Biscuts versus glue joint bit opinion wanted

[Dave]

What's your opinion of these joints, is one any better than the other? Thanks

[dpb]

Different, in style, purpose...

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[SonomaProducts.com]

I suppose you are thinking of the glue joint bit as a tongue and groove type connection. Many times glue joint shapes are used on opposing (edge-to-face) joints like a drawer corner and T&G (which is usually deeper) is used for edge-to-edge but the glue joint bit is mostly the same.

Biscuits are mostly about alignment. The glue joint or T&G adds much more gluing surface area plus alignment.

If you are doing a panel glue up a simple edge butt joint will do fine if milled and clamped correctly, biscuits will help with alignment and a T&G type joint will be much stronger.

Another option is using a spline and you can use a stopped spline and get the advantage of a long added glueing surface area and hid it from the ends of the panel if they'll be exposed.

BW

[Phisherman]

There are pros and cons to each. One can be better than the other, depending on the joint desired.

[Lowell Holmes]

I've had a biscuit jointer for 15 years. IMO the only thing biscuits do is help with alignment. They can create problems for you if they shadow through and show on the finished joint. My experience is that they add no strength to the joint.

[Leon]

"If" the glue joint bit can be used it will be infinitely stronger than a joint reinforced by biscuits.

[Toller]

Specifically, what experiences lead you to that conclusion?

I have found that if you glue nothing but the biscuits you get a strong joint. That suggests that gluing both the buiscuit and the edge will give a stronger joint than if you didn't use biscuits. But I have never tested that suggestion.

[Chris Friesen]

Are you sure about that?

It's accepted fact that a properly executed glue joint is stronger than the surrounding wood. Given that any failure will already be in the wood rather than the joint, how can a T&G type joint be any stronger at all?

Chris

[Leon]

If you use biscuits on mitered corners and or butt joints, end grain to side grain the biscuit will add tremendous strength to those joints.

[Upscale]

Increased surface area with glue joints is one reason. Also, a T&G to fail completely would require failure several areas.

[Chris Friesen]

But the increased surface area doesn't buy you anything. In either case the failure will be in the wood, not the glue joint. There is the same amount of wood in either case, therefore the strength will be identical.

As for the "failure in several areas" argument, it doesn't really make sense. Any failure will be in the wood, not the glueline. What difference does it make if the wood fails beside a simple edge joint or beside a T&G joint?

T&G could make alignment simpler because the boards can't slip relative to each other. But that's the only advantage that it has. There's no way that it can possibly be any stronger than a well-made edge joint in a panel glue-up, because the edge joint is already as strong as the wood itself.

Chris

[dpb]

That conclusion, of course, depends on the wood and the glue as well as the quality of the joint itself.

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[SonomaProducts.com]

Its always interesting to see what statements will cause controversy.

I have not done any purely scientific tests but I did once have the opportunity to do karate kicks of glued up panels some of which were edge butted and some were T&G. The T&G's seemed about twice as hard to break.

Yes, everyone has bought into the concept that a well executed butt glue joint with modern wood glues is stronger than the wood. Interestingly in the case of PVa types (like Titebond II) there are really only generally accepted therories about how it actually bonds but no real provable science at the molecular level, just via physical testing. However, break apart any butt glued joint, or any joint for that matter and you'll see the wood fracturing in some places and the glue joint breaking in others.

Things to consider in the non-perfect world we live in.

1. Strenght of the glue joint: Imperfections in the glue joint may cause weak spots so some mechanical advantage could add strength.

1. Strength of the wood: Even if you just consider the wood strength, involving the tongue of one piece inside the groove of another can compensate for any weak areas in the grain of either. Where one might split from direct downward force, the other might be strong enough to hold

2. Strength of the glue joint (again): With a much expanded surface to surface connection between the mating parts, at least double I assume, you have greatly increased the odds of good contact all along the joint. You have greatly lowered the possibility that the two pieces are joined at a coincedient weak point.

Adding a spline with an opposing grain orientation will add even more strength than the tongue concept and still includes the benefit of added glue surface area.

BW

[Toller]

Leaving aside the fact that both are exactly as strong as the wood, and no stronger; "infinitely stronger" is rather absurd.

[Leon]

Normally not true if gluing mitered corners or end grain. In that case the biscuit adds significant strength because the glue does not bond well to end grain.

[Leon]

HUH?

[Toller]

But you can't use a glue joint bit there, so it is outside the OP's parameters.

[Leon]

HUH?

[Toller]

I'm sorry, I thought you could read. I'll just block your posts so this doesn't happen again.

[Leon]

I wish you would.