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.
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.
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
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
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.
What? And screw up a good argument with FACTS?
LMAO!
Greg G.
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
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.
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
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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