How to edge-joint bowed stock
It's a basic woodworking rule: Boards need one straight edge before you can work with them. If your board is warped by less than 1/2" or so, it's simple enough to power up a jointer and straighten the edge. But when you're faced with a seriously bowed work piece, try the tricks here to straighten things out.
1. Before you joint a board that's bowed by more than 1/2" along its length, cut the board to shorter lengths, if possible. This may reduce the bow of each work piece to 1/2" or less, allowing you to straighten the work pieces with your jointer in the typical fashion. And, you'll get more usable stock out of the board. In the example below, a board with 3/4" bow yields three boards each with 1/4" bow.
2. If you can't cut your board to shorter lengths, use this method to joint long, bowed stock. (We do not recommend you try this procedure with work pieces longer than 6' if you have a 6" jointer, or longer than 8' if you own an 8" jointer. For these pieces, use the procedure described in Step 4.)
First, set your jointer for a 1/16" cut. Then, place your board on the jointer infeed table with its concave edge down. Make several passes on one end of the board. Each pass should remove more material along the edge than the pass before it. As the jointed surface of the edge approaches the center of the board, turn the board around and repeat this procedure on the other end. The jointed edges should nearly meet in the center. By this point the overall bow should be less than 1/8", shown below. Make one or two complete passes to straighten the edge.
3. Next, read the grain of the board to determine which end to feed first so the grain runs "downhill" and away from the cutter head, as shown below. This helps reduce grain tearout.
Now, decrease the cutting depth to 1/32", and make a final pass along the entire edge of the board. If the grain runs every which way, slow down your feet rate to reduce tearout to a minimum.
4. For boards too long for the method described in Step 2, or for short boards bowed 1/2" or more, try this technique. With a straightedge or chalk line, mark a line as shown at right. Then, cut along this line with a portable circular saw or jigsaw. If you closely follow the line you should be able to joint the edge straight and smooth in one pass, as described in Step 3.
5. Because of the position of defects in a board, you may find it necessary to straighten its convex edge. This could happen when the best wood in a board is along its convex edge, as shown below. Then, it often makes sense to straighten that edge first so you can join the good edge to other work pieces or rip thin strips from the best wood and work toward the lesser-quality wood.
Do not attempt to straighten a convex edge on a jointer, even if the convex edge is bowed by less than 1/2". To straighten any convex edge, follow the method described in Step 4.
The grain of highly figured woods, such as bird's-eye maple, can tear out quite easily, especially if your jointer knives aren't as sharp as they should be. At these times, slow down your feed rate to a crawl (just an inch or two per second) and take light cuts (1/32" or less).
It may be difficult to tell if you're getting a complete cut at these shallow depths, so try this trick. Mark a wavy pencil line along the entire edge, as shown right, and make the cut. Any remaining pencil marks tell you that you need to repeat the cut.
Here's a good rule of thumb that may actually save your thumb. Whenever you edge-joint a board that is not as wide as your jointer's fence is high, use a push stick, as shown at right.
Remember to reposition your jointer's fence across the width of the tables from time to time. You'll get more life from your knives because they will wear more evenly along their lengths.
Written by: Bill Krier with Jan Svec Illustrations: Brian Jensen