Glue-line Router Bit For Strength?- Question of the Week
This week’s question comes from Alexander. He writes:
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?
And here was my reply:
“Hey Alexander. The short answer is no. The long grain glue bond in a tabletop glue-up is stronger than the wood itself. So if its going to break, it should not break along the joint. Now biscuits do help with alignment but don’t add much in the way of strength. A glue-line bit will also help in alignment, but some people believe it actually decreases the strength of the joint. So if you don’t have alignment issues, all you really need is glue and clamps. If you do have alignment issues, I recommend a set of cauls or just a biscuit every 8-10 inches. Good luck!”
*Note*- For those of you who aren’t familiar with glue-line bits, check out this article on NewWoodworker.com. Also, be sure to read the comments section of this post for further clarification on the weakened glue-line issue.
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Marc – I am confused by your answer. Why would the joint be weaker in some cases? I would think that (and the article states this) the increased surface area would increase the strength of the bond.
Mind you this is all theoretical knowledge. But I have read on several occasions where things like splines and glue-line bits weaken the joint (although more so with splines than with the glue line bit). The idea usually stems from the fact that the joints have the potential for little breaks in the glue line. If the glue line bit joint doesn’t bottom out, there will be a slightly larger gap where the glue will pool. This will then be a weak point. Same thing with a splines. When most people makes splines, very rarely do they fit perfectly. So whereever the inconsistent and more “open” glue line exists, you will have a weak point.
But even in the worst case scenario, the joint will be solid for all practical purposes. But the real take home message is that you dont really need more strength than a long-grain glue bond provides.
Now since this is not from personal experience, I should change the text to read, “some people believe…..” In fact I will change that right now to avoid people thinking I am making a claim from my own experience.
Thanks for the question Trey.
A quick cure for long grain glue up joint alignment is a 30″ wide belt sander! We would always mill 1/16″ over, and get the edges as close as you can..then sand up from 60 grit through 120 into a perfect surface.
My guess is that the glue joint bit might create voids where the wood surfaces might not get good contact with the glue/other surface, and therefore have poor adheasion. Two boards, jointed and ripped properly with good even squeeze out along the joint will work perferct every time!
Wow… Learn something new everyday. I had never heard of such an animal before, so what an educational question.
Thanks for posting the link, Marc. I read the question three times and still didn’t have a clue. I followed the link and now I’m smarter! :-)
I would just be concerned that if you didn’t get that thing aligned exactly, you could force ridges on your glue up. Seems like more of a hassle to me. Easier to use nothing or biscuits.
An excellent question Alexander and a fabulous answer Marc. I have been watching your podcast and blog for weeks without any need to speak up, but this I must say has been very informative for me. I have used glue joint bits thinking I was getting a better bond however your logic makes plenty of sense. Thank you Marc for saying it striaght up and making me think deeper about the glue joint bit benifit. I suppose this could be classified as a woodworking paradigm.
glue line bits make your furniture look cheap and very nasty!
If you want it to look like some kind of mass produced Swedish cr*p, go ahead.
A well jointed edge with a few biscuits for alignment (whats not to like about biscuits?) gives a virtually seamles join and nice end grain finish.
A bit of a rant, but its Friday and I’ve had a bad week with the “client from Hell”
Where’s the beer?!
I just made a table top using 15/16″ thick, 6 1/4″ wide black ash boards, edge glued. I used a wedge glue joint bit (supposed to line up the edges) with a router table and feather boards both to hold the boards down and against the router table fence. With this thickness of boards, the feather boards failed to take out all of the little bit of bow in the boards. As Brad Nailer said above, a 36″ wide belt sander took care of the ridges. I should have used biscuits to line up the boards, not a wedge glue edge router bit. If your feather boards won’t completely flatten your boards, I would not advise using a glue router bit on the edges.
I am troubled that so many wood working people point to the example of glue line failures that show pieces of wood still bonded to the glue, and then say “The long grain glue bond in a tabletop glue-up is stronger than the wood itself.” This is false. Perhaps the bond between the glue and wood is strong enough to cause the pull out of wood fibers that it bonded with, but to my knowledge all of the failures these wood workers point to occur adjacent the glue line. Stress points build within material at changes in properties, ie. stiffness, shear and tensile strength, density, etc. The glue line does just this, and the failure mode that people are pointing to supports this. If the glue line is as “strong”, then the failures should occur with equal frequency some distance away from the glue line. Glue joint and finger joint bits provide a distribution of stress over a relatively large width of the board, and will provide more strength. The question should be, is this necessary? To that I would say no. A glue line joint provides more than enough strength if done right.
Hey Rolf. I totally see your point, but I have to disagree with you. I suppose I interpret it differently than you do. I agree that the change in rigidity caused by the glue bond will usually result in a crack or split somewhere adjacent to the glue line. I still interpret that result as “stronger than the wood itself” which supports my original assertion. I dont understand how you interpret this as disproving that claim.
Obviously, ANY joint will result in a weak point. Its inevitable. The point is that if the wood is going to break, it will not break at the joint. So when answering questions about reinforcing a long grain joint, my answer is that it is not necessary simply because the long-grain glue bond will not fail before the wood itself fails.
At least we both agree that in general, these extra steps are not necessary for a standard tabletop. :)
Obviously the joint area should be the weakest portion of a typical board. However if the joint were equal to the strength of the wood, then the board is likely to fail at the point of most tension. If you happen to be testing just the glue joint, you will predominantly get a break very near or at the joint. However, does the same occur when the tension is applied away from the joint in order to test the wood? Where does it break then? If just the wood breaks then can we actually say the glue joint is stronger than the wood? Or do we repeat the test multiple times to determine the percentage of breaks for each category? Oh and we must include the other variations like types and brands of glue and of course many species of wood with each glue. etc. etc etc After all the figuring, just glue it using proper technique and tell everybody not to lean on it!