This was the catapult we made for our biomechanics camp. James designed the whale walls and most of the intricate designs while I made and replicated the supports and lever. We used the Sketchup program as well as DFX files converters and V-Carve in order to cut the design into a 4X8 piece of plywood.
- 4'X8' board
- Sketch Up
- Illustrator (if necessary)
- V Carve
- Shop Bot
- Nail gun
- A lot of wood glue
- Plastic cup
Step 1: Sketchup
The first step in this process was creating a design in a 3D environment. We created the whale walls, supports, lever arm, knobs, and decorative fins in Sketchup and virtually assembled it. The supports were created by making rectangles with dimensions of 1'X.75"X1.5". All supports going in between the whale walls, including the lever stopping ones, were made the same size. The lever was created using the same technique, however the dimensions were changed to 2.25'X3"X.75". We then made two knobs both being 1.5" in diameter and .75" thick. The decorative fins were then made to no specific size. We just made to a scale which would allow them to ensure stability to the knobs while still being aesthetically pleasing. Finally the whale walls were virtually cut out of 10'X10' square and the scaled down to size to match the other components.
Step 2: Converting to 2D
After virtually assembling the pieces, we then converted them to 2D. We did this by laying all the components on their side (on a 4' X 4' X .75" rectangle to replicate the piece of plywood that the components would eventually be cut out of); then going to the camera menu and selecting "parallel projection"; and finally selecting the top view under the "standard views" menu.
Step 3: Exporting
After converting to a 2D view the design has to be exported because the Shop Bot doesn't read Sketchup. Normally one would export the design in .pdf form, however due to some complications, a .dxf file was used.
Step 4: V Carve
After exporting the file, it is then downloaded to the program V Carve which is what the Shop Bot reads.
If certain aspects of your Sketchup design cannot be read in V Carve you may need to edit, or "clean it", up in a different program called Illustrator.
Step 5: Chop It Up!
Now that the file is perfect and ready in V Carve, we synced it with the Shop Bot, loaded the piece of plywood, ran it, and watched our components get carved out!
Step 6: Construct
The final step is obviously the construction. Preassemble the catapult before screwing and glueing. First glue two supports together. The first support was then glued near the front bottom of the whale walls the second support was glued near the back bottom. Glue the third support (double support) near the tops of the whale walls. In addition to the wood glue, screw all the supports to the whale wall. The lever arm ended up being cut in half lengthwise to reduce the weight. The remaining half of the lever arm and the remaining support were then cut to the same size and had identical wedges carved into them. The two pieces were then screwed on the inner rear of the whale to stabilize the whale. Two holes were then drilled on each side of the whale walls for the torsion string to weave through. Two holes were then drilled into the lever arm, one above the other for weaving. The torsion string then weaves through all the holes and into a knob on each end. The string should then complete a whole circle in an order of left wall, lever arm (bottom), right wall, right wall, lever arm (top), left wall. Knot the string on the side in which it was inserted to complete the loop.Then twist the knobs in the same direction until the string is ridiculously tight. Then the decorative fins are stapled to the know and a plastic cup is screwed to the top of the lever.
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