Biscuit Joinery

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I just bought a biscuit joiner. Need to join (salvaged) 3/4" pine boards to make kitchen cabinet.
What size biscuits to use? I'm guessing #20s. How to space them? How far center of first biscuit from edge of board? After that. how far to center of next one etc. ?
Thank you.
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JacksonD wrote:

Basically, all the biscuits are for is an easy alignment. More than one 8" is definitely overkill.
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They've *got* to add strength. What about all that additionl gluing surface ?
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http://www.sawmillcreek.org/showthread.php?t=7651
R
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Eh? No. Modern wood glues and good clamping produce a joint who's strength exceeds the strength of the wood around it. So failure will occur in the wood not the glue joint. -- Doug
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I space them about every 12" to 15". Keep away from the edges in case you have to trim and leave the biscuit exposed if the end is exposed. Don't go crazy with measuring. Put the two boards in place where you will join them. Make a pencil line across the two boards and that is the center line for the joiner.
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Use the biggest you can get away with. # 20 is good.

Every foot or so if your boards are straight. Every three inches if they're crooked. Of course, if they're that crooked, you should get new supplies. Biscuits are just for aligning edges. They add very little strength.

Far enough that you won't saw into it. Of course, if the edges are hidden, put them anywhere

Note: Keep your biscuits sealed up. They'll absorb water from the air and swell, then they won't fit in the slot.
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into it.
The primary use is alignment; getting your boards together evenly so you don't have to plane/sand 1/8" off to get them even. If your boards are perfectly flat, 2 or 3 biscuits will be fine. If they are not (and even properly jointed and planed board can warp pretty quickly) then you will need more than that to keep things reasonably even. I can't tell you from here what you need. But get them as close to the edge as you can because that is where alignment is needed the most. If the edges will be seen, be careful because biscuit scars can't be hidden if you cut through one.
I ran out of 20s and have been using a big box of 10s I got for nothing at an auction. If you only want them for alignment, I can't see that it makes a difference. For strength... oh lets not go there.
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Oh come on lets go there. Somebody move the 'do biscuits add strength' thread to rec.woodworking. It will be fun.
To me the depth difference between a 10 & 20 isn't that much so if biscuits do add strength the added strength a 20 would give over a 10 is insignificant. Use whatever you have.
If the edges of your boards aren't straight it won't matter what size you use because the joint will most likely fail.
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If biscuits add no strength, and are solely for alignment, then why do we glue them in? Why not just let them "float"? What is the glue on the biscuit for then?
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JacksonD wrote: ...

They only replace wood which was removed to insert them; how can they add significant strength over the solid wood which was there previously?
I didn't say "no strength" earlier, I said "mostly for alignment".
It also depends on how you define and test the joint strength and how the joint is made. In a well-fitting edge joint, the glue bond will be as strong or stronger than the wood, anyway, in bending. To see this, make a few and then break them by bending--almost all will fail away from the joint, not at the joint itself (again, assuming a well-fitting joint and adequate clamping pressure).
If, otoh, the joint doesn't fit particularly well or isn't clamped properly, then the biscuit may be essentially the only thing actually holding the two pieces together.
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"They only replace wood which was removed to insert them; how can they add significant strength over the solid wood which was there previously? "
Because the biscuits *span* the joint. The wood they replaced didn't. I'm assuming the fibers in the biscuits run across the joint. Otherwise I would expect biscuit could fracture lengthwise just like the wood.
Granted that the work will ultimately break along the fibers parallel to the joint, the biscuit ought to hold the fibers immediately adjacent to the joint together. So at least the failire won't be within an inch from the joint on either side.
This is all really more than I wanted to know. My intent was to use the biscuits for alignment, as intended.
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JacksonD wrote: ...

But the glue is stronger than the wood in most cases anyway (again, well-made, etc., etce., ...)

Actually, they're laminated (normally 3-ply) but they're quite thin so there's not much material there. It is that additional cross-grain that is what little actual strength difference there is which owing to the size isn't really all that much...

Well, you asked... :)
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dpb wrote: ...

And, of course, in the case of pine or similar, that the biscuits are birch or similar wood of somewhat stronger properties--but they're still small in comparison so it's a minor effect.
One last note is that they can be used in places for mechanical strength where otherwise there wouldn't be any--end to side grain, or other places where a long to long grain joint isn't possible...
--
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That is not correct. They are made from solid wood with the grain running at 45 degrees to their length. They are compressed as part of the manufacturing process so that when water based glue is used, they expand to make a tight fit. When used it long grain to long grain joints, they do not add significantly to the strength of the joint, but in but joints or joints where one surface is end grain, they do add considerable strength.
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There are no stupid questions, but there are lots of stupid answers.

Larry Wasserman - Baltimore Maryland - lwasserm(a)sdf. lonestar. org
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Larry W wrote:

Those I have are clearly laminated (and compressed) -- one can see it on the edges. Who manufactured these I don't know; they were a generic 3000 mixed selection of 1000/1000/1000 0/10/20 and I couldn't even say where I got them anymore. I don't use biscuits much so this is the only stash I've had in years so made assumption they all are.
The rest I also said in one post or another...
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I don't know if biscuits add strength or not. I've read articles both ways. I suppose the application determines if strength is added or not. Either way a 10 & 20 are so close in size it doesn't matter which one you use.
Pine boards properly glued are going to fail along the grain long before the joint fails - biscuited or not. This may not be true if you have some old growth pine but it still doesn't matter. I use 20s when gluing panels and the smaller biscuits when 20s won't fit.
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Limp Arbor wrote: ...

Tests consistently show the same to be true for well-made joints in virtually all woods for anywhere from 60% up of all joints. FWW has published many tests of various types from the simple butt to M&T in various woods w/ varying manufacturing tolerances, gluing pressures, glues, etc., etc., ... Good joints w/ PVA (yellow) glue are as good as or better than the wood in virtually all cases -- that is, the wood fails before or in conjunction w/ the glue rather than the glue alone failing at the joint line(s).
Hence my assessment the question of biscuits and strength is moot anyway as the joint is likely already as strong as the material.
--
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wrote:

> bought a biscuit joiner.

It adds *some* strength, but it is the glue that does the holding power. The glue on the biscuit causes the biscuit to swell inside the slot helping the lock the joint in place. For thinner stock biscuits should not be used as this may cause a raised area from the swelling. There have been situations where I left the biscuit float due to possible excess wood movement (eg, breadboard ends).
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Phisherman:
Please explain why breadboard end is a special case. Thank you.
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