Solid Wood Panel Joinery
When making solid wood furniture you will need to glue-up a number of pieces to width to make up any panels. Most hardwood lumber is not less than 4" wide and not more than 14" wide. Anything wider has become more and more rare.
I recently bought two Bubinga planks, 2" thick x 42" wide x 14' long, with live edges. I can usually find woods like Cherry, Walnut, and Mahogany up to 36" wide but these are not common sizes and take a lot of research, and I pay a premium price for that large width. So most work will require some glue-ups.
There are a number of different joining methods and all require glue. Butt, dowell, and biscuit joining are the most common today. Also used are splines, through bolting and glue joints.
I am going to start by explaining the worst joining methods and progressing to the best ones and explain their advantages and disadvantages.
1. Through bolting is usually used in thicker glue-ups (2" and greater) when making cutting boards and slab doors, for example. It is actually worse than doing no joinery at all. Boards are drilled through their width and all-thread is inserted through the holes, washers and nuts placed on both ends and tightened. Sounds strong, right? Well, it is strong, except for the problem that wood is always changing dimensions. Higher humidity expands wood, lower humidity shrinks it. Ten years after your glue-up the panel will still be changing dimensions with the seasons. So when the panel expands against the tightened bolts, the panel warps because it has no where to go. And when the panel shrinks, the bolts are doing nothing. So what you have is a butt jointed panel which will eventually fail.
2. In butt jointing you are joining pieces with glue only. With softer, more porous wood, and using all the proper glue-up techniques, it's pretty strong because of our modern glues. But over time, some joints will starts to fail. With hard wood, and especially hard oily wood, the failure rate becomes higher and can be quite severe.
3. Splines are grooves cut in the edges of the boards to be glued up. A plywood or solid-wood spline is cut slightly smaller than the depth of the two grooves combined, for the glue to squeeze out. Glue is placed on all surfaces and clamped up. The glue space creates a weakness in the joint, making the joint 20% to 30% weaker than the solid wood itself. All joinery should strive for a joint that is as strong as the wood itself, or stronger.
4. Both dowels and biscuits are very similar in strength but they use just slightly different techniques. They both interlock the joint and add glue surface that butt joints lack, without causing a weakness like splines. But they should not be used in large glue-ups or in exterior doors because of the stress.
5. Glue joints are usually run with a shaper but can be run with a router as well. It's a single cutter that is offset allowing for a single set-up as opposed to a tongue-and-groove set-up requiring two cutters and two set-ups. What a glue joint does is interlock the joint and double the glue surface, making this joint the strongest in any type of wood and without any of the drawbacks of the other joints.
As outlined above, #5, glue joints, are the best way overall when constructing solid wood panel joinery. The other methods are widely used, but should be used with caution, and only in certain applications. Furniture and doors should be built to last a couple of lifetimes, in my opinion, and if you build it right, it will.
