Many woodworkers use splines, dowels and biscuits in an attempt to make their joints stronger. They believe that will make them stronger than using yellow woodworkers glue only. Although this is a very common belief, it isn’t correct.
A glue only joint, when done correctly, is stronger actually. In fact, when you put a biscuit, spline or dowel in an edge to edge joint, you might actually weaken it.
Now of course there are some joints that can benefit from having these reinforcements added. For example, end grain joints need assistance from a mechanical aid like a biscuit so that there is a good glue surface.
Edge to edge joints need to be be almost perfectly mounted. Otherwise the joint could possibly weaken.
Many people think yellow glue makes use of mechanical adhesion. This refers to when the glue molecules absorb into the pores of the wood and form “fingers” to hold the wood together.
They’re right in the sense that the glue does penetrate into the wood fibers. However, the way that yellow glue really works is by specific adhesion. That’s when wood molecules and glue molecules bond together. A layer on the wood adheres to the thin glue layer on the piece on the opposite side.
When this is done correctly, it results in the glue bond being stronger than the actual wood. If a piece does break off of the project, the glue line won’t break, but the wood will.
That’s why after you’ve got something glued up, you don’t have much time to make your changes. For example, I built a garden bench. It was to be a wedding gift. The man who bought it wanted to have the newlywed’s name engraved on the bench.
So I did the engraving. However I spelled their name wrong. After he told me about that misspelling, I told him I would take off the board that had the engraved name on it and replace with with a new board with the name engraved correctly.
The bench had a rectangular back and slats that ran up and down. The bench’s slats were sitting in a dado on the bottom and top board. They were also glued in. I used a small brad for holding them in place while the glue was drying.
Anyway, about one hour before I made the mistake, I had finished the bench’s back. I was thinking I could just take the top board off and then replace it with a new one. However, what happened was I wound up ruining the entire back of the bench when I attempted to remove the slats from the back.
By that time, the glue was stronger already than the wood. Here is the secret to make sure that the joints are strong when you glue them. You need to ensure that you have as flat of surfaces as possible. You then need to apply your glue as consistently as you can.
It also works better when you clamp the joint, if the clamping pressure is even all along the glue line. When applying the clamping pressure, you should see beads of glue come out. If you see a big gush of glue, it means you’ve used too much.