I prepare the bamboo backing pretty much the same way I do on the bamboo backed ipe build along (i.e. cutting the inside away with a bandsaw, then grinding it by hand with the belt sander). There were just two things I did differently.
First, whereas on the BBI build along, I cut the bamboo to the shape of the bow before glue up, in the case of the recurve, I leave the bamboo 1.5 inches wide through its whole length. (It's 2 inches wide when I buy it, so I cut 1/4" off both sides.) The reason is because tip alignment is crucial in recurves. By leaving the tips wide, I have some flexibility after glue up.
Second, I leave the bamboo for the recurve a little bit thicker than I do for the BBI. The reason is because after heat bending the bamboo, it'll warp a little, and the gluing surface will no longer be flat.
As you can imagine, that will ruin any chance you might have of getting a good glue line. By leaving the bamboo a little thicker, you can re-flatten the area that warped after you're done heat bending it.
Usually, I'd use a string and weights to get the bamboo straight with the tips lined up through the middle. But in this case, my bamboo was already pretty straight, so that wasn't necessary. Here's a picture showing, from left to right, the bamboo at its full 2 inch width, after narrowing it to 1.5", and then after flattening it.
Ideally, a recurve would be 1.75" wide. That helps with stability. You see, the limb tips of a longbow are a little more rigid than the limb tips of a recurve. That extra width in the limb of a recurve helps keep the tips rigid. The reason I'm going 1.5" instead of 1.75" is because that's how wide my form is. I suppose I could spend the extra money and go to the extra trouble of getting plywood or something to make a thicker form, but the 2x6 is easier to acquire and much easier to work with. Of course the down side is that it might warp while it's in the oven.
Now, we've got to pre-bend the bamboo with heat because otherwise it won't bend around the recurve without breaking. I took one of the recurve cut-outs, sanded it smooth and round with the belt sander, then cut it out in such a way that I could bend the bamboo around it and apply clamps.
I used a cheap $10 heat gun from Harbor Freight to do this. The heat gun is a good investment because there are all kinds of uses for it. I've had mine for several years, and it still works great.
The idea is to heat the bamboo and bend it gradually, starting at the first clamp. You don't want to apply too much pressure to the bamboo because the idea is to let the heat render it pliable. As you get it to bend a little, apply another clamp. Keep moving down the bamboo until you've got the whole curve clamped down.
There are two things you have to be careful of. First, you don't want to heat the bamboo so much that you scorch it. I really don't know how to explain how much pressure to use and how much heat to use, but I can tell you that without anybody explaining it to me, I figured it out, so I'm sure you can, too.
Second, you have to be careful with the nodes. The nodes will be stiff, and that tends to create a hinge on either side of the nodes. There are a few ways to deal with that. One way is to cut a groove in the form to accomodate the node. But then you'd have to do that for each piece of bamboo you bent, and your form would be full of grooves. Another idea is to put some padding on the area between the nodes, like maybe a piece of thick leather, a t-shirt, or a thin piece of wood. Another idea is to attach a strip of carpet to the form. It'll absorb the nodes. In my case, thankfully I only had one node to deal with. I just placed a clamp anywhere it looked like there was about to be a hinge, and that took care of it.
From bending a few pieces of bamboo, I've found that having the bamboo a little thicker than final thickness also helps prevent hinging while you're heat bending it. So there's a third advantage to keeping the bamboo kind of thick. And in case you're wondering, mine is about 0.2" (i.e. 1/5") thick. I'll make it a little thinner after I take it off the form, like less than 1/8" thick.
Once I've got it all clamped up, I go over it a little more with the heat gun to relax it in shape so it'll hold when I take it off. You want to give it plenty of time to cool off. I'd give it at least a few hours. Over night is even better. It'll spring back some when you take it off, but the longer you leave it on there, the less spring back there'll be.
As you can see, I got a lot of spring back. But that's okay. It doesn't need to hold the shape perfectly. Having it bent that much will prevent it from breaking when I clamp it to the form, but it'll still take a little pressure to get it pressed against the form.
When I did the other end, I got a few cracks near the node.
You probably can't see that very well, but they're there. That's why I said you have to be careful around those nodes. If you see a hinge starting to form, put a clamp on it. It might help to put a flat piece of wood between the bamboo and the clamp to spread the force a little.
