Our objective is to eliminate the need for an elastic rubber band. In order to make the bow work with a non-elastic string, the bow must be designed in such a way that it can act elastically and provide the power needed to shoot an arrow itself.
The initial CAD design is based on our original concept. After downloading the STL files from thingiverse, we converted them into CAD files and then assembled the components to get the final product. By doing so, we were able to identify a few areas for improvement. For example, the assembly fit between the grip body and the track was too tight. We were not sure if the printers which we have would be able to print such an accurate geometry. We are planning to optimize and change our design to match the printer’s ability so that the final print fits well.
After converting the stl files, we started working on the design for our bow. We borrowed key principles from the website and also a few design concepts from grabcad.com to get our design started. After a few modifications and calculations, we came up with the design shown below, called “bow design 1”. http://crossbow.wikia.com/wiki/Bow_design
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The two main parameters considered during the design were the draw length and the draw weight.
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Draw length in our case is limited by the length of the track.
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Draw weight is in our case is the weight of the pusher.
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Since we needed more draw length to increase the energy absorbed by the bow, we added a stiff middle section and increased the length of the bow string. The middle section was also supporting the track, so having it thick was very essential
- The second method was to decrease the stiffness as we were printing with plastic. The stiffness of a limb is proportionate to cube root of it’s thickness. So we reduced the thickness so that it would bend
As per the dimensions of the arch of the bow, theoretically it should be able to return to its original shape after releasing the sting that pulls it back. Further calculations are required to determine if the plastic material which we will be using in the lab will be able to withstand the forces.
