A spring joint takes advantage of wood’s natural flexibility. Use a spring joint when gluing board edges together to allow for the expansion and contraction of wood and to prevent cracking and separating. A spring joint is achieved by planing a curved cut into the middle edge of both boards so that when the boards are glued and clamped together the outside edges are brought tightly together. Watch this Video!
Clean mill marks from the material
Use a hand plane to clean the mill marks off the material. In this instance a high-angle #4 is used, but this choice is up to the individual. It is important to note that the goal is not to take off a lot of material but simply to remove mill marks.
Make a couple of passes along the board edges with the plane
Be sure the hand plane blade is not offset to the right or left. Make a few even passes with the hand plane along both boards to ensure both edges are flat. By the final passes the wood shavings should be long. The goal is not to take too much material off, but simply to level the edges.
Create the concave cut in the wood edges
The small curve in the middle of the board edges is achieved by first making a cut with the hand plan over a smaller middle segment of both edges. Then make a longer pass over a slightly longer segment. Finally, make a third pass over an even longer segment (but not quite the full length of the board edges).