We decided to redesign the bow for the current printed size of the track and grip to be able to print another working prototype of the bow quickly and evaluate the snap connection as well.
Since we designed completely new bow designs, the snapping of our bow to the channel was also a problem. Because, no changes could be made to the STL file from the Thingiverse. We tried designing our own snapping system by taking measurements of the channel using vernier caliper.
We printed the Capture with the Stratasys printer to see if the tolerances are good enough and if our design fits to the track.
Printing parameters:
Material: ABS
Infill percentage: 20%
Layer height: 0.013 mm
Building time: 86 minutes
As the picture shows the connection part doesn’t fit to the track since it was slightly too small. We changed the tolerances and enlarged the dimensions of the connection part a little bit and included the design into our new bow design.
The “bow design 2” we came up with is shown in the picture below. It includes the new designed connection part.
We printed that design with the Stratasys printer.
Printing parameters:Material: ABS
Infill percentage: 20%
Layer height: 0.013 mm
Below is a picture of the printed and assembled bow.
After printing the new prototype, we evaluated the forces, connection and rubber band attachment.
As you can see on the picture the pins for the snap fit connection of the bow to the track broke, so we had to redesign the snap fit connection to ensure a good fit of the bow on the track. We want to add a radius at the snap fit pins and design them thicker in the shape of triangles to decrease the stress concentration.
We also noticed that the pins for attaching the rubber band are too small and have to be higher and include a groove to hold the rubber band. Also the current bow doesn’t bend at all since it is very thick and not very curved. Our final design should be thinner and more curved.
We printed a new bow design that included all the modifications mentioned above (left figure). The modified snap fit pins are now strong enough to resist the force caused by the pusher when it hits the bow. It was found that the bow was still too think and stiff to proviede energy for the shooting.
A final optimization step resulted in the bow design on the right. As can be seen the bow is much thinner and bends as required. Furthermore a reduction of building material and printing time could be achieved, as shown in the table below.
| Part | Built time | Building Material Volume [in³] | Support Material Volume [in³] |
| Modified bow | 5:30 | 2.98 | 1.10 |
| Final bow | 4:54 | 2.23 | 1.03 |