This turned out not to be a problem because it wasn't that deep. Once I was done heat bending, I used the belt sander to grind the bamboo and make it more thin. All the splinters went away.
Although I used cedar in my previous two recurves, I'm using all bamboo flooring for this one.
***Important note on bamboo flooring***
Speaking of bamboo flooring, I need to tell you something really important. If you're going to use bamboo flooring in a bow, you need to get vertical grain bamboo. Do not get horizontal grain. Horizontal will delaminate on you. It doesn't matter whether you get carbonized or natural. Paul Kloster used to say natural was better because carbonized takes more set, but I haven't noticed a difference.
You can get bamboo flooring from ifloor.com. I used to buy it in 6' pieces. I don't know if they sell it that way anymore. 3' is fine. You can just butt splice them together. I actually got a box for really cheap on craigslist. Somebody had installed bamboo flooring and had some left over. Score!
Bamboo flooring usually comes with a really hard finish on it. I find it easier to grind the finish off with the elbow of the belt sander before I cut out my lams. Just grind the smooth side and cut the lams from that side.
From back to belly, there'll be the bamboo backing I prepared above, then a 21" power lam, tapered on both ends, then two lams of carbonized bamboo tapered at 0.001 inches per inch, then the belly will be parallel. I put the tapered lams on the inside and the parallel lam on the outside so that if I need to, I can tiller the bow from the belly.
I could use my belt sander as a lam grinder, like I did with the fiberglass bow build along, but why do that when my brother-in-law has a big drum sander? I made three sleds for use with this drum sander to grind my lams. One of them is 6 feet long and has no taper. A second is 3 feet long and has a 0.001 inch per inch taper. A third is 3 feet long and has a 0.002 inch per inch taper. I labeled the tapered sleds so I won't get them mixed up.
I used to have a small piece of wood glued to the high end as a back stop for the lam when I pushed it through the drum sander, but when the lams got really thin, I'd have to hold it down so it didn't buckle while it was going through. But then I decided to glue some 100 grit sand paper with a spray adhesive, and now it works a lot better. I can just set it on there, and it runs itself through without slipping. I catch it on the other side. This is a whole lot easier than using the belt sander as a lam grinder, but I understand not everybody has a brother-in-law like mine. :-(
If you're going to make sleds, you should use some kind of hard wood, like oak. You need something that isn't going to warp. Anyway, here's the drum sander with the long parallel sled grinding my belly lam.
Here's all the lams after I got them ground.
From top to bottom, that's my power lam, then my tapered lams, then the belly lam. I used carbonized bamboo for the tapered lams and blonde bamboo for the belly lam. I just did that for looks. With a blonde belly and blonde backing, I thought the darker bamboo in the middle look kind of cool--like a pin stripe.
I wasn't shooting for any particular thickness. I just eyeballed it. Whenever I'm making an experimental bow, I try to err on the side of too weak so it'll be easy to string and unlikely to break. Once I've made a bow out of a new form, then I'll have a better idea of how thick my lams need to be to get the weight I want for the next bow.
But in case you want to follow along, here's the thickness these came out to:
power lam: 0.144 inches
tapered lams: 0.140 inches at the thick ends
belly lam: 0.102 inches (a little thinner than I meant, but oh well)
The power lam needs to be tapered on both ends, and it needs to be really thin, like the ends of the riser on a glass lam bow. I do that by holding it against the belt sander with a block of wood.
Notice how I'm holding it from the back of the belt sander which has the effect of sharpening it like a knife. This seems to work better than holding it from the front, but you have to have a good hold of it or it'll fly out of your hand. Here's the end result
Yeah, I realize it's not the best picture, but trust me. It's thin. You can see light through it. Having it nice and thin like that will help with the glue line.
Oh my goodness! I haven't even explained why we have a power lam in the first place, have I? Well, it's just to strengthen the bow near the fades to kind of even out the tiller so the outter limbs will work. It also stiffens up the area just before the glued on handle, which will keep the handle from popping off. If I did this like a fiberglass bow where the lam runs up the fade, a power lam would be unnecessary.
For the tapered lams, I used a butt splice the same way I did on the fiberglass bow build along. I used epoxy to glue them together.