Well I can't seemed to get home and it looks like it will be another week or two before I do, I am going to try to explain decrowning using my faithful crayons. But first, in regards to the walnut, what is the diameter in the center of the stave in question? How long is it? How clean and straight is it? What draw length are you shooting for and at what weight? All important, need to know information.Also , we love pictures! The more info. you can give, the better we can help.
Now on to the decrowning of your stave. First of all, lets go through the reasons why you want to decrown you small diameter stick. First reason is that if your stave is choke cherry, like I think it is, choke cherry is not very strong in tension. With a high crown all the tension forces are concentrated in a narrow strip up the back of the bow. While the belly of the bow will be alot wider and flatter. To really simplify how a bow works, you have two opposing forces at work while at full draw. You have roughly 10% of the thickness of your limb being stretched on the back, or under tension. The opposing force is 10% of the thickness of the limb on the belly undergoing compression. These two opposing forces are what is storing the energy and what forces the limbs back forward after release. The wood in between for the most part is just adding leverage to the opposing forces. The thicker the limb the more leverage the two opposing forces exert upon each other. That is really over simplifying it, but good enough for now. Now back to the original thought, by this premise, if you have a narrow strip of wood on the back trying to take the tension exerted from a wide flat belly, the belly is going to overpower the back and you have a tension failure(explosion). The reason why is the 10% thickness on the flat wide belly is alot more wood than the 10% thickness of the back.
The other reason to decrown this stave is to get your limbs out of the widest portion of the stave( the center portion). This gives you a wider limb to work with, which spreads the workload over more surface area. This usually translates into less set and better longevity of the bow.
The first picture is the end view of a small diameter stave(I wasn't kiddin' bout the crayons). The green is the surface of the wood, the red is the sapwood rings and the brown is the heartwood. I use a pretty simple method to decrown. first I pick out the cleanest, knot free side of the stave. and flatten it out by taking wood off down to the first heartwood ring and then chase that ring all the way down the length of the stave. You don't chase the ring all the way across the width. You just sort of stay in contact with it so that it is a continuous stripe down the middle as you flatten out the crown. Obviously, not all woods have heartwood and not all woods will have the first heartwood rings exactly where you need to make the back of your bow. These pictures are just to give you an example of what I'm talking about. The first sketch is the end view of the stave showing the rings. the second sketch is supposed to be the end view of the stave except where the red line cuts across it will be the new back of the bow. The removed material is the dotted lines above the red line. If you faithfully chase a ring down the center of the stave it should look like the third sketch with the growth rings running parralel the entire length of the stave. Even if the stave is as snakey as the one illustrated.
If your not planning to back, there can be no twist in the grain and no knots or this won't work! I assume that you are going to back this so if the twist is somewhat mild, it won't matter much. Also, I should mention very carefully avoid tear outs! I usually draw knife down close and use a scraper to take off the last 1\8" or so. It's alot more work that way, but one tear out can mean your bow stave is firewood.
The fourth sketch is the side view of the stave after decrowning. As you can see there is a bit of character here as well. This is to show that if you decrown properly the new flat back will still run true with the stave, keeping grain violation to a minimum.
Man, I truly hope I haven't thoroughly confused you. This is the best I can do While on the road. Josh
