Wednesday, 22 November 2017

Prod Failure Mode

The experimental crossbow pod is too strong. To reduce weight, improve the tiller and increase draw I've been reducing the width, however it came to a point where the back was too stressed and it popped a splinter with a satisfying "crack!"
Of course the splinter started at a node and I can see that maybe I'd rasped down the node a bit too much. The fibres seem to flow up at the node and as I peeled back the splinter it ran up and out of the bamboo at the next node. This gave me the idea of stripping off the bamboo and re-backing the bow. It might be another iteration of the design before a completely new version. After all it would be good to see how far I can get it back at 120#

I've learned enough from this to make a better version, a tad longer, some built in deflex, a bit less core thickness, and a thicker Yew belly to allow removal of wood from the belly for tillering rather than making it too thin in the limbs.
the final test before it broke got it to a 10" draw (measured from the belly) at 120#  which would extrapolate to about 168# at 14" . Ideally I want 120# at 14" draw at 120#

Now I have a better idea of dimensions I can cut my various components for the prod closer to finished size.
To have the prod still in one piece is hand too as a pattern.
Update:- As I started planing off the bamboo it occurred to me that I could try it again. I'd rasped the limbs such that the back was narrower, and as I planed some off I found the back was returning to it's wider dimension. I sanded it up on the belt sander and gave it another go, getting it back to about 13" at 90# . All very promising, the outer fibres of the bamboo are the strongest, but maybe going down about 1mm still leaves decent material, and maybe the back and belly strength is better matched... mind it could all just mean it performs sluggishly.
Anyhow it's all good dimensional info for the mk2 . I'll clean it up further and try for the 14" draw!

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