Biscuit Joinery Question

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Hello all,
I am making a simple set of shelves out of 3/4" plywood with the shelves attached to the sides with biscuits. My question is: Do I need to put glue on the ends of the shelves in addition to the biscuits, or will just the glue in the slots work good enough?
Thanks in advance,
Ronnie Aldrich B'ham, AL
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With most any joint of this type, glue on all mating surfaces is stronger than not.

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"Ronnie Aldrich" wrote in message

With plywood, glue _both_ the edges, and the biscuits, before clamping.
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I have made joints with glue only on the biscuits just for fun and they have been plenty strong. (In fact I have used just water and they have been pretty good.) Since you will have mainly shear stress, it ought to be fine if you don't want to glue the ends for some reason; especially since end grain gluing isn't worth much anyhow.
Is there a reason you don't want glue on the ends?
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The only reason I have for not wanting to glue the ends is so I would not have to deal with glue squeeze out.
Thanks for all you help.
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"Ronnie Aldrich" wrote in message

Got a table saw and dado set, or a router with a guide? It's pretty hard to beat a dado/groove for non-adjustable shelves in bookcases and cabinets. ("housed" joint, patriarch ;>) )
A simple blind dado/groove is hell-for-stout, will hide the joint, and it'll last until your grandkid's grandkid's cows come home.
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Ronnie Aldrich asks:

Biscuits are primarily alignment devices, adding very little extra strength in most cases. Add glue to the end of the wood, too, as well as to the slots and the biscuits.
Charlie Self "Half of the American people have never read a newspaper. Half never voted for President. One hopes it is the same half." Gore Vidal
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snipped-for-privacy@aol.comnotforme (Charlie Self) wrote in

Can I disagree here, Charley, without starting a religious war?
In this case, I believe these are much more analogous to a floating tenon, and that what is being made here is much closer to a housed joint. The added shear strength is important.
If we were discussing edge gluing, we would agree.
Patriarch
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patriarch responds:

I'd like to test that some day when I have time. Not this year. But three or so biscuits in the end of a 12" wide shelf do not seem to me to add anything much to overall strength, though they do add a little. That doesn't reduce the need for glue on the end of the board where it meets the vertical side of the shelf carcase.
Charlie Self "Half of the American people have never read a newspaper. Half never voted for President. One hopes it is the same half." Gore Vidal
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snipped-for-privacy@aol.comnotforme (Charlie Self) wrote in

That's true.
I'll admit also that my tendency is to find a means of hiding some of McFeeley's best in there as well. Over-engineered, since these things tend to get overloaded. And somewhat abused, when moved around, during the course of life.
The last set has pocket screws, biscuits, glue, and "brads 'til the glue dries." Won't likely end up in a museum. Might not even sell in a yard sale. But it's holding up 6 cases of engineering texts right now in my office, and is 'seismically stable'.;-) I needed it quick, since one of the kids needed to move back home 'for a while'.
What's Brother Roy say? 'Twill do. 'Twill serve.
Patriarch
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"patriarch wrote in message

Not without a dado/groove, eh? It appears he was talking butt joint to me.
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What I was thinking was a constructed mortise and tenon. Perhaps I used the wrong term. Wouldn't be the first time.
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snipped-for-privacy@aol.comnotforme (Charlie Self) wrote in message

Charlie,
Somewhere I recall reading an article comparing the strengths of various end-to-edge joinery techniques. Included were a plain butt joint, dowels, M&T, two biscuits, three biscuits, and maybe others (all were glued). I was surprised to read that the two and three biscuits provided as much strength as the M&T (additional strength from the third biscuit was insignificant). The article implied that, properly used, biscuits add _a lot_ of strength. As a relative newbie (have yet to use biscuits), I am left wondering what to believe. Perhaps I'm misinterpreting your position. If so, I'd appreciate a little light on the subject. TIA.
Cheers, Mike

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Ronnie Aldrich wrote:

It's actually more important that glue be spread across the mating surfaces than in the slots. Biscuits will aid in alignment but add little strength.     mahalo,     jo4hn
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Dang, I better change professions.......all of my work is about to fall apart...
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Well, you surely won't be alone. All the people who've driven a nail or screw into the edge of a shelf will be right behind you.
Actually, just happened to be reading Choosing the Strongest Joinery for Doors by John Wagner, in FWW Practical Design Solutions & Strategies. A little different as it's not shear resistance, but he compared bolts, T&G, dowels, M&T and biscuits for failure in door joints. On pure strength, biscuits came out on top.
GerryG

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I can believe that! I was looking at some of the practice joints in the scrap/firewood box that I did while learning to use the biscuit jointer and they will NOT break loose as my old "screw & glue" or "dowel & glue" joints do...
A problem with biscuit joints at my current skill level is that the mistakes and rejects could out live me...
Mac
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Actually biscuits do add strength but not in all cases. If gluing long grain to long grain, they are better for alignment purposes. However as you start to introduce more end grain into the joint, 45's and butt joints, the biscuit adds significantly more to the joint in terms if strength. The biscuit adds the long grain to long grain glue surfaces within the biscuit slot to join to the long grain of the biscuit surface.
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True, but the type of stress the joint must handle is still the starting point. Then look at the biscuit (dowel, tenon, or whatever) and apply any mechanical advantage, then finally the glue bond.
And an obvious mechanical advantage of biscuits is their swelling. Although that decreases as they dry, the glue tends to prevent this. Further, even without the glue, they will not return to their original thickness (try this at home; it will vary with different makes of biscuits).
From that analysis, if we were to look instead at edge joining boards with biscuits, it can be seen that the biscuits will aid in alignment, but add little strength as compared to the long grain glue joint between the edges. (Assuming, of course, that you haven't used umpteen biscuits.)
In other applications (such as the start of this thread), this can work out quite differently.
GerryG
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On Mon, 27 Sep 2004 15:07:34 GMT, "Leon"

no kidding!! after reading this thread yesterday, I went into the scrap bin last night and played with a 18 x 8" frame that i'd made for practice... Made of 1 x 3" pine and butt jointed, 1 biscuit per joint... glued in slots and face/grain of joint..
I locked it in the bench vice and started a sideways rocking, like the folks in the thread described as what would most weaken a box during use...
After 4 or 5 minutes, I got one end (2 joints) to break loose... one joint had slivers of biscuit and pine on both sides, the other joint had an almost perfectly 1/2 biscuit shaped piece of pine attached to the biscuit of the other part of the joint..
I don't know if I described that right, but there is a 1/2 oval of pine, from the side of the biscuit to the face of the board, and the other half of the joint has a matching 1/2 oval missing from the end grain...
After breaking up several glue joints and some glue and dowel joints, I'm impressed with the biscuits!
Mac
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