3) Manufacturing and Prototype Iterations

CAD and Development of First Physical Prototype

         Once we were able to replicate the velocity profile generated from our experiment with the linkage created in MotionGen, we created a solid model of our mechanism in Solidworks. Initially, this model simply served as a way to visualize our mechanism as a 3D assembly at a specified scale (which we purposefully kept as small and compact as possible for the demo) as opposed to the 2D linkage created in MotionGen. Beyond creating the links for the CAD model, this involved designing a pan that would be able to be 3D printed for the prototype (in MotionGen, the pan was modeled as the right-most link in the mechanism).

         Once the linkage was created and we verified that the correct general motion and velocity profile was generated by the mechanism, we focused on updating the model with a physical prototype in mind; this included altering the links to account for the thickness of the wood available for the laser cutters at TIW and altering the pan design to better integrate it into the link it is attached to. We then printed out and laser cut our first versions of the pan and links, respectively, in order to start to test tolerancing for press fits and ensure that our scale was adequate for the prototype.

Immediately after inspecting the links and pan, we realized that our pan geometry was too shallow and, as a result, we would not be able to replicate the ability to flip an object through pure rotation, which requires a greater outer pan radius than our initial printed pan did. After increasing this outer radius to something closer in scale to the actual pan we had used in our experiment, we reprinted the pan, and its second iteration is shown below.

Next, we gave thought to how the entire mechanism would be assembled and updated our Solidworks model accordingly. We included bearings in the joints, which led to an increase in the width of each link due to the size of the bearings available to us. Once the size of the links was changed, the pan model also had to be adjusted to account for the increase in link thickness due to the nature of the press fit we decided to use to fix the pan to the output link. In addition, we designed a base plate and anchors (both shown below) that were laser cut along with the adjusted links. In order to position the motor shaft at the correct height for our mechanism to work as intended, a block with an extrusion to press fit into a slot in the base plate was 3D printed to mount the motor to. Finally, the shaft protruding from the motor we used is extremely short and also has a unique geometry, so rather than adjusting the input link to be able to fit onto the shaft directly, we designed a shaft which we 3D printed, and press fit onto the shaft of the motor. The 3D printed shaft has the same outer diameter as the other shafts used in the assembly, so it allowed us to keep the geometry of the input link consistent with that of the others.

When assembling the new set of laser cut pieces together, we realized that the sheet of wood we had was not exactly as thick as was labeled and as a result, the press fits between the base plate and the anchors were too loose. We accounted for the actual thickness and laser cut a new base plate. We then assembled all of our 3D printed and laser cut components using shafts and shaft collars (which were ordered) to produce the prototype shown below. Once our assembly was complete, we used an Arduino and motor controller to program the power delivered to the motor in order to deliver enough torque to effectively power the mechanism without inducing a velocity too high for safe/realistic operation.

First Prototype Reflection

After completing our initial prototype, we learned a number of different things that will aid in the creation of the final prototype. Firstly, we will need to use a much more powerful motor and a much more stable motor mount. While this change between the initial and the final prototype was expected, our expectation was confirmed in the operation of our first prototype. The motor we used was readily available and supplied enough torque to achieve the full range of motion of our mechanism while also being small enough to keep our design compact. However, after we scale the size of our mechanism for the final demo, we will need a more robust motor. 

On the topic of scaling, we learned that we would not be able to successfully flip an object with our initial prototype due to its size. Our plan was to make the first prototype at a relatively small/compact scale with the goal of learning about how the mechanism could be assembled while also demonstrating that we could flip an object using pure rotation, as we did in our experiment with a real pan. However, while we were able to replicate the desired angular velocity profile which we found to be sufficient to flip an object through pure rotation, we did not account for the insufficient linear velocity of the model egg due to the small scale of our prototype. Even though we intended to build a larger mechanism for the final prototype from the beginning, we did not realize that our first prototype would not be sufficient for demonstration of flipping a model egg. A larger mechanism is necessary in order to generate a linear velocity large enough to flip an object. 

