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A Crooked Bow

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vtsteam:
After making a kid's bow, I got kind of hooked on building bows, and since I like trying unusual, but loclly available materials for making things, I decided to try a black (sweet) birch sapling, as a start. I have just an unlimited supply of these. It's not a generally recognized bow wood, but I know from FPL tables of wood properties that it has a very high modulus of elasticity (stiffness, in other words) -- higher than most recognized bow woods. Not that that means everything, but I figured, it's everywere here, might as well give it a try.

The first sapling i cut was straight an pretty clean for 14 feet and then less so above that. I cut it into three 7 foot sections and peeled the bark and sealed the ends. I figured it would take a couple months at least to dry the pieces, but it wouldn't hurt to take the worst, upper, section and split it with the bandsaw to dry faster.

vtsteam:
When I checked it with a moisture meter a week later I was very surprised to see a reading of about 13% -- down from 50% green, and no checking. I decided to trim it down further, and lined out a bow roughly as shown above. The lines are a centerline that follows the grain and high point of the stave, and the two outer profile lines that more-or-less follow that outline's curves, and taper. This is all done freehand.

I also followed the grain around knots -- there were two medium sized ones near the center handle area, and one rather large one (toward bottom of the photo below). The knots and curves of this piece would be a real challenge for a beginning bowyer like me, but I figured what the heck, the forest is full of birch, and it's something to do.

 I trimmed to the rough outlines with a hatchet.

vtsteam:
The bow's "belly" (the part that faces you). The "back" faces the target. You can see the knot all the way through and how the lower limb curves to follow the grain.

vtsteam:
The bow set up on the tillering tree. I've tapered the belly in thickness to get it to bend. The taper isn't even, but must be thicker where the knot is to compensate for the reduced strength there. that's on the left side in this photo. It will be the lower limb. You can also see that the limbs aren't the same shape either. the left one bends forward in "reflex" then back again in "deflex" much more than the right (or upper) limb.

I've also laid out the handle off center. The bottom limb is 2 inches shorter than the top, to compensate for how we moderns tend to pull the bowstring with 3 fingers. Index finger on top of the arrow and two fingers underneath. That tends to pull the limbs unevenly, unless compensated for in the construction -- you can either make the lower limb stiffer, or shorter than the upper limb.

The bow is here set up to tiller "with the long string" -- a string longer than the usual bowstring. I did not realize this when I was working on this bow, but the long string should actually be shorter than I show it here -- this is too long. It should be tight to the bow, but not bending it.

There's a scale and a rope to a pulley and back to me at the camera so I can bend the bow.

vtsteam:
Here I'm bending the limbs a little to check the curve (or "tiller"). The limbs should bend equally if the same length, and no knots or defects, but it gets a little more complicated if the limbs are unequal intentionally, and there are weirdnesses in reflex/deflex, grain, etc. You kinda have to use intuition and estimation to get what you think is right under the circumstances.

Here the bend is small, because it's early on in tillering. You don't want to overstress the limbs -- never pull them down past the intended draw weight for the bow (in my case 40 lbs), because you will add set that way (permanent deformation, not elastic change) or worse, break a limb.

Actually, you probably should not pull more than a little over half the draw weight early on. I tried to stay at about 25 lbs. before shaving more wood off of the belly side of the limbs. You gradually remove wood and tiller on down until you can bend the limbs evenly all the way to your intended draw length -- 29" in my case.

It's easier to tiller to a shorter length (like say 24") than a long one -- but I have long arms, and that's my draw length.

The draw below is showing 20", but since it's the long string, it must be calculated minus the slack, which was about 7" --so this was really only drawn to 13". Long way to go to get to 29"!

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