Use Spring Joints to Improve Your Panels Glue Ups
A spring joint simply means a joint with some spring to it. It is used in a panel glue up where multiple boards are glued together on edge. A spring joint is a slight concave edge in between the two boards which will become one.
But why? Wouldn’t it be easier to have them flat? Yes it is, but wood gains and loses moisture, mostly through it’s end grain. Now most modern glues are water based. As the glue dries the wood absorbs some of that moisture and has to lose it, sending it out to the ends of the boards to be lost through the end grain. But that means the ends of the boards will shrink more compared to the centers. And as the seasons change these ends will suck up and lose moisture.
Amazing illustrative skills I have, but you get the picture. The top represents a spring joint. The bottom, after it is glued and clamped. Notice how the inside curves are clamped out to form a straight line but the outside of the boards now have a curve to them. That’s fine, you’ll cut your panels to final size post glue up to square everything off.
A spring joint anticipates this moisture loss and accounts for it. The slight hollow is pulled out by the clamps and held by the glue, the board is naturally trying to go back to resting state, putting tension inward on the ends. As moisture is lost the tension relaxes. As it’s gained the tension resumes, keeping the boards where you want them.
When should you use a Spring Joint?
I don’t think it’s necessary to use a spring joint in every edge glue up, but there are times when a little extra tension is nice.
In this case, I am making a Lazy Susan. This panel will have no support holding it flat, therefore something else has to. As the wood expands and contracts I don’t want the Lazy Susan turning into a potato chip, sliding everything toward the center, so a spring joint makes sense.
The Lazy Susan panel. At roughly 48 inches across and a mix of 4, 5, and 6 inch wide boards it was tough to spring this, but it will be helpful to keep the end shape steady throughout the seasons.
The wood, held in tension, will fight against itself linearly. It will want to go back and forth, not up and down. That’s what I want it to do.
Were this a table top with an apron to hold everything flat, I’d say the extra effort of creating a spring joint isn’t worth the time.
How much tension?
The gap you leave sets the tension. The bigger the gap, the more pressure your clamps have to apply to squeeze it out. Don’t go too overboard with this. A small gap will put hundreds of pounds of pressure on the ends of the boards. I like to keep it small, 1/32nd of an inch or so, but consider the material when making this decision.
Just about 1/32nd and that was enough. Any bigger and I don’t know that I could apply enough pressure to bring these boards together.
The wider the boards the harder it will be for them to bend. The shorter a board is, the less it will give in the middle. A combination of short and wide boards require a small spring vs long thin boards where a spring joint likely will have little to no effect.
When cutting a spring joint I simply take a couple of swipes on the interior of the board with a hand plane on each board. The gap should be easily pulled out with clamps. If all your might can’t close the gap you’ve gone too far and need to take a little off the edges of the boards to relieve the strain.
So consider a spring joint for your next flat panel and see if it helps keep your project flat.