I had the unfortunate experience (not to be repeated) of ordering my balsa from Nationalbalsa.com. I had grown accustomed to straight and even balsa, so I was more than a little pissed when I got my order and fully half of the sheets I recieved were so warped as to be completely useless. On top of that, the width of these "3 inch" sheets varied as much as 1/8", further complicating the process of laying them together to make skins.
Rather than let them all go to waste, I figured I'd try to get them all even and straight, even if that meant losing some overall width.
I made a simple jig using a flat chunk of plywood and some 90-degree pieces I cut from a 2x4. Very low-tech stuff. I attached the 90* corners with a single drywall screw drilled in at an angle. The holes through the corner pieces were oversize so the threads of the drywall only grabbed the plywood underneath. By angling the screw, it supplied clamping pressure as it was tightened down. I could then easily loosen and tighten my "clamps" with just a couple of turns of a screwdriver.
Here's what the balsa strips looked like before working them over. Like I said, so warped as to be useless.
So, I clamped them in and sent them through the thickness planer until all the edges were parallel and straight. I discovered that the pressure of the roller in the thickness planer would actually press out some of the warp as it the wood passed through. Then it would spring back as it came out the other side, leaving much of the curve as it was when it started. I got past this by making sure the balsa strips were clamped firmly together, by taking shallow passes, and by placing the warped sheets with the direction of the curve in such a way that the ends were higher than the center when doing the first side. It seemed to have less tendency to press the curve out that way.
Here's the first side done.
After the first side is flat and indexed, I just loosened my clamps and flipped all the balsa strips over. In this step, it is very important to make sure all the sides that were just cut are firmly against the plywood base. That's the only way to ensure the sides are parallel, so I took extra care to make sure everything was flat.
Once they're clamped in again, just send them a few more times through the thickness planer to flatten the second side. Since I was doing several batches, I watched my ruler on the planer carefully to make sure they were all coming out the same width on the final pass through.
The final result, some still have a little bit of warp to them, but nothing that can't be worked out as the skins are being laid up. They're certainly a hell of a lot easier to work with than when I started.
I think this could also work using a hand-planer, it would just require a steady hand and close measuring to make sure successive batches all came out at the same width.
I hope some people find this useful.
