Using 2x4s as flush clamp bars when gluing up boards edge to edge can easily result in misaligned boards toward the middle of the panel.

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Using 2x4s as flush clamp bars when gluing up boards edge to edge can easily result in misaligned boards toward the middle of the panel. The problem occurs because nontapered boards exert more pressure at the ends of the assembly than in the middle. The means to a solution: your jointer.

Mark the center of the length of a 2x4 on its face side and set the infeed table of your jointer 18 " below the cutterhead as illustrated in the inset drawing above. Lower the workpiece over the cutter at the center mark and feed it off the table so it makes a tapered cut from the middle to one end. Keep downward pressure on the infeed side of the table throughout the cut. Turn the piece end for end to make an identical cut from the center to the opposite end. This will create a clamping bar that gently bows and that has a central crown that's 18 " higher than its ends.
—From the WOOD® magazine shop