Glue Joint Myths

[edma194]

Being new to woodworking, I’m curious to know how and if time would affect his joint tests. Would the movement of wood (expansion/contraction) affect the glue bonding over the years? I would expect there might be some impact on the end-to-end joint since there is more movement there. Or do you think that would be negligible?

PVA glues are water soluble and over time in humid environments or with continued exposure to water the joint may eventually fail. I have seen Cutting boards that people have wet washed repeatedly come apart at the seams. Those usually have mostly side grain joints but often have edging that is glued end-grain to side-grain.

As for movement due to temperature and humidity changes, I don't think there is substantially more movement of the wood over time in the end grain at the joint as opposed to side grain. A board expands and contracts substantially more along along its length than side to side because boards are usually much longer than wide. A small square cut piece like one used in the video would not have a noticeable change in size in any direction. You'll find numerous chess boards that are very old and still very tight even if they have been made with alternating grain orientations.

Of course anything not well glued in the first place could have problems no matter what grain orientation is used or what the conditions are. I'll bet most failures are due to things like gaps between the mating surfaces, too much or too little glue, lack of sufficient clamping or drying time, dirty mating surfaces, and other issues besides the grain orientation.

[RFGuy]

I think what he 'proved' is the same thing glue purveyers have said recently(?) that the glue joint IS stronger than the wood(lignin).

Where end joints are weak is that usually that end grain is at the end of a longish slenderish piece that is likely subjected to leverage.(something he did eventually mention)

Such a 'joint' WILL fail far before a more substantial 'joint' would, but it is not relevant to the glue strength, but rather the structural strength of the joint itself.

Supporting the glue with essentially nothing causes forces that create the failure of the wood itself. Those large forces are concentrated in close proximity to the glue 'joint'.

I see nothing controversial other than the implied(not stated) conclusion regarding conventional wisdom re end grain 'joints'.

FWIW I do NOT consider a glued butt 'joint' to be a joint, but rather wishful thinking.

JPG,

Yeah, I agree with you. I personally don't like butt joints even though many claim they are fine, e.g. for panels like table/desk tops. I still prefer stronger mechanical joints in addition to the glue, even if they are only there for alignment purposes. Maybe this is my own failing, but I had assumed that end-grain to end-grain joints were too weak to even consider based on the false messaging saying that open end grains would soak up all the glue you could throw at them and NEVER bond well. Clearly, when done properly, end-grain butt joints are at least as strong as the glue bond strength which is quite significant. This is a new revelation for me at least because a lot of the online content on this topic tends to indicate that a good bond is impossible for end-grain joints. Of course, this presumes that you can get a good amount of surface area for the joint, so a slender piece would undoubtedly be weaker than a joint with higher cross sectional area.

[JPG]

I think the surface of end butt joints being not finely sanded contributes to failure. With perfectly flat surfaces, the glue adhesion becomes the weak link.(apparently stronger than the lignin) When rough surfaced, the wood fibers inter mingling become the weak link.

Lotta conjecture there.

[BuckeyeDennis]

Being new to woodworking, I’m curious to know how and if time would affect his joint tests. Would the movement of wood (expansion/contraction) affect the glue bonding over the years? I would expect there might be some impact on the end-to-end joint since there is more movement there. Or do you think that would be negligible?

On an end-grain to end-grain joint, assuming the same wood type and grain orientation, the two pieces will expand and contract pretty much the same amount in response to humidity changes. End grain to side grain is where I’d expect to see expansion stresses. Wood movement is generally negligible along the length of the wood fibers, but can be very substantial in the directions orthagonal to the fibers.

[rlkeeney]

Why not post your concerns/opinions on the youtube page and give the guy some feedback?

[rlkeeney]

I have unscientifically tested end grain gluing using the method described in Nick Engler's gluing tips video. I could not break the joint.

https://youtu.be/Wmw5rayNIV4

[RFGuy]

Why not post your concerns/opinions on the youtube page and give the guy some feedback?

Robert,

Thanks. I thought about it, but I checked a few of his videos and it looks like he never responds to comments on his YouTube videos, so I really doubt he would respond. I would be interested to see his data just to see how the different orientations affect the joint strength.

[RFGuy]

Steve Ramsey did a quick test to put that Patrick Sullivan glue myth video to the test. It is a crude, but effective test that quickly confirms that yes end grain glue ups are stronger than side grain glue ups. As Mythbusters would say, this myth (end grain glue ups are weaker) is "BUSTED".

[davebodner]

I think it's sloppy to say that end-grain joints are stronger than side-grain joints. He didn't prove that.

He proved that end-grain joints are stronger than side grain, itself (not side-grain joints). Those side grain joints didn't fails. The adjoining wood failed (which surprises none of us).

We don't know if the end-grain joint is stronger or weaker than the side-grain joint, since we never saw a side-grain joint fail. Still it was surprising just how strong the end-grain glued butt joint could be. It appears that a properly prepared, end-grain butt joint could replace a scarf joint. Not that I think I could ever convince the building inspector!

[RFGuy]

I think it's sloppy to say that end-grain joints are stronger than side-grain joints. He didn't prove that.

He proved that end-grain joints are stronger than side grain, itself (not side-grain joints). Those side grain joints didn't fails. The adjoining wood failed (which surprises none of us).

We don't know if the end-grain joint is stronger or weaker than the side-grain joint, since we never saw a side-grain joint fail. Still it was surprising just how strong the end-grain glued butt joint could be. It appears that a properly prepared, end-grain butt joint could replace a scarf joint. Not that I think I could ever convince the building inspector!

Well now we are into semantics. If the wood directly adjacent to a side grain joint fails, then I consider that a joint failure even though the glue didn't fail at the reference plane of the joint. It is still a premature failure. There have been quite a few joint tests done online; often joining 2 boards at 90° like you would have in a face frame carcass. Even when joined with M&T joinery, it is still considered a joint failure when the wood breaks adjacent, but very close to the joint. Fair? Maybe not, but this has been fairly common practice for some time now.

According to the Patrick Sullivan video (top of this thread), the PVA glue bond itself is stronger than the Lignin that holds the wood fibers together. So the fact that the end grain joint breaks evenly at the reference plane of the joint (glue bond failure), whereas the side grain joint breaks in the Lignin holding those side grain fibers together proves that end grain joints are stronger than side grain joints. In my opinion, Steve's video also confirms, albeit crudely, that this is the case. This has been a long standing myth in woodworking, i.e. end grain joints are weaker, so I am sure this will continue to be controversial for quite some time...