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Brilliant idea that allow you to use the original bore-above-plunger setup without the tube-related losses.
ManiacalCoyote0
Member Since 09 Apr 2020Offline Last Active Feb 02 2024 11:22 PM
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In Topic: Gutenberged Crossbow: The Morrigan
15 October 2020 - 03:09 PM
Postmortem: Morrigan Mk 1
Performance: Stock-to-Superstock performance (Velocities ~ 100 fps) until catastrophic failure of plunger rod.However, the other printed parts (trigger, catch, plunger tube mounts, spring rest) remain intact.
Reason for failure: Brittle fracture of plunger rod 0.125" from plunger head face, presumably growing from #8-32 threaded hole in center of plunger rod, following the sweep of the threading.
Crack growth presumably accelerated by lack of damping due to sub-optimal plunger head construction and spring choice. The main rubber seal was too small to engage the plunger tube, leaving the only source of grip being the fender washer. Also, the spring had ~ 1" of pre-compression, which led to harder shocks on firing. Last but not least, I used a bolt that only barely reached into the plunger rod, increasing the stress concentrations near the front of the plunger rod.
Evaluation: In an effort to make the form the most appealing possible, I sacrificed function and selected inferior materials for the ignored the advice from the print-shop advisors that transparent filaments were weaker than opaque filaments in an effort to make a blaster that was aesthetically pleasing, and I was too caught up in the idea of having almost all of my Crossbow's mechanical parts being 3d printed to consider my alternatives.
Plan for improvements: replace PLA plunger rod with PVC rod and a separate handle, which can be 3d-printed. This will result in all components being small enough to be printed on a small or standard-size printer.
Silly, I think you might have been right.