Finally, we learned the importance of proper dimensioning and tolerancing for press fits through the process of building this prototype. Our guess-and-check method worked for the quick manufacturing of this initial prototype, but the tolerancing error for each press fit propagated throughout the mechanism, leading to more out-of-plane motion than is ideal for stable mechanism operation. For the final prototype, we will laser cut a 10 mm x 10mm square and measure it with calipers to determine the percent error of the cut dimensions. Also, we will 3D print a 10 mm x 10mm x 10mm cube, measure again using calipers, and determine the error. With a better understanding of the actual dimensions of our manufactured parts, we will refer to the ANSI Standard Limits and Fits data to ensure we have the best press fits possible for our application.

Development of Final Prototype

Following our first prototype, we moved on to make adjustments based on what we learned, starting with updating our CAD model in Solidworks to reflect the changes we hoped to make for our final design. The first and most readily apparent of these changes is the size of our final prototype. The size increase revolved around being able to use a life-size pan in our linkage, so our final mechanism turned out to be roughly two and a half times the size of our initial prototype discussed above. 

Other changes to the design of the mechanism include the addition of two saddled links, doubling the thickness of each of the non-saddle links, changing the design of the pan, designing a mount for a new motor, and adjusting the design and placement of the link anchors as well as the base plate to reflect changes to the links making up the mechanism.

The changes to the links of the mechanism, including increasing thickness and adding saddle joints, were all made with structural integrity in mind. The goal was to increase the stability of the system to mitigate any issues that might present themselves with the mechanical load of the system increasing with its size. Following this same line of thinking, we made the decision to change the material of the input link (connected to the motor), output link (connected to the pan), and ternary link from wood to acrylic, as we believed these links to be the most important structurally and thought acrylic would be able to handle the stresses on the mechanism better than wood would. 

As discussed previously in the First Prototype Reflection section, we had an issue with not generating enough linear velocity in our first prototype. As a result, we knew we wanted to extend the distance between the location of our egg to the linkage arm. Thus, we created a longer detachable pan arm design which connected with bolts and nuts. This allowed us to iterate on the design, such that if we wanted to increase the pan arm length or adjust pan geometry, we could print each part while reutilizing the other part that didn’t need changing. We also made slight adjustments to the pan geometry itself, decreasing the thickness of the walls and increasing the curve radius to improve flipping motion.

Having learned our lesson with regard to the importance of determining tolerancing ahead of time to make the manufacturing process smoother from building our first prototype, we spent time taking data for the tolerance of one of the laser cutters available at TIW. We found each laser cutter to be slightly different in terms of the tolerance they would cut parts to, so we made sure to cut our parts for the final prototype on the same laser cutter for which we took all our measurements for determining the tolerances. We determined the tolerances for square cuts as well as circular cuts for both wood and acrylic on our chosen laser cutter by cutting multiple small squares with circular cutouts in the middle. 

After determining all required tolerances for our mechanism, we proceeded to laser cut all of the components, including the links, anchors for the mechanism, and base plate. We then took these components and assembled them together with the 3D printed pan and motor mount, using bearings, shafts, and shaft collars at each joint. Finally, we wired and integrated our electrical components (arduino, motor controller, motor, and power source) into the assembly with a laser cut acrylic cover providing protection for the arduino, motor controller, and the wires between them. 

After initially assembling the final mechanism, our motor struggled to power the weight of our system, resulting in it outputting a non-constant angular velocity of the motor shaft. This meant that we were able to achieve our targeted motion profile, but not the ideal velocity profile. In order to attempt to counteract this, we decided to remove half of the input link (connected to the motor shaft) and the ternary link, since both of these were acrylic and removing the extra thickness lowered the overall weight of the system. After halving the thickness of these two links and reassembling, our motor continued to struggle to power the system, albeit to less of an extent as before. 

Bill of Materials 

• Small non-stick fry pan
• Motor 
• DC power supply
• 12 x shaft collars
• 8 x 6 mm ball bearings
• 7 x 6 mm shafts (95 mm length)
• 20 x washers
• 10 x bolts
• 10 x nuts
• Model egg
• Arduino
• L 298N Motor Controller
• TIW approved wood and acrylic for laser cutting links
• PLA used for 3D printed parts