Thursday, 26 April 2018

Another Try at the Crossbow Prod

A while back I bought some nice Maple off Ebay and cut it on the bandsaw to form the core of the prod. Some heat bending got it roughly shaped for glue up on the former. This lay around in the workshop for a while then eventually I cut some Ipe belly laminations and put a bit of shape into them (each limb being a separate lamination). The bamboo was planed up in the usual manner.
Once I'd started the glue up I realised my clamps weren't quite big enough... bugger! Too late to trim down the plywood former to allow the clamps to fit (although I s'pose I could as the pot life of the glue is a couple of hours). Anyhow I pressed on with plenty of rubber strapping. The prod came off the form pretty well, a couple of tiny gaps in the glue line, but they are near the centre where there will be a block glued for the mounting arrangement.
Oh, dear it was too stiff! |When tried on the tiller the draw weight rocketed up towards 150# while the draw was still a bit short. I felt that if I drew further the weight would be too high and it would start to take a set (I was checking that by, putting it back on the former, and I could feel there was a slight change already)
What can I do? I'd narrowed the bow a tad already, I can't take wood off the back without weakening the fibres of the bamboo, and I don't want to take any off the rather thin Ipe belly strip...!

Hmm... I know I'll take it out of middle! What? Eh?
I ran it through the bandsaw cutting along through the centre of the Maple core removing a saw blade's width! This effectively gave me two thin prods, I lightly sanded the two sawn faces and glued them back together now minus the thickness of the saw cut!.
At first sight, this sounds bonkers, but it's actually quite clever. If the saw wanders a bit it doesn't matter because the two halves will still match virtually perfectly! Now the prod is glued back together it is now about 1.5mm thinner, not only that but losing an even amount along the limbs weakens the tips more than the centre which should improve the tiller as it was mostly bending in the middle (1mm removed from a 10mm section is a smaller proportion than 1mm removed from a 5mm section at the tip).

This was all carefully calculated beforehand by sticking a wet finger in the air and going for a fine narrow bandsaw blade. The proof of the pudding is trying it on the tiller and yes, it feels more supple and the weight is down to just over 100# rather than approaching 150# . These figures are at a sort of guess work draw length with a string that will just fit on without flexing the prod.

It looks good enough to proceed with further work.

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