Glue Joint Myths

[algale]

Gee i wonder why scarf joints were 'invented'.

Because there's more surface end grain surface area, but, again, I'm still surprised end grain-to-end grain ends up being the strongest joint.

[JPG]

I wonder if an end to side joint is stronger than a half lap?

Not fair! Again increased joint area.

Keep in mind all the tests were a 3/4 x 3 area.

Also keep in mind the direction of the 'breaking' force(s)(3).

[edma194]

Because there's more surface end grain surface area, but, again, I'm still surprised end grain-to-end grain ends up being the strongest joint.

Scarf joints have many times the surface area as simple side to side joints so they'd be very strong no matter which way the grain was oriented.

The experiment doesn't tell us how strong side-grain to side-grain glue joints are because in each of those cases it was the wood that failed, not the glue joint. We only know that an end-grain to end-grain glue joint has about twice the strength of the lignin in the wood samples tested. A side-grain to side-grain glue joint might still be stronger or weaker than an end-grain to end-grain glue joint, but it won't matter as long as it is stronger than the lignin. This could be tested with a different configuration of wood, larger stronger pieces that are only glued with the same small surface area as used in the video's test.

[Ed in Tampa]

I have used end grain to end grain glue up for years! Has something to do with me never using the measure twice and cut once rule. I usually measure once cut glue and cut again.

Hey I have even gotten better than fair at hiding end grain to end grain glue joints.

[RFGuy]

I have gone back and re-watched this video a couple of times to try to fully dissect it. Again, I wish he presented more of his data. This is the key statement that he said in the video that blew my mind. He said "the force required to break end grain joints was considerably higher than the force required to break side grain joints instead of being vastly weaker as we've been taught, end grain joints turn out to be about twice as strong as side grain joints on average". To put this into perspective, I have heard over and over again that end grain joints are taboo in woodworking. The end grain will soak up all the glue due to grain orientation and lead to a much weaker joint so end grain joinery should be avoided at all costs, or at least that is the jist of what I have seen repeated online ad infinitum in the woodworking community. What the testing in this video shows is that end grain glue joints are 2x stronger than side grain joinery. Unfortunately the guy in the video doesn't really clarify whether he is referencing end-grain to end-grain, or does he mean end-grain to side-grain, so we don't really know which one is 2x stronger than side grain joinery. Again this is where I wish he presented more of his data and/or presented the results a little more clearly. IF what he presented is accurate then it dispels a common myth in woodworking.

[JPG]

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.

[algale]

I wonder if an end to side joint is stronger than a half lap?

Not fair! Again increased joint area.

Keep in mind all the tests were a 3/4 x 3 area.

Also keep in mind the direction of the 'breaking' force(s)(3).

Seemed fair to me. All edges, whether end grain or long grain were the same are, all pieces all the same size. Presumably even if scaled up and thicker, as long as the glued surfaces are equal in area the same results should obtain, i.e. the end-to-end joint would be stronger than the long grain to long grain joint or end grain to long grain joint.

[algale]

Because there's more surface end grain surface area, but, again, I'm still surprised end grain-to-end grain ends up being the strongest joint.

Scarf joints have many times the surface area as simple side to side joints so they'd be very strong no matter which way the grain was oriented.

The experiment doesn't tell us how strong side-grain to side-grain glue joints are because in each of those cases it was the wood that failed, not the glue joint. We only know that an end-grain to end-grain glue joint has about twice the strength of the lignin in the wood samples tested. A side-grain to side-grain glue joint might still be stronger or weaker than an end-grain to end-grain glue joint, but it won't matter as long as it is stronger than the lignin. This could be tested with a different configuration of wood, larger stronger pieces that are only glued with the same small surface area as used in the video's test.

That's kind of the point, isn't it? The failure is with the wood on side-grain-to-side-grain but the failure was still at a lower force than on the end-grain-to-end-grain.

[edma194]

That's kind of the point, isn't it? The failure is with the wood on side-grain-to-side-grain but the failure was still at a lower force than on the end-grain-to-end-grain.

Yes, that is what is high-lighted here. It illustrates that the failure of side-grain joints is likely a failure of the wood, and of course the design. The glue itself has been a MacGuffin, and led to the stories about end-grain joint glue failures. JPG stated the actual problem with such joints quite well:

"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."

[Majones1]

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?