James Ruby and Joe Seifert
Mechanical Engineering Department, University of Wisconsin-Madison, USA
Abstract
This project outlines the process for designing and fabricating a steering wheel for the first car that the UW-Madison Badgerloop team will produce for the American Solar Car Challenge. The primary method of fabricating this steering wheel will be additive manufacturing. Once the wheel was modeled in NX, testing was conducted to help determine the requirements of the materials. After testing and literature analysis the additive manufacturing method was narrowed down to fused filament fabrication (FFF) using acrylonitrile butadiene styrene (ABS).
Introduction
The goal of our project is to produce part of the steering wheel for the Badgerloop team. Badgerloop is the solar car team at the University of Wisconsin – Madison. The team competes in the American Solar Challenge (ASC). The steering wheel will be part of the first car the team has ever built. The steering wheel design has multiple components: the front and back plates, the body, the buttons, the screen, and the fasteners. The only component that will be manufactured for this project is the body. The body consists of handles that are connected by two connecting bars. These components are a singular solid body that will be printed at once. A CAD model of the part is shown in figure 1 below.
Figure 1: CAD model of the steering wheel assembly.
Fabrication Process
Testing
We knew going in that this project would use a lot of whatever material we chose. In order to not waste as much material and time, we wanted to run some simulations in a structural analysis software. We elected to use Ansys as the software. Within Ansys, we pinned the center of the wheel so it could not translate or rotate, then applied vertical forces to the handles similar to how they would be if the wheel was being turned. The results of the first analysis test are shown in figure 2.
Figure 2: Results of the first analysis on handles.
The resulting maximum stress of the first analysis appears in the connecting bars at the top and bottom of the part. This maximum stress was around 33 MPa which was a bit higher than we were hoping for. This led to a design change where the connecting bars were thickened to reduce the stress on them. The same test was run on this new design and the results of that test are shown in figure 3.
Figure 3: Results of the second analysis on handles.
The maximum stress appears in the same location as in the first trial. However, this stress is now a much more manageable 9 MPa. We proceeded forward with this redesigned version for the duration of the project
Method
The main two fabrication methods identified for fabricating our wheel were stereolithography (SLA) and fused filament fabrication (FFF).
FFF is a process where the material is heated and ejected onto a heated bed through a moving nozzle. The material is deposited in different shaped layers to make the shape of the part. The process is relatively fast and has a decent surface finish. It allows us to print in ABS or PLA for materials.
SLA is a process where the build plate lifts out of a container of UV-cure resin. This resin is cured by a UV laser and then lifted up the height of one layer, then the next layer is cured. This process has a very good surface finish and since it is UV cured, it would have excellent thermal properties in the heat of the driver cabin. The problem with SLA is that the build plate is much too small for our part. This led us to choose FFF. We have access to the Fused Form 600
Materials
For our project, we identified three main material types to potentially use for our fabrication process. These materials were Polylactic acid (PLA), ABS, and polyamide.
PLA is a versatile, biodegradable polymer that is very widely available [1]. It is the most sold material in the world [1]. Due to the large supply and demand of this material, it is also a very economical material for additive manufacturing purposes. It is commonly used in a wide range of applications from the agricultural to the biomedical industries [1]. Due to its availability and price, this was immediately one of our best options from the start.
ABS is used in many of the same applications as PLA [1]. It also has a similar market share as PLA, which means that it is also widely available [1]. ABS is also a thermoplastic polymer [2]. ABS is not biodegradable like PLA, but it can be recycled [2].
When comparing these two materials, PLA has higher yield strengths and flexural strengths, but ABS experiences more elongation before failure [2]. This material property data can be summarized in figure 4 below.
Figure 4: Material property data of PLA vs ABS [2].
Both of these materials are commonly used in conjunction with FFF printers. One study was conducted to compare the flexural stresses of each of these two materials using FFF to manufacture them. The results of this experiment can be seen in figure 5 below.
Figure 5: Stress data of PLA vs ABS [1].
ABS also has slightly better thermal qualities than PLA, which is one of the primary requirements for our wheel [3]. ABS is also a durable material, suggesting that it is suitable for a car that will be used for years.
Polyamides are also commonly used in FFF processes. Many polyamides are also infused with carbon fiber for increased stiffness and thermal resistance [4]. These types of materials are not widely available and are considerably more expensive than ABS and PLA [4]. Due to the higher cost and lower availability, we decided to proceed with ABS for our project.
Once the material was selected, we were able to determine many of the printing parameters. One of the first printing parameter to consider is the infill percentage. From figure 5 above, we can see that a higher infill percentage generally results in a stronger part, regardless of the infill shape [1]. In order to simplify our design choices, we decided to do a 100% infill percentage. We also were not constrained by the weight of the part, so we were able to maximize the strength of the part.
Some of the other printing parameters were the bed temperature, nozzle temperature, layer thickness, and nozzle diameter. We decided to go with the standard values for all of these parameters with ABS using FFF. The bed temperature was 100 degrees celsius, the nozzle temperature was 230 degrees celsius, the layer thickness was 0.2 mm, and the nozzle diameter was 0.8 mm.
The last main printing parameter that we focused on was the layer orientation. The two main factors that we considered while determining the layer orientation were print time and part strength. one study was conducted testing these factors using ABS and FFF and the main results are summarized in figure 6 below.
Figure 6: Layer orientation data for ABS using FFF [5].
As seen from the data from this experiment, the 0-degree orientation was the strongest [5]. This is fairly intuitive and backed up our initial guesses, but now it is safe to proceed knowing that we have data to back this choice up. One notable thing about this data is that the strength of the part for pretty much any orientation is well above the planned stress that our part will undergo (9 MPa).
The other factor, print time is an important factor to consider. Because the part is the shortest in the direction perpendicular to the forces on the wheel by far, the 0-degree orientation will also be the shortest print time. This wrapped up our main printing parameter deliberation as we proceeded with this orientation.
Discussion
We proceeded with the ABS in an FFF printer and used the Fused Form 600 available to us. The first print trial resulted in some almost hilariously poor results. These results can be seen in figure 7.
Figure 7: The first trial print on the Fused Form 600.
In this initial trial, we believe the part began to peel off of the print surface because it wasn’t hot enough and eventually was pulled off by the nozzle. Once the base of the part was pulled off the print surface, the material had nothing to stick to and ended up just “spaghetti-ing” all over the bed.
This of course led to a second print. We noticed a corner of the print start to peel up again so this print was stopped immediately in fear of the same thing happening as the first trial. The part peeling up can be seen in figure 8.
Figure 8: The second print trial, in which the part peeling up can be clearly seen.
We then proceeded with a third print after rotating the part and placing it in a different position on the bed. This resulted in our most complete print, but about 60% of the way through the print must have shifted. Some top layers of the print are clearly shifted off to one side, and this made the third print unsuccessful. The result of the third print can be seen in figure 9.
Figure 9: Result of the third trial showing some shifted layers.
After seeing the results of the third trial, we decided to print the part in two pieces, the front, and back. These two pieces would be connected in the center using a solvent-based adhesive. This print was much more successful as we were able to print both halves. One of the halves can be seen in figure 10.
Figure 10: Result of the fourth print trial, one half is shown in this figure.
After two successful prints, the next step is to purchase the necessary adhesive and connect the two halves. The two halves will also be bolted together to carbon-fiber plates on either side, ensuring a secure connection between the two pieces.
Conclusions
In conclusion, we determined the best method for fabricating a steering wheel for the UW-Madison Badgerloop team’s American Solar Car competition to be FFF using ABS. Despite identifying the best method for this process, fabrication proved to be challenging due to the size of the part. Many trials were conducted and eventually, a successful print was performed for two different pieces of the wheel to be connected in the future.
References
[1] E. Provaggi, C. Capelli, B. Rahmani, G. Burriesci, and D. M. Kalaskar, “3D printing assisted finite element analysis for optimising the manufacturing parameters of a lumbar fusion cage,” Materials & Design, vol. 163, p. 107540, Feb. 2019, doi: 10.1016/j.matdes.2018.107540.
[2] J. A. Travieso-Rodriguez, R. Jerez-Mesa, J. Llumà, G. Gomez-Gras, and O. Casadesus, “Comparative study of the flexural properties of ABS, PLA and a PLA–wood composite manufactured through fused filament fabrication,” Rapid Prototyping Journal, vol. 27, no. 1, pp. 81–92, 2021, doi: http://dx.doi.org/10.1108/RPJ-01-2020-0022.
[3] “White MH Build Series ABS Filament – 1.75mm (1kg),” MatterHackers. https://www.matterhackers.com/store/3d-printer-filament/175mm-abs-filament-white-1-kg (accessed May 02, 2022).
[4] “ColorFabb PA-CF Low Warp Filament – 1.75mm (0.75kg),” MatterHackers. https://www.matterhackers.com/store/l/colorfabb-pa-cf-low-warp-filament-175mm-075kg/sk/MCKRCTXY (accessed May 02, 2022).
[5] B. Rankouhi, S. Javadpour, F. Delfanian, and T. Letcher, “Failure Analysis and Mechanical Characterization of 3D Printed ABS With Respect to Layer Thickness and Orientation,” J Fail. Anal. and Preven., vol. 16, no. 3, pp. 467–481, Jun. 2016, doi: 10.1007/s11668-016-0113-2.