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For corrective action, we should use a more precise method of drilling the bores and counterbores into the wood (in lieu of the current hand drill press method), and after that iteration, attempt to create a metal structure (column/base apparatus) so there would be no skewing when assembled together. As for the links, we would stack two plates on top of each other and drill downward to ensure the alignment of the centerline. Lastly, for press fitting, we would use a press or an assisted device with a vice to better align the normal force on the rod.
Listed below are areas of improvement in greater detail:
Tolerances:
We had a lot of trouble with tolerances during our manufacturing process. 3D printed parts had a lot of error in dimensioning values because of the mesh size, printing quality, and the feed rate of the end effector. For example, if we were to make the cams and cam followers out of machined aluminum instead of 3D printing, we likely would have seen a much better valve actuation. With the 3D printed parts, we had trouble creating large valve actuations. While the aluminum parts would likely also have to be lubricated, they would have provided a better example of valve movement.
Increased Friction:
In addition, we also had a lot of friction between the piston and cylinder wall. This was caused by a few factors. Once the belt was placed onto the pulleys, the camshaft was pulled up while the crankshaft was forced downwards. This adjustment in the rotating shafts cause the piston to traverse upwards in a slightly different path than we expected. Because of this, the piston would hit the cylinder wall on upwards motion and try to fall out of the cylinder on the downwards motion. Another source of friction was the linkages being misaligned and creating contact friction. This was a factor of very slightly misaligned crankshaft input holes. Because these holes were where the crankshaft was press-fit into, the linkages did not move together at the exact same time. This also contributed to the friction between the cylinder wall and piston. The solution to this would be to CNC these parts instead of manually making holes.
Material Choice:
Another issue that contributed to the misalignment of parts was the material selection. The wooden base was subject to movement and misalignment with the metal shafts, as well as poor press-fit with the hex bearings. If the base was made from aluminum, it is likely that we could have avoided these poor fitting issues. Another issue was the PLA used throughout the machine. Often, we would have to modify these parts after being printed to work the way they were designed. PLA also has rough edges that must be sanded down to use for heavy contact parts.
In summary, these three factors combined and did not allow our robot to work completely as intended. It was interesting to see the difference between designing in theory and manufacturing, every seemingly small issue was magnified in the final operation. In future attempts, the robot parts should be CNC machined from aluminum to eliminate tolerance error and misalignments. We realized this mistake during the manufacturing process and made the linkages from aluminum, but our project would have benefited if most components were also made from aluminum.
Further Improvements and Advancements:
Adding a motor input with feedback control:
If we could add a motor input with an encoder to sense the crank angle, we could project how the valves actuate at different RPMs, as well as show how an Internal Combustion engine works at rated speed. Furthermore, implementing a closed-loop PID control would be an interesting way to see how the motor speed adjusts for the different cams, and give insight into how variable valve timing works within the engine assembly.
Variable Compression:
Automakers are just starting to implement motors with variable compression. This technology basically allows for an engine to change compression ratio based on engine load. This can be achieved via controlling crank motion as well as valve timing. It would be interesting to see if this sort of variable compression could be achieved mechanically through linkage analysis or feedback control.