parts, but it can also cause issues for fitting multiple parts together and disassembly. This should either be done more carefully to avoid these issues or avoided. Another thing to consider is the forces that a part will encounter in its final position. If the part is printed with the grain along the same line as the forces applied to it, it may very well break. It is also possible to have issues with the surface quality of the part based on there the supports are attached based on the print orientation.
Final Prototype Assembly and Testing
Figure 17: The prototype assembly
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Figure 18: Prototype Testing
Final Design Decisions
We arrived at our final design by making a few key adjustments to our prototype. These were the implementation of a rail system to avoid out of plane motion in the follower, the addition of the motor and electronics, and the design of the gear train and drawing cylinder.
As can be seen in Figure 18, despite the use of the keyed shaft to avoid rotation, the moment placed on the follower by the turning of the cam would still result in out of plane motion. To compensate for this we initially considered and tested a dual shaft design, wherein the keyed shaft would be replaced with a regular 8mm shaft with a linear bearing on it to prevent binding and the shaft holding the follower passed between two parallel rails that would prevent the shaft from moving out of alignment with the slider. While this partially solved our issue, it meant that a taller overall design would be needed as the longer shaft would need to pass between the rails before reaching the cam. To solve for this issue the dual rail system and the linear bearing slider were consolidated into a single set of parts. This meant that we could avoid out of plane movement, prevent binding, and keep the shaft short to prevent large moments and material waste.
Figures 19 & 20: A prototype dual rail system with linear bearing rail holder (left), a consolidated rail holder design (right)
The second major issue was that, as of the design of our prototype, the device was still powered by the user. To automate the process of drawing the figure on the cam, we decided to use a stepper motor (and accompanying motor driver and arduino) that would be able to turn the helical cam a set number of rotations forward or backward with replicable results, so that the follower would draw the full figure and the device could be reset mechanically. By positioning the motor in line with the cam, we were able to take full advantage of this rotation and keep the design straight forward.
The final major design consideration that would lead us to out final design was the inclusion of the gear train and drawing cylinder. The major concept that we proved with our prototype was that we could get the follower to travel along the track of the cam, but this would represent a limited success without the ability to inscribe the data of the cam into an image. As such we needed a way to get our motor to turn the cylinder we had designed alongside the cam. Thus, by iteration we conceived of system by which we would turn the cylinder using a gear train consisting of multiple sets of spur gears to turn the cylinder using the power of our motor and refined it into a design which used a belt and pulley system alongside a set of bevel gears.
Figures 21 & 22: An early spur gear and cylinder design (left), a final belt and pulley+bevel gear cylinder design
With these design decisions made, we were ready to fabricate and assemble our final device!
Bill of Materials
For the final prototype, our team has discussed both a gear train and a belt pulley to determine the optimal system for connecting the helical cam and the cylinder to the motor, taking into account factors such as the friction and size of our entire system. Once all dimensions of our final prototype are finalized, we will move on to purchasing either several gears or pulleys with belt. Materials on the last three rows are the tentative materials we haven't finalized yet.
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