Truth be told, straightening bowed stock isn’t as hard as you would think, especially with hand tools. Unlike power tools, no special jigs are required, only the skills you already possess. The following is the way I generally go about it.
The first thing you need is a reference line. Since there’s no straight surface to start from, a straightedge of suitable length or a chalk line is the tool for this job. I always start with the concave edge, but that’s just my preference. In the drawing above, the curve is exaggerated for clarity. If I really had a board that bowed, I’d go back to the lumberyard. Once you have a straight reference line, it’s simply a matter of removing the waste. Conventionally, this is done with hand planes. However, if the waste is wide enough, I see no problem with removing most of it with a ripsaw.
Either way, you eventually wind up using one or more hand planes to finish the job. For initial stock removal, I like a scrub plane or jack plane, switching to a jointer plane as you get closer to the reference line. In the drawing above, I show starting from the far end and working backwards. However, depending on the grain, you can start from the near end and work forwards. In either case, the body of the plane bridges the gap of the bow, and said gap grows smaller as your cut progresses downwards. Do this equally on both ends, paying attention to the grain, and you’ll end up being able to take one long shaving the entire length of the board. Check with your straightedge – your edge should be straight.
Once one edge is straight, go back to your marking gauge or panel gauge and mark a line parallel to the first one. This ensures parallel edges. Whenever possible, mark directly from your reference edge to reduce the chance of error.
Convex surfaces are generally considered more difficult to plane straight than concave ones. As shown above, a concave surface allows the plane bed to “bridge” the gap, restricting cutting to the higher areas at the end. With a convex surface, the plane can follow the curve without reducing it. If there’s enough room, I highly recommend removing most of the waste with a ripsaw to gain a flat area parallel to the line. If not, a scrub plane or coarse-set jack plane will get you close. In this case, start in the middle, establish a flat area, and work outwards and downwards, trying to stay parallel to the line. When you get close, switch to a jointer plane.
I mentioned earlier that all of the parts for these two sawbents comprise a series of pairs. The last step is to clamp a pair together, and plane them to matching width. I use small handscrews to hold them together, and then clamp the whole assembly in the tail vise.
With this operation completed, I have a stack of matching parts to assemble two sawbents. There are, left to right, two pairs of uprights, two bottom stretchers, two top stretchers, and two pairs of feet, all 3″ wide. Next time, I’ll start foot joinery. Stay tuned!