Biscuit Joinery Question

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Hey Mac - thanks for posting this. There's a lot of speculation that takes place surrounding a lot of questions that pop up here and it's good to see some real documented results.
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-Mike-
snipped-for-privacy@sprintmail.com
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What? And screw up a good argument with FACTS?
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Edwin Pawlowski said:

LMAO!
Greg G.
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wrote:

no way! there is no place for facts or logic in a good thread...
I love the one's that begin with someone asking "how do I find the center of a circle" or something and end up being a debate on quantum physics..
Mac
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Mac I've been a commercial furniture builder for years, the biscuit is my method of construction. The only time I now use a mortise and tennon is when it's visible. Biscuits are available in several sizes including the S-6 which measures 1 1/4" X 3 3/8", use a S-6 in a table leg apron joint and I defy you to make it fail in normal use. Double stack it and it's almost impossible to break the joint.
--
Rumpty

Radial Arm Saw Forum: http://forums.delphiforums.com/woodbutcher/start
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wrote:

I think there is a use for what I consider "high end" joints like dovetails and tenons, but mostly for looks or the woodworkers pleasure/challange.. both methods are fine with me, I just don't have the skill level involved for fancy, precision joints yet..
Mac
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    Greetings and Salutations....
On 26 Sep 2004 10:47:26 -0700, snipped-for-privacy@camvideoproductions.com (Ronnie Aldrich) wrote:

    Hum...I would use polyurethane glue (gorilla glue, etc), and, would glue the end of the shelves as well as the biscuits.     However, since I am distrustful of shelves...I would also put a slight dado (1/8" or 1/4") into the sides and inset the shelves into them, and, skip the biscuits entirely.     If you put a back on the shelves, your plan will work ok. However, if no back, then the shelves may rack a bit, and break the shelf to side joints loose...dropping the shelves out.     Actually, I also just think that butt joints like that are kind of ugly, and I prefer the dado look...and it IS a lot stronger and stiffer.     Regards     Dave Mundt
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Mix of answers, what? The biscuits resist shear - the downward weight of the books, in this case, very well. They don't do as well against racking - the bump on the top corner or the attempt by someone to drag the case on one end while partially unloaded. That is best served by a full back, fastened to both sides and shelves.
The glue on the shelf ends would help a bit in keeping the joint butted square and tight, aiding in both shear and rack. Probably won't be much, but wouldn't hurt, certainly.

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Wow, did you get some replies that hit most of the possible range! Maybe we can look into this a bit more. Now, gluing end grain to face will be a very weak glue joint. And if the end grain is plywood, it'll tend to be even lower density and even weaker (depending on the plywood being lumber core, hardwood ply, or mdf core). Also, any even small racking in the frame will likely break that glue joint.
While biscuits are primarily used for alignment when edge gluing, that's not the only way they can be used. Adjustable shelves, for instance, can hold quite a bit of weight with even a 1/4" dowel used in four holes at or below the ends. That's because you're relying on the shear strength, which is pretty high. If your shelves are fairly tight fitting, I would suspect a biscuit would have about the same shear strength, and that the shelves would hold up with nothing but dry biscuits in them (assuming the frame prevented racking).
Gluing the biscuits adds little to the shear strength, but holds the edge of the shelf against the face of the side, helping to stabilize the frame against racking. I suspect the strength of this joint is such that gluing the sides won't help significantly. Of course, it really won't hurt, either.
For more information on this subject, try it at home. Prototype a shelf with dry biscuits and load on alot of weight. If the shelf's a couple of feet long, I'll bet it bends first. Then take two plywood scraps and glue one to a face with biscuits, and the other just with glue on the edge. Let dry overnight, then break each to see the difference, and conclude for yourself if it's worth gluing the edge.
BTW, remember that you are free to use as many biscuits as will fit, and four on an edge are much stronger than two, of course.
GerryG
On 26 Sep 2004 10:47:26 -0700, snipped-for-privacy@camvideoproductions.com (Ronnie Aldrich) wrote:

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On 26 Sep 2004 10:47:26 -0700, snipped-for-privacy@camvideoproductions.com (Ronnie Aldrich) wrote:

Hello Ron,
Are the shelves for Video Casettes for your business or something heavier? Providing additional glue to the edge and face of a 90' plywood joint without a dado or T&G joint will fail over time. Using Lamellos with glue may be sufficient ( and probably will be). Please note...the potential of glue "squeeze-out" may compromise your possible intentions of staining the shelving unit(s) Clean up that excess glue.
BTW...Opps. I just read through my post prior to sending and saw that I used the word Lamello. It's a brand name from the Swiss who developed the first biscuit joiner machine I think. Most folks look at me funny in the shop when I ask them to fetch the #20 Lamellos...LOL Good Luck Ron.
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snipped-for-privacy@camvideoproductions.com says...

Anything that increase glue area adds strength to a joint. So, no you don't need too but the joint won't be as strong as it could be.
Is it good enough for your application. You got me, my crystal ball is out for a tune up.
--
MikeG
Heirloom Woods
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