BRIDGE
Time Period: March 2017 - May 2017
Course: Introduction to Manufacturing and Tolerancing
Skills Developed: Tolerancing, 3D printing, Design thinking, Product testing
Software Used: SolidWorks, Cura Type A
Project Background
The task of this project was to design a bridge that, attached only to two vertical poles, could withstand a load. The main focus was on tolerances related to fused deposition modeling. As a team, we were to 3D print the bridge with PLA on Type A printers. The final task was to test the bridge's specific strength and specific stiffness, designing to optimize both ratios.
My team designed and printed two different bridges with different designs. Both bridges performed exceptionally. The lighter bridge had a higher specific strength and specific stiffness value; however, the results for both our bridges surpassed that of the majority of the class. We presented the bridge and the test results at the Jacob's Hall Design Showcase at the end of the semester.
BRIDGE 1
Design
As a team, we designed a simple, flat bridge. The span of the bridge is fairly small; this combined with ease-of-printing of a flat structure makes this shape ideal. Using a basic understanding of truss structures, the sides are made up of with horizontal, vertical, and diagonal trusses to maintain strength while reducing weight. The bridge would be held up on the vertical poles with collars that would be held up using a screw to clamp the pole.
BRIDGE 2
Design
We decided to give this bridge a more complex shape. There is an X shape made of trusses, to maintain strength while reducing weight, that is cut in half vertically. This makes an arched center which is useful because the bridge will want to flatten with an applied load in the center. This load applies a sideways force on the location of attachment to the pole, helping to stop it from sliding down. A collar is used to apply additional resistance to counteract the downward force. Furthermore, a platform sits on top of the cut X to unite the two pieces.​
Printing
When printing this bridge, it was important to consider the orientation of the print on the print bed because it affects the structural rigidity of the unit. Although printing it on one of the square surfaces would have been the best option, this would allow a high chance of print failure. So, we decided on printing the bridge as it would be attached to the vertical poles. It took many tries to get a successful print, adjusting and learning about all the different settings and what the optimal setting would be for such a print. The collars and screws for this bridge were also difficult to print due to the thread and the precision required for these parts.
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Printing
When printing this bridge, considering the orientation of the print mainly relied on simplicity due to the complex shape. We decided to print the bridge with all the flat sides touching the bed simply because the chances of the print failing increased exponentially by changing the orientation. It took many tries to get a successful print, adjusting and learning about all the different settings and what the optimal setting would be for such a print. The collars and screws for this bridge were also difficult to print due to the thread and the precision required for these parts.
Testing
We tested the bridge by applying a force and measuring displacement until the bridge failed. The results for this bridge were exceptional. It had the higher specific stiffness and specific strength values between the two designs and was one of the highest in the class.
The data resulted in:
Specific strength = 5532 N/kg
Specific stiffness = 1,636,661 N/(kg•m).
Testing
We tested the bridge by applying a force and measuring displacement until the bridge failed. The results for this bridge were very impressive. It had a lower specific stiffness and specific strength value between the two designs. However, it was able to withstand more load than the other bridge.
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The data resulted in:
Specific strength = 5,426 N/kg
Specific stiffness = 469,814 N/(kg•m)
Conclusion
The design process from CAD to 3D printing to testing brought a lot of factors to our attention that we were able to better understand by the end of the process:
The initial design relied on the flexibility of PLA and the fact that it is easy to print and install.
Post-printing, we realized that the resolutions of the Type A printers were too poor and the material was not flexible enough to provide enough compressive force to withstand as much load as we had hoped. The tolerance of the Type A 3-D printer was concluded to be ± 0.04 in.
Regarding orientation of printing, the difference was most noticeable when printing screws and nuts because the thread becomes more accurate if the screw or nut is printed vertically. If it is printed horizontally, supports ruin the threading.
The initial screw size required a significant amount of filing before it was able to fit, so we ended up increasing the nut thread and increased infill of screws to be ~25% to prevent screw from bending/snapping.
Reflection
In hindsight, it would have been best to avoid air gaps between bridge parts as these gaps become the weakest points and the location at which the bridge is most likely to fail first. Printing the bridge in the optimal direction will prevent it from splitting along layers. Relying mostly upon screws to stop a downward force is not ideal. Increasing the surface area of the inner surface of the collar to provide more frictional force may help the bridge to withstand more load. With access to only the Type A 3D printers, designing a collar with relatively small screws is not ideal as they tend to snap easily; increasing the width may be beneficial. Print the screws facing up, and decrease the screw thickness layer; print them at smaller diameter than standard bolts to account for the printer's resolution.
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From this project, I learned a lot about design, Fused Deposition Modeling, and Tolerancing. When 3D printing, designing and printing in multiple parts increases the likelihood of a successful print. It is important to print in the optimal direction to strengthen the object.​ It is important to change the printer settings based on the geometry and use of the product to help the print succeed. Tolerances are important to consider when manufacturing anything, and when doing so, you must consider the tolerance of the manufacturing method you are using as some are more precise than others.​​