Biscuits for a table top?

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I am making a table top out of hard Maple (36" X 70"). I plan on using bisuits to join six sections together. The thickness of the table top is about 1 1/8" so its pretty heavy. My question is, how many bisuits should I use, how far spread apart? I was also wondering if I should just use one row of biscuits centered throughout the sections, or if I should use double or stagger them? Is there like a rule of thimb for biscuits, especially on a pretty heavy table top?
Thanks,
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The biscuits are only there for alignment - they'll add minimal strength. Use Gorilla glue or Titebond and that will be all the strength you're going to need.
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I'd use a single row, about six inches apart, starting two or three inches in from the end - this would work out to biscuits at 2, 8, 14, 20, 26, ..., 56, 62, and 68 inches, or twelve biscuits per joint.
No need to double them - they don't add significant strength to the joint. They're just there for alignment. Staggering them just complicates things unnecessarily.
It will take a while to glue that many biscuits. _Do_not_ attempt to glue up the entire tabletop at once: that's a big panel, sixty biscuits, and the glue will begin to set up on the first joint while you're still spreading it on the third one.
Your best bet on something that long, IMO, is to glue and clamp only one joint at a time.
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Doug Miller (alphageek at milmac dot com)
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No need for a double row; biscuits are basically for alignment so you don't fight keeping the boards aligned. Good glue will keep your table top together regardless of biscuits being used or not.
Dave
gwoodwork wrote:

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As everyone has pointed out, a properly glued joint is stronger than the wood it is made off. You can verify that by breaking the scrap you get when you even off the end. It is plenty strong, and never breakes at the glue line. Biscuits are primarily for alignment; which is extremely important for panels as big as you are making.
But, biscuits add considerable strength to the joint, as you can verify by making a panel and gluing nothing but the biscuits. The result is also stronger than the wood. But of course, the question then is why bother to strengthen something that is already stronger then the wood?
If, though abuse perhaps, you managed to break a glued up panel, it would break right next to the glue line; my scraps always do. That is because it has to break somewhere, and will do so at an irregularity. The glue line, however strong, is an irregularity, and serves as a locus for failure. Biscuits strengthen that locus and make the whole panel stronger. (and of course, it is only properly glued joints that are stronger than the wood; we all do things improperly now and then)
As anyone who has taken mechanical engineering knows, the entire strength of a beam lies at the top and bottom; the inside only prevents buckling. By placing two biscuits together you get them closer to the top and bottom, and so maximize their effectiveness. Just besure to let everything dry thoroughly before sanding, as biscuits near the surface can telegraph their location if you sand before everything is dry.
The one thing you have to watch out for is that a six board panel takes a while to glue up, and will take forever if you use many biscuits. I will use extended set time glue when doing something like that. It is also important to get each board in tight; if you wait until the end to get them together tightly, the biscuits will swollen and it will be difficult to crank everything together. Unfortunately, this takes even longer.
Have I completely confused you about what to do? Sorry.
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Although it's not too hard to peel a thin layer off a piece of wood along the grain, like the one the glue is stuck to.

It's not the irregularity that causes the wood to break.

Biscuits hold the wood together so that bending the wood loads the wood's face in compression and the inside with the biscuit in tensions.
It's like ripping a piece of corrugated cardboard - gripping the sides in their middle and pulling them apart does nothing. Starting at an edge it's easy to tear apart.
Where bending strength isn't important biscuits and dowels don't matter.
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Naw, the top half of the wood is in compression and the bottom is in tension; as it is in the biscuit if it is centered. Since the biscuit grain is at an angle to that of the panel, it resists the tension better than the panel does. (If the biscuit grain ran the same as the panel, it would probably weaken the panel.) That is why stacking them is so useful, it puts 100% tension on the top biscuit and 100% compression on the bottom one.

You are correct about that; strength probably isn't the correct word to use; but what word really means "resistance to breaking"? Toughness?
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wrote:

I would use a single row... maybe 6-8 inches apart...
The biscuits do absolutely nothing as far as strength is concerned But they sure make it a lot easier to align the boards...
I have only owned a Biscuit Jointer for about 3-4 years and to be honest it took me a good year before I really learned to appreciate just how useful it is...
How I managed without one for 40 years has me scratching my head...
Bob G..
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Maybe it's just me but I wouldn't use them at all. I have used biscuits on many projects and was very disappointed in one situation. Maybe it was just the wood (poplar) but at one point after the project was finished, the wood 'caved in' around the biscuit and I could see a perfect outline of all the biscuit joints in the project. Since it was a 'v' shaped bookshelf and the books would be sitting on the joint, I didn't sweat it. However, I have vowed to only use biscuits in 'hidden' joints. I will never use them in applications such as tabletops.
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What was the wood thickness?
Did you sand or scrape the surface before allowing the swelling to go down?
Dave
D Steck wrote:

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3/4"
Did not sand or scrape it for quite some time. It was quite odd. I've never had that happen but this my first time using biscuits on poplar.

