What are Spring Joints?
Spring joints are panel joints where we intentionally create a slight hollow in the middle of the glue joint. This hollow is slight enough that under clamping pressure the gap between the boards is closed. This puts the ends of the panel joint in compression which will resist the greater movement cause by loss and gain of moisture at the end grain.
A happy by product is that you can glue up a panel with a single clamp right across that hollow. But this is not the reason to make a spring joint. The reason is that compression that will resist dynamic forces (wood movement) and prevent that joint from opening up over the years and decades.
The amount of material you remove from the center of the joint will vary with the species you use and the length of the total joint. The method I use is simple to apply across these variables but in the end you want to test that joint and you should be able to close it with medium to light clamping pressure. If not then you will want to lessen the amount of the hollow. If you can close the gap with just hand pressure then you really aren't springing the joint enough.
When to Make a Surface NOT Flat
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