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Glue Joint Myths No.2 - Miter Joints

algale

Another Patrick Sullivan video. Here testing the myth the miter joints are inherently weak.

Turns out they are a little stronger than side-to-side or edge-to-side but weaker than end-to-end.

RFGuy

Thanks Alan. Interesting video and conclusion. I agree with him pointing out in the beginning of the video that he is performing relative comparisons which is most useful for woodworkers. In other words, viewers shouldn't get too invested in the absolute force numbers that he shares, but rather the comparison between different types of joinery. Several prominent woodworking influencers online came out to give their rebuttal of his first glue joint myth video. I agreed with the points made by most of them, except for what the Wood Whisperer posted. Marc specifically compared joint strength numbers from other sources (FWW test from 2009) against the numbers from Patrick. I have seen joint strength numbers all over the map depending on who did the testing, the orientation, wood species used, glue used, equipment used for testing, etc., etc., etc., so I think it is naive for him to compare absolute numbers from different studies. In fact, I have seen joint strength tests that show M&T joints come out on top, others show pocket hole screw joints coming out on top, yet others show Domino joints coming out on top. Clearly the testing methodology as well as the joint size/orientation matter, so it is more instructive (IMHO) to compare relative joint performance inside the same test. I have to agree with Patrick here when he says that we should leave the absolute numbers to the structural engineers. Also, Marc has been one of the main ones I have seen online in recent years trying to explain that end grain glue joints are weak because the end grain is like a set of straws and will soak up all of the glue leaving a dry and weak joint, i.e. you can't get enough glue in there to hold. Clearly Patrick busted that myth from his 1st video. Don't get me wrong, I like Marc and the content that he produces, but I was definitely disappointed with how he went about his rebuttal to Patrick's video when he has been the main instigator online selling the myth that end grain glue joints are weak and should NEVER be used.

Again, I am NOT saying that end grain glue joints are superb and nothing else should be used. Clearly you have to consider the dimensions and how the project will be used. Whenever possible I think it is an advantage to add other means to lock a joint together, but for small dimension pieces I now believe that end grain glue joints are totally viable. IF not, then all of those segmented woodturners better start working in pocket hole joinery into their turnings!!! :D

edma194

No myths busted here, miter joints are clearly weaker then end joints. He also shows how they are more susceptible to breakage by leverage when they join long pieces. So it isn't just the glue that's a factor, and of course they need a spline of different type of joint.

The side-to-side joint is still being described as weak even though the glue has never failed in tests with these joints. It is important to remember that a single piece of wood the same size and shape as the piece created by gluing two boards together side-by-side would also fail. The weakness is the lignin bonds in the wood and nothing at all to do with the glue. Maybe Sullivan will do the tests to find the strength of the glue in side-to-side joints. It requires reinforcing the wood or using larger pieces and only the smaller area of the original test being glued. I'm not sure the results would be significantly different from the end-to-end glue joint tests.

JPG

All those naysayer are guilty of confusing apples and oranges.

He proved the glue is NOT the deciding factor. The joint itself is the limiting factor.(as is the strength of the wood).

RFGuy

JPG wrote All those naysayer are guilty of confusing apples and oranges.

He proved the glue is NOT the deciding factor. The joint itself is the limiting factor.(as is the strength of the wood).

Well said...and much more concise than my rant. :)

algale

JPG wrote All those naysayer are guilty of confusing apples and oranges.

He proved the glue is NOT the deciding factor. The joint itself is the limiting factor.(as is the strength of the wood).

Correct. But people (including me) nevertheless worried about the end-grain-to-end-grain glue joint failure when in reality the wood was going to break first even if it was side to side grain.

RFGuy

algale wrote Correct. But people (including me) nevertheless worried about the end-grain-to-end-grain glue joint failure when in reality the wood was going to break first even if it was side to side grain.

Yeah, it seems to me that too much focus is getting taken away from that initial end grain myth that is now busted. I mean I know I can make a stronger joint than an end grain butt joint, but that really isn't the point. The original myth, as I know it, was that end grain glue joints are weak and should never be used (possibly related to outdated info related to older glues that weren't as strong as today's glues). I don't like it when options are taken away from me, so now I will reconsider this joint as a possibility. Also one of the arguments used against end grain glue joints is by gluing together 2 long, but thin boards and then breaking them over the knee. How applicable is that really to what might happen to a particular woodworking project? In woodworking there are many joints available to pick and choose from, just like there are many fasteners available. As an example, I know that drywall screws are a no no for strength in joining 2 pieces of wood, but in a pinch am I going to use them when I run out of wood screws? Yes, of course, I will, though I will think twice if I need maximum strength. I really do question at times where some of this "advice" comes from in the online woodworking community. Another shift in ideology has been with regard to panel glue ups. I learned to always use biscuits or dowels to join boards for panel glue ups, but recently prominent online woodworkers (influencers) like to go on and on about how this is unnecessary as it is only providing alignment during glue-up and modern glues are sooo strong. Of course, perhaps these guys were prescient and realized that side grain is the weakest so why bother adding additional mechanical advantage to a panel glue-up. Jumping up and down on that table would have just broken the side grain anyway! :D

Ed in Tampa

I think the real message in the glue up controversy has been missed. From everything I have seen the discussion is just about using glue only. NOBODY denies that dowels, biscuits, dominos, splines or carefully cut special joints do not reenforce the joint. A mechanically enhanced joint will always be stronger than just a glued up joint. Glue holds wood together but does little to supply structural support that would normally damage wood. In other words if you have two 1x2 boards 4 feet long and glue them end grain to end grain making a 1x2 board 8 ft long and support it at the ends while pushing down on the middle the board, it will break but probably not on the glue joint.