Does a Glue Line Bit Add Strength To a Joint?
Article - August 16, 2007
This article was inspired by a question from Alexander.
“Do you have any opinions on the use of glue-joint bits for the use of making thick table tops? I am talking about gluing boards at least 1-1/8 inch thick for a project I am working on. I hate biscuits and want the extra strength that glue-joints offer. Other than self-aligning properties. Is it really needed?”
The short answer, in my opinion, is no. The long grain glue bond in a panel glue-up (such as a table top) is said to be stronger than the wood itself, and no further reinforcement is needed. If the joint is going to fail, it will usually fail in the wood fibers adjacent to the joint. You can actually test this yourself with scraps. Just edge joint two pieces of stock and glue them together. After an overnight dry time, try to break the joint apart. In most cases, you’ll find the glue joint remains intact while the wood splits along its weakest point in the grain. So in most cases, a simple panel butt joint requires no reinforcement at all.
You mentioned biscuits and while I don’t hate them, I do agree with your lack of trust in their strength properties. In my shop, biscuits are used for alignment purposes only.
As for the glue-line bit, I don’t even own one. Given what we know about the simple long grain butt joint, I just never thought something like a glue-line bit was necessary in my shop. So for full disclosure, I can only speculate on its usefulness. Clearly the bit would be quite handy for alignment purposes. And we know it creates extra glue surfaces by cutting interlocking mating profiles on both work pieces. More surface area is generally considered a good thing for a wood joint as the long-grain to long-grain contact is at the heart of the joint’s strength. So on the surface, our instincts tell us this would be stronger than a simple butt joint, even if that extra strength is overkill. But here’s the rub: some people believe the mating profiles actually decrease the strength of the joint. I wish I could remember the exact magazine article I read on this topic, but here’s the gist. Since the glue joint is stronger than the wood itself, if breakage occurs it will usually take place just adjacent to the rigid glue line. The profile that is cut into the wood essentially things the wood out in those areas thereby creating several weak spots. Those weak spots are now even weaker than the adjacent wood grain.
Again, I am only speculating here and more testing would need to be done to prove or disprove this theory. But to be safe, I recommend going with a good old butt joint. If you want some alignment help, go ahead and use biscuites, Dominos, dowels, or a spline. Or simply use a set of cauls positioned over the joint itself.
Want some more information on Glue Line Router Bits? Check out this NewWoodworker.com article!