Upon completion of our first prototype, it was clear that our components were oversized which led to a long build time of approximately 13 hours. To optimize the design we reduced the number of parts, made parts smaller, thinner, and removed unnecessary material from the center of the parts. The major changes are outlined below:
The previous frame design included 4 parts (vertical track, cam, and two side frames). This was necessary for initial testing to be able to move the cam to different position holes and find the optimal cam location and shape. With this information, we were able to design the second prototype frame which combined the old 4 parts into one part. To reduce material and build time, unnecessary material was cut out of the center of the frame and cam. Also, the old vertical track was 2 cm square, while the new vertical track portion is 1 cm square.
For the second prototype, the outrigger leg design is not as wide. We kept them at 1 cm thick because we were concerned a smaller thickness would be too flimsy, but we removed material from the center of the part to reduce material and build time. I semi-circular design was used since it is the strongest possible shape to support a vertical load.
The overall architecture of the prototype designs are shown below. The design improvements to the second prototype have reduced the build time to around 8 hours. We are continuing to make improvements to the design to reduce the material usage and build time. Our next focus is on the arm, hook, arm carrier, and counterweight holders.
Comparison to other AM Technologies:
Our current parts have been design specifically to be made using a FDM machine. We have considered parameters such as the build orientation in the design and layout of our parts to reduce build time, optimize the strength of the part, and to eliminate the need for support material. We believe FDM is the best process for this design but would like to point out some advantages and disadvantages to using other technologies.
Fused Deposition Modeling:
Advantages – Inexpensive, shorter build time, wider availability for home users, good mechanical strength
Disadvantages – Assembly/fitment modifications required, poor surface finish
Powder Bed Fusion:
Advantages – We could print the entire assembly including the pivot joint all at once and have a working trebuchet without the need for assembly, good mechanical strength, high precision
Disadvantages – Long build time, high cost
Stereolithography:
Advantages – Smooth surface finish would reduce friction of the sliding components, high precision
Disadvantages – The material would be more brittle and prone to failure, high cost
EDIT (12/6/2015):
Keeping the movable/removable cam would be beneficial if the user wanted to maintain the ability to easily adjust the projectile firing height and distance based on the location of the target. Since the goal of this project was to have a simple trebuchet with a short build time that shoots the furthest, we decided to fix the cam in the optimal distance location. This allowed us to combine 4 of the parts into one and reduce build time. The estimated build times and parameters used for design 1 and 2 are shown below:
Design 1: 12hours 25min
Design 2: 5hours 37min
For both designs we used a layer height of 0.3 mm, 20% infill and a shell thickness of 1mm.
Thus we reduced the build time by 55%.
The unique opportunities with additive manufacturing allowed us the freedom to make complex shapes that would be time consuming or impossible to make with other manufacturing methods. This is seen mainly in the legs and frame where inner material is removed to reduce build time and material usage. Additive manufacturing also allowed us to have relatively hollow parts with a 20% infill which reduces build time, material usage, and weight compared to a fully dense part. Another benefit of additive manufacturing is that we could make multiple parts all at once during the same build as shown below. For the long, thin parts such as the vertical track and arm it is important to have a bead direction parallel with the sides for maximum strength. Parallel orientation of the vertical track also reduces friction losses since the counterweight carrier is sliding in the same direction as the bead orientation. Surface quality and friction losses could further be improved by a smaller layer height and bead width, but this would negatively impact the build time.
