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.
Mark the faces of the boards
Use a pencil to mark the faces of each board.
Secure the boards in a vice
Position the boards together in a vice so that the two edges to be joined both face up. Make sure the boards are lined up and flush.
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).
Check the cut
Remove the boards from the clamps and set them up on the bench to check the cut. When the edges are put together there should be a very small space between the two boards in the middle section.
Dry fit the boards together at the clamping station
Position the boards together at the clamping station. Consider using a piece of cardboard under the panels to contain the glue. Clamp the middle section of the boards together first. The outside edges should be sprung together nice and tight.
Apply glue to one board edge
Apply wood glue to the edge of one of the boards.
Spread the glue along the entire edge of the board. Try to keep most of the glue towards the inside of the edge, rather than simply mushing it down.
Apply glue to the edge of other boards to be joined
Follow steps #8 and #9 for remaining boards to be joined.
Clamp the boards into place
Place the boards down and position the edges together. Tighten the middle clamp first. Check to make sure the boards are flat and face is flush. Tighten up the outside clamps. Allow the wood to dry for the prescribed amount of time.