Since the first printed prototype version of the grip and trigger was unwieldy, too small, and the resolution was not high enough to print all the details, we scaled everything up to 2 times the original size and used a smaller layer height for printing.
In the picture you can see the comparison of the old (small) and new (bigger) printed grip parts. The assembly of the new enlarged version is much easier since all the parts fit together very well. Also the details like the small pin for attaching the trigger are printed very precisely and the plug-connection does work well now.
Printing parameters for the new (bigger) grip, which is 2 times the scale of the original one, printed with the Stratasys printer:
Material: ABS
Infill percentage: 20%
Layer height: 0.013 mm
| Part | Built time | Building Material Volume [in³] | Support Material Volume [in³] |
| Grip Body | 3 hrs, 32 min | 3.70 | 0.88 |
| Grip Cap | 59 min | 0.94 | 0.14 |
| Trigger | 45 min | 0.31 | 0.17 |
| Grip | 5 hrs, 16 min |
Built Time
The printing time for the parts depends on the infill percentage and the layer height as well as on the amount of support structure used. A higher infill percentage and a smaller layer height lead to an increasing built time for the parts. We also strive for less support structure to decrease the built time of the parts. That could be achieved by changing the design as well as changing the built direction. For choosing the optimal print parameters for a fast built time, the strength of the part also has be taken into consideration, which is influenced by the infill percentage as well as the built direction.
The built time for all 5 parts of the initial small Snap Fit Mini Crossbow design was about 2 hours. Since we scaled everything up to double the original size, the printing time increased dramatically to about 9 hours, because of the higher built volume. For example the printing time for the grip, which includes the grip body, grip cap and trigger was about 55 minutes for the old smaller design and is more than 5 hours for the new, bigger design (see table above).
Optimization for support structure and built direction:
In addition to the Grip we also printed the other parts of the 2-times up scaled Snap Fit Mini Crossbow design with the Stratasys printer.
Material: ABS
Infill percentage: 20%
Layer height: 0.013 mm
| Part | Built time | Building Material Volume [in³] | Support Material Volume [in³] |
| Pusher (vertical) | 1 hr, 10 min | 0.68 | 0.09 |
| Pusher (horizontal) | 1 hr, 26 min | 0.71 | 0.20 |
| Track | 2 hrs, 40 min | 2.54 | 0.78 |
| Crossbow | 9 hrs, 6 min |
We compared different built directions to find the optimal solution. We decided to print the vertical version since it has a lower built time, as well as less support material.
In a second step we printed the new “bow design 1” with the Stratasys printer, which is larger since we needed a larger bow for the enlarged crossbow version.
Print parameters:
Material: ABS
Infill percentage: 20%
Layer height: 0.013 mm
Built time: 63 minutes
Build material: 1.053 in³
Support material: 0.212 in³
With this new design of the bow we want to evaluate the forces and ability to bend. As the picture shows the new bow design has some very thin and weak areas with just two layers of material. Since the arch is very elastic, it will be too week to resist the forces and will break very fast. After bending it a couple of times you can already see that the thin areas are nearly broken. In additional to the bow design we have to design the connection part to connect the bow with the track. In the thingiverse models this connection is realized by a snap connection on both sides of the track.
Evaluating our “bow design 1” we figured out that we need a much thicker and solid bow to enable the bow to absorb the energy. Furthermore we need some more holes to reduce the printing time and material volume and a slightly different shape of the bow. Moreover we need pins or holes at the ends of the bow to attach the rubber band.
Beyond this there were new requirement concerning the volume of the bow and the maximum size, which is limited. The print volume has to be within 220mm x 140mm x 140mm. Therefore, for the future designs and optimizations, we have to take this volume limitation into account.
Having these requirements we developed two new bow designs.
1.) The fancy version:
2.) The serious version:
Due to the volume and maximum size requirements we have some issues with the size of the bow. Since the track and grip of the bow are somewhat large the snap connection part at the bow also has to be very big, which makes the whole bow too big for the size limitations we have.
We discussed the two options to avoid this issue:
– scaling down everything to have a smaller connection part at the bow
– change the connection design to connect the bow to the already printed track of the current size.
We decided to redesign the bow for the current printed size of the track and grip so that we could print another working prototype of the bow quickly and evaluate the snap connection. Thus we don’t have to print the grip, track and pusher again, since these parts do already work very well and are not part of our optimization loop.