I was going to cut a tree down for my self bow today, I found one that is nice and straight and tall but its only 7" acrossed, is this going to be big enough for my self bow? It's black locus, maybe honey locus, but the beens look like black locus tree. Or do I need to find a tree that is 12" or bigger?
It should work. The ideal size for a tree is about 6-12" diameter, iirc. It really depends on how much crown there is.
more questions: What design considerations did you have in mind? Length? Draw weight?
Black locust has a reputation for chrysalling, so I'd keep that in mind. wide, flat, and long may the way to go. Also, if the wood is prone to chrysalling, it won't be forgiving of tillering errors. Tiller slow and careful.
I was thinking of making it 1 1/2"-2" wide limbs 5/8"-1/2" at tips 40-45#@28 and around 70" long. Does that sound like a good setup?
1 1/2" wide to the mid-limb and tapering to under 1/2" at the tips would be perfect. Your weight range is very reasonable for black locust. I figure this would be an American style flatbow (stiff handle)? 70" would be a bit long, you could do 66-68" wold work just fine for a 28" draw. 70" would give you a good margin for overbuilding the bow
If you don't have the book, "The Bent Stick" by Paul Comstock, I'd spend $12 and get it. It'll answer most of your questions. Spending a weekend with that book will get you on a good track.