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I seem to remember Norm saying a couple of times now that he's more careful about using biscuits as he's encountered the same problem. And he wasn't using popular... don't remember what, but i do remember him saying it was a problem...
Seems he'd know not to scrape/sand before it dried, but...
anyway, just some more info...
-phiip
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Philip Hallstrom says...

My guess would be that biscuits aren't the problem so much as excess glue. I've heard that yellow/white glue will cause depressions in the wood if used in excess. It makes sense that soft, weak poplar would be more at risk than most other woods. Norm really pours on the glue when he uses biscuits. I'm using epoxy more and more because of all the headaches with yellow glue.
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I've found that using biscuits in a tabletop glue-up can cause problems if you surface the glued up panel too soon. The added moisture of the glue makes the area around the biscuit swell slightly, so if you surface the panel right after the glue sets, there will be a slight depression in the surface in the area of the biscuit after the moisture fully leaves the glue and biscuit. Whenever I'm going to make a table or cabinet top I've now learned to let the panel sit unsurfaced for about a week before continuing with the surfacing. This lets the area shrink and stabilize so when it's surfaced it stays flat afterwards.
If I'm in a hurry and don't want to wait to prep the panel, I avoid water based glues and lean toward epoxies for my glue-ups. Even then it takes at least 24 hours for the biscuit pocket and epoxy to settle to a stable thickness before it's wise to surface. If you rush it, you will likely find depressions in the surface around the biscuit areas when the project is completed.
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An opinion based on such extensive experience...
Wood swells when it is wet, such as by glue. Accordingly, the biscuit area swells. If you sand while swollen, it will be depressed after it dries. Poplar swells about average, so it wasn't the wood.
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If you are using biscuits "only for alignment", which is an entirely reasonable choice for an edge grain panel glue-up, you can NOT glue the biscuits at all, that is insert them in their slots dry. This will eliminate the depressions caused by the wood swelling and shrinking. It also makes the glue-up easier and quicker.
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JeffB
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Ummmm.... perhaps you meant to say that you don't need to glue the biscuits; that, I wouldn't argue with. But to say that you must not glue them is simply not correct.

No, it won't - if you're using water-based glues, the wood will still swell at the joints, and if you sand or plane too soon, you'll still have depressions at the joints. The only difference is that those depressions will be shallow valleys running the entire length of the joints, instead of biscuit-shaped craters.
The problem with the depressions is caused *solely* by sanding or planing too soon after glue-up with a water-based glue.
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Doug Miller (alphageek at milmac dot com)
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I did not intend to convey the meaning that gluing biscuits is wrong. I pointed out it was a possible choice to alleviate the problem of biscuit shaped depressions showing up in this situation. It is certainly not the only option.

That is true, however it's easy for "too soon" to sneak up on you. It takes much longer for the moisture to dissipate from all the glue in a biscuit slot than from the area along the joint that would cause the "valley" to which you referred. Many people have run into the problem of biscuit shaped depressions appearing along joints. I just pointed out a simple way to alleviate that.
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This happened because the biscuits, and the wood immediately around them, absorbed water from the glue and expanded - and you sanded or planed the panel without waiting long enough for the moisture to dissipate. When it did, and the wood shrank back to its previous size, you were left with outlines of the biscuits.
There are two ways to avoid this: wait longer before you smooth the panel (24 hours works for me), or use a solvent-based glue such as Gorilla Glue.
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Doug Miller (alphageek at milmac dot com)
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Lots of good advice here but I'll chime in with my opinion anyway.
1. Yes use biscuits to help with alignment.
2. I would only use 3 or 4 along the length. They are just to help alignment and 5/4 + material is not going to unalign over even a few feet so 6" spacing is useless.
3. Be careful not to place them too close to the ends so when you trim to final size they won't be exposed.
4. You could easily not use them at all if the wood is good and flat and you have a good flat table to layout and clamp on.
5. It migh be a good idea to do this as two panels first, then glue the 2 panels together later.
6. I glue up maybe 20-50 panels of various types per year. Most smaller and some bigger than this project. I mostly don't use any buscuits anymore. For material this thick and long (I hear that alot) I'd probably use 3 biscuits just to avoid slipping as I clamped and I'd do 2 18"+ panels first, then join them later.
7. Use Titebond or similar, not Gorilla or Poly glues.
BW
